diff --git a/pkg/calc/number.go b/pkg/calc/number.go
new file mode 100644
index 0000000..feb6661
--- /dev/null
+++ b/pkg/calc/number.go
@@ -0,0 +1,19 @@
+package calc
+
+import "math"
+
+// Ceil3 rounds num UP to three decimal places. The ship-class
+// calculator displays every computed value (and every goal-seek
+// back-solved input) through this so a result is never shown lower than
+// it really is — e.g. a speed of 5.0003 reads as 5.001, not 5.000, which
+// matters when a fraction of a light-year decides whether a ship clears
+// the gap to a planet. It is display-only and lives here (rather than in
+// the engine's round-to-nearest util.Fixed*) so the UI bridge can reach
+// the one implementation through WASM.
+//
+// num is pre-rounded to nine decimals before the ceil so float64
+// representation noise does not push an exact value up a step (e.g. a
+// computed 5.0 stored as 5.0000000002 stays 5.0).
+func Ceil3(num float64) float64 {
+	return math.Ceil(math.Round(num*1e9)/1e6) / 1000
+}
diff --git a/ui/core/calc/number.go b/ui/core/calc/number.go
new file mode 100644
index 0000000..4a01475
--- /dev/null
+++ b/ui/core/calc/number.go
@@ -0,0 +1,10 @@
+package calc
+
+import "galaxy/calc"
+
+// Ceil3 wraps `calc.Ceil3` (`pkg/calc/number.go`): round up to three
+// decimal places. The calculator formats every displayed number through
+// this bridge so the UI and the canonical Go implementation agree.
+func Ceil3(num float64) float64 {
+	return calc.Ceil3(num)
+}
diff --git a/ui/core/calc/number_test.go b/ui/core/calc/number_test.go
new file mode 100644
index 0000000..10fca37
--- /dev/null
+++ b/ui/core/calc/number_test.go
@@ -0,0 +1,27 @@
+package calc_test
+
+import (
+	"testing"
+
+	source "galaxy/calc"
+	bridge "galaxy/core/calc"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCeil3Parity(t *testing.T) {
+	t.Parallel()
+	cases := []float64{0, 5, 5.0003, 4.2761, 139.29503, 0.0001, 1.9999999998}
+	for _, c := range cases {
+		assert.Equal(t, source.Ceil3(c), bridge.Ceil3(c))
+	}
+}
+
+func TestCeil3Values(t *testing.T) {
+	t.Parallel()
+	assert.Equal(t, 5.0, source.Ceil3(5.0))
+	assert.Equal(t, 5.001, source.Ceil3(5.0003))
+	assert.Equal(t, 4.277, source.Ceil3(4.2761))
+	// Float noise just above an exact step stays put.
+	assert.Equal(t, 5.0, source.Ceil3(5.0000000002))
+}
diff --git a/ui/docs/calc-bridge.md b/ui/docs/calc-bridge.md
index 7e5e624..8a036eb 100644
--- a/ui/docs/calc-bridge.md
+++ b/ui/docs/calc-bridge.md
@@ -49,6 +49,7 @@ on the JS-side `globalThis.galaxyCore` (registered in
 | `shieldsForDefence` | `calc.ShieldsForDefence(targetDefence, sTech, restMass)` | `number\|null`  | calculator goal-seek (defence → shields) |
 | `cargoForEmptyMass` | `calc.CargoForEmptyMass(targetEmptyMass, restMass)`     | `number\|null`  | calculator goal-seek (mass → cargo)      |
 | `loadForFullMass`   | `calc.LoadForFullMass(targetFullMass, emptyMass, cTech)` | `number\|null`  | calculator goal-seek (loaded mass → load)|
+| `ceil3`             | `calc.Ceil3(value)` (`pkg/calc/number.go`)              | `number`        | calculator display rounding (round up to 3 dp) |
 
 `BombingPower` and the per-turn build loop are no longer engine-only:
 Phase 30 extracted `BombingPower` from
diff --git a/ui/docs/calculator-ux.md b/ui/docs/calculator-ux.md
index 376d8f1..81646c2 100644
--- a/ui/docs/calculator-ux.md
+++ b/ui/docs/calculator-ux.md
@@ -28,8 +28,11 @@ in as a per-ship result rather than a separate mode.
    icon once overridden; clicking it resets to the default.
 2. **Calculator area** — derived results: empty/loaded mass, empty/
    loaded speed, attack, defence, bombing (per ship), cargo capacity.
-   A load toggle (empty / full / custom) sets the cargo load that the
-   loaded-column results use.
+   A load toggle (empty / full / custom) sets the cargo load (in cargo
+   units) that the loaded-column results use. At **full** the toggle
+   shows the ship's cargo capacity; a **custom** load over that capacity
+   is flagged as an error. With a zero cargo block there is no hold, so
+   the load is pinned to empty and the toggle is disabled.
 3. **Planet area** — when an own planet is selected on the map, shows
    its MAT (overridable) and the single-turn build rate (ships per turn,
    turns per ship). The realistic multi-turn forecast with CAP/COL
@@ -61,7 +64,24 @@ appears once a value is pinned, click to release):
   solved block that fails the value rules — leaves the locked cell in a
   red error state and does not apply. Inverse solving lives in
   `pkg/calc/solve.go`; the bisection for defence → shields is the only
-  non-analytic case.
+  non-analytic case. Locking a speed is disabled when the drive block is
+  zero (a deliberately immobile ship has no speed to back-solve).
+
+## Validation and display
+
+Every numeric input is validated independently and an offending one gets
+a red border and a hover/tap tooltip with the reason: no value may be
+negative, the five blocks follow the engine value rules
+(`pkg/calc/validator.go`, surfaced per-field by
+`shipClassFieldErrors`), and a custom load may not exceed cargo capacity.
+
+Every displayed number — the derived results and the goal-seek
+back-solved input — is rounded **up** to three decimals through the
+shared `pkg/calc/number.go.Ceil3` (bridged as `core.ceil3`), so a value
+is never shown lower than it is (a speed of 5.0003 reads 5.001). The
+engine keeps its own round-to-nearest `util.Fixed*`; `Ceil3` is a
+display-only helper that lives in `pkg/calc` so the UI and Go share one
+implementation.
 
 ## Create / load / delete
 
diff --git a/ui/frontend/src/lib/calculator/ship-design-area.svelte b/ui/frontend/src/lib/calculator/ship-design-area.svelte
index 0d5c02e..6eb0473 100644
--- a/ui/frontend/src/lib/calculator/ship-design-area.svelte
+++ b/ui/frontend/src/lib/calculator/ship-design-area.svelte
@@ -8,7 +8,11 @@ The component is presentational — the parent owns the state and the
 calculator math — so the ship-group upgrade flow can reuse it later.
 -->
 <script lang="ts">
-	import { i18n } from "$lib/i18n/index.svelte";
+	import { i18n, type TranslationKey } from "$lib/i18n/index.svelte";
+	import {
+		shipClassFieldErrors,
+		type ShipClassValueInvalidReason,
+	} from "$lib/util/ship-class-validation";
 	import type { ClaimedInput } from "./calc-model";
 
 	export interface DesignBlocksState {
@@ -49,6 +53,29 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 		onResetTech,
 	}: Props = $props();
 
+	const VALUE_REASON_KEY: Record<ShipClassValueInvalidReason, TranslationKey> = {
+		drive_value: "game.calculator.invalid.drive_value",
+		armament_value: "game.calculator.invalid.armament_value",
+		armament_not_integer: "game.calculator.invalid.armament_not_integer",
+		weapons_value: "game.calculator.invalid.weapons_value",
+		shields_value: "game.calculator.invalid.shields_value",
+		cargo_value: "game.calculator.invalid.cargo_value",
+		armament_weapons_pair: "game.calculator.invalid.armament_weapons_pair",
+		all_zero: "game.calculator.invalid.all_zero",
+	};
+
+	// Per-block validity (independent of which one failed first) so every
+	// invalid input is highlighted, not only the first.
+	const blockErrors = $derived(shipClassFieldErrors(blocks));
+
+	function blockError(key: keyof DesignBlocksState): string {
+		const reason = blockErrors[key];
+		return reason === undefined ? "" : i18n.t(VALUE_REASON_KEY[reason]);
+	}
+	function techError(key: TechKey): string {
+		return techs[key] < 0 ? i18n.t("game.calculator.invalid.tech_value") : "";
+	}
+
 	const BLOCK_ROWS: {
 		key: keyof DesignBlocksState;
 		label: () => string;
@@ -92,6 +119,8 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 					min="0"
 					bind:value={blocks[row.key]}
 					readonly={blocksReadonly}
+					aria-invalid={blockError(row.key) !== "" ? "true" : "false"}
+					title={blockError(row.key)}
 					data-testid={`calculator-block-${row.key}`}
 				/>
 			{/if}
@@ -105,6 +134,8 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 						min="0"
 						bind:value={techs[techKey]}
 						oninput={() => onTechInput(techKey)}
+						aria-invalid={techError(techKey) !== "" ? "true" : "false"}
+						title={techError(techKey)}
 						data-testid={`calculator-tech-${techKey}`}
 					/>
 					{#if techOverridden[techKey]}
@@ -167,6 +198,9 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 		color: #9fb0ff;
 		background: #11162a;
 	}
+	input[aria-invalid="true"] {
+		border-color: #d97a7a;
+	}
 	.tech-cell {
 		display: flex;
 		align-items: center;
diff --git a/ui/frontend/src/lib/i18n/locales/en.ts b/ui/frontend/src/lib/i18n/locales/en.ts
index 61d7239..f315b81 100644
--- a/ui/frontend/src/lib/i18n/locales/en.ts
+++ b/ui/frontend/src/lib/i18n/locales/en.ts
@@ -395,6 +395,10 @@ const en = {
 	"game.calculator.invalid.cargo_value": "cargo must be 0 or ≥ 1",
 	"game.calculator.invalid.armament_weapons_pair": "armament and weapons must be both zero or both nonzero",
 	"game.calculator.invalid.all_zero": "at least one value must be nonzero",
+	"game.calculator.invalid.negative": "value cannot be negative",
+	"game.calculator.invalid.tech_value": "tech level cannot be negative",
+	"game.calculator.invalid.load_over_capacity": "load exceeds the ship's cargo capacity",
+	"game.calculator.lock.no_drive": "set a non-zero drive before locking speed",
 
 	"game.table.sciences.title": "sciences",
 	"game.table.sciences.column.name": "name",
diff --git a/ui/frontend/src/lib/i18n/locales/ru.ts b/ui/frontend/src/lib/i18n/locales/ru.ts
index efe436a..77fdf4e 100644
--- a/ui/frontend/src/lib/i18n/locales/ru.ts
+++ b/ui/frontend/src/lib/i18n/locales/ru.ts
@@ -396,6 +396,10 @@ const ru: Record<keyof typeof en, string> = {
 	"game.calculator.invalid.cargo_value": "трюм должен быть 0 или ≥ 1",
 	"game.calculator.invalid.armament_weapons_pair": "вооружённость и оружие должны быть оба нулевыми или оба ненулевыми",
 	"game.calculator.invalid.all_zero": "хотя бы одно значение должно быть ненулевым",
+	"game.calculator.invalid.negative": "значение не может быть отрицательным",
+	"game.calculator.invalid.tech_value": "технологический уровень не может быть отрицательным",
+	"game.calculator.invalid.load_over_capacity": "загрузка превышает грузоподъёмность корабля",
+	"game.calculator.lock.no_drive": "задайте ненулевой двигатель, прежде чем фиксировать скорость",
 
 	"game.table.sciences.title": "науки",
 	"game.table.sciences.column.name": "название",
diff --git a/ui/frontend/src/lib/sidebar/calculator-tab.svelte b/ui/frontend/src/lib/sidebar/calculator-tab.svelte
index 0b64afc..6b432e0 100644
--- a/ui/frontend/src/lib/sidebar/calculator-tab.svelte
+++ b/ui/frontend/src/lib/sidebar/calculator-tab.svelte
@@ -140,6 +140,13 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 		}
 	});
 
+	// With no cargo block there is no hold to load: pin the load to empty
+	// and disable the toggle.
+	const cargoEmpty = $derived(cs.blocks.cargo === 0);
+	$effect(() => {
+		if (cargoEmpty && cs.loadMode !== "empty") cs.loadMode = "empty";
+	});
+
 	const planetBuild = $derived.by(() => {
 		if (selectedPlanet === null) return null;
 		const emptyMass = result.outputs?.emptyMass;
@@ -228,11 +235,54 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 		return { perBlock, total };
 	});
 
+	// Display every computed number rounded up to three decimals via the
+	// shared `Ceil3` bridge, so a value is never shown lower than it is.
 	function fmt(value: number | null | undefined): string {
 		if (value === null || value === undefined) {
 			return i18n.t("game.calculator.unavailable");
 		}
-		return value.toLocaleString(undefined, { maximumFractionDigits: 3 });
+		const rounded = core !== null ? core.ceil3({ value }) : value;
+		return rounded.toLocaleString(undefined, { maximumFractionDigits: 3 });
+	}
+
+	// The goal-seek back-solved block, shown in its read-only cell, is
+	// ceiled the same way (only the claimed block's cell is displayed).
+	const resolvedCeil = $derived.by(() => {
+		if (core === null) return result.blocks;
+		const c = (v: number) => core.ceil3({ value: v });
+		return {
+			drive: c(result.blocks.drive),
+			armament: result.blocks.armament,
+			weapons: c(result.blocks.weapons),
+			shields: c(result.blocks.shields),
+			cargo: c(result.blocks.cargo),
+		};
+	});
+
+	// A custom load must stay within [0, cargo capacity]; beyond that the
+	// ship cannot hold it.
+	const customLoadError = $derived.by(() => {
+		if (cs.loadMode !== "custom") return "";
+		if (cs.customLoad < 0) return i18n.t("game.calculator.invalid.negative");
+		if (cs.customLoad > result.cargoCapacity) {
+			return i18n.t("game.calculator.invalid.load_over_capacity");
+		}
+		return "";
+	});
+	const matError = $derived(
+		cs.matValue < 0 ? i18n.t("game.calculator.invalid.negative") : "",
+	);
+
+	// Locking a speed back-solves the drive block; with a zero drive the
+	// ship is deliberately immobile, so disallow it.
+	function lockDisabledReason(output: LockableOutputId): string {
+		if (
+			(output === "speedEmpty" || output === "speedLoaded") &&
+			cs.blocks.drive === 0
+		) {
+			return i18n.t("game.calculator.lock.no_drive");
+		}
+		return "";
 	}
 
 	function onTechInput(key: TechKey): void {
@@ -346,15 +396,18 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 			</button>
 		</span>
 	{:else}
+		{@const extra = lockDisabledReason(output)}
 		<span class="cell">
 			<span class="val" data-testid={`calculator-out-${output}`}>{fmt(value)}</span>
 			<button
 				type="button"
 				class="lock"
-				disabled={cs.lock !== null || value === undefined}
+				disabled={cs.lock !== null || value === undefined || extra !== ""}
 				title={cs.lock !== null
 					? i18n.t("game.calculator.lock.max")
-					: `${LOCK_LABELS[output]}`}
+					: extra !== ""
+						? extra
+						: LOCK_LABELS[output]}
 				aria-label={LOCK_LABELS[output]}
 				data-testid={`calculator-lock-${output}`}
 				onclick={() => lockOutput(output)}
@@ -429,7 +482,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 
 	<ShipDesignArea
 		bind:blocks={cs.blocks}
-		resolved={result.blocks}
+		resolved={resolvedCeil}
 		bind:techs={cs.techValues}
 		techOverridden={cs.techOverridden}
 		computedInput={result.computedInput}
@@ -445,6 +498,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 					<button
 						type="button"
 						class:active={cs.loadMode === m}
+						disabled={cargoEmpty}
 						data-testid={`calculator-load-${m}`}
 						onclick={() => (cs.loadMode = m)}
 					>
@@ -459,8 +513,18 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 					min="0"
 					class="custom-load"
 					bind:value={cs.customLoad}
+					aria-invalid={customLoadError !== "" ? "true" : "false"}
+					title={customLoadError}
 					data-testid="calculator-custom-load"
 				/>
+			{:else if cs.loadMode === "full"}
+				<span
+					class="full-capacity"
+					title={i18n.t("game.calculator.out.cargo_capacity")}
+					data-testid="calculator-full-capacity"
+				>
+					{fmt(result.cargoCapacity)}
+				</span>
 			{/if}
 		</div>
 
@@ -531,6 +595,8 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 							min="0"
 							bind:value={cs.matValue}
 							oninput={onMatInput}
+							aria-invalid={matError !== "" ? "true" : "false"}
+							title={matError}
 							data-testid="calculator-planet-mat"
 						/>
 						{#if cs.matOverridden}
@@ -580,6 +646,10 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 							step="0.001"
 							min="0"
 							bind:value={cs.targetTech[row.key]}
+							aria-invalid={cs.targetTech[row.key] < 0 ? "true" : "false"}
+							title={cs.targetTech[row.key] < 0
+								? i18n.t("game.calculator.invalid.negative")
+								: ""}
 							data-testid={`calculator-target-${row.key}`}
 						/>
 					</span>
@@ -592,7 +662,6 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 			{/each}
 			<div class="rrow total">
 				<span class="label">{i18n.t("game.calculator.modern.total")}</span>
-				<span></span>
 				<span class="cell">
 					<span class="val" data-testid="calculator-modern-total">
 						{fmt(modernCosts?.total)}
@@ -814,6 +883,20 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 		text-align: right;
 	}
 	.rrow.total .label {
+		grid-column: 1 / 3;
 		color: #cdd3f0;
+		white-space: nowrap;
+	}
+	input[aria-invalid="true"] {
+		border-color: #d97a7a;
+	}
+	.seg button:disabled {
+		opacity: 0.4;
+		cursor: not-allowed;
+	}
+	.full-capacity {
+		font-variant-numeric: tabular-nums;
+		font-size: 0.8rem;
+		color: #9fb0ff;
 	}
 </style>
diff --git a/ui/frontend/src/lib/util/ship-class-validation.ts b/ui/frontend/src/lib/util/ship-class-validation.ts
index 67e3a8e..f9ffd61 100644
--- a/ui/frontend/src/lib/util/ship-class-validation.ts
+++ b/ui/frontend/src/lib/util/ship-class-validation.ts
@@ -170,3 +170,35 @@ function isValidDWSC(value: number): boolean {
 	if (!Number.isFinite(value)) return false;
 	return value === 0 || value >= 1;
 }
+
+/**
+ * shipClassFieldErrors returns the invalid reason for each offending
+ * block, independently, so the calculator can mark every bad input
+ * (not just the first failure `validateShipClassValues` reports). The
+ * weapons/armament pairing rule flags both fields. The all-zero rule is
+ * a whole-design condition and is left to `validateShipClassValues`.
+ */
+export function shipClassFieldErrors(
+	values: ShipClassValues,
+): Partial<Record<keyof ShipClassValues, ShipClassValueInvalidReason>> {
+	const errors: Partial<
+		Record<keyof ShipClassValues, ShipClassValueInvalidReason>
+	> = {};
+	if (!isValidDWSC(values.drive)) errors.drive = "drive_value";
+	if (!Number.isFinite(values.armament) || values.armament < 0) {
+		errors.armament = "armament_value";
+	} else if (!Number.isInteger(values.armament)) {
+		errors.armament = "armament_not_integer";
+	}
+	if (!isValidDWSC(values.weapons)) errors.weapons = "weapons_value";
+	if (!isValidDWSC(values.shields)) errors.shields = "shields_value";
+	if (!isValidDWSC(values.cargo)) errors.cargo = "cargo_value";
+	if (
+		(values.armament === 0 && values.weapons !== 0) ||
+		(values.armament !== 0 && values.weapons === 0)
+	) {
+		errors.weapons ??= "armament_weapons_pair";
+		errors.armament ??= "armament_weapons_pair";
+	}
+	return errors;
+}
diff --git a/ui/frontend/src/platform/core/index.ts b/ui/frontend/src/platform/core/index.ts
index 8d23b9f..e853ba3 100644
--- a/ui/frontend/src/platform/core/index.ts
+++ b/ui/frontend/src/platform/core/index.ts
@@ -145,6 +145,10 @@ export interface LoadForFullMassInput {
 	cargoTech: number;
 }
 
+export interface Ceil3Input {
+	value: number;
+}
+
 export interface Core {
 	/**
 	 * signRequest returns the canonical signing input bytes for a v1
@@ -311,6 +315,13 @@ export interface Core {
 	 * below the empty mass.
 	 */
 	loadForFullMass(input: LoadForFullMassInput): number | null;
+
+	/**
+	 * ceil3 wraps `pkg/calc/number.go.Ceil3`: round a value up to three
+	 * decimal places, for display so a computed result is never shown
+	 * lower than it is.
+	 */
+	ceil3(input: Ceil3Input): number;
 }
 
 export type CoreLoader = () => Promise<Core>;
diff --git a/ui/frontend/src/platform/core/wasm.ts b/ui/frontend/src/platform/core/wasm.ts
index 88749d6..124220e 100644
--- a/ui/frontend/src/platform/core/wasm.ts
+++ b/ui/frontend/src/platform/core/wasm.ts
@@ -11,6 +11,7 @@
 import type {
 	BlockUpgradeCostInput,
 	BombingPowerInput,
+	Ceil3Input,
 	CargoCapacityInput,
 	CargoForEmptyMassInput,
 	CarryingMassInput,
@@ -73,6 +74,7 @@ interface GalaxyCoreBridge {
 	shieldsForDefence(input: ShieldsForDefenceInput): number | null;
 	cargoForEmptyMass(input: CargoForEmptyMassInput): number | null;
 	loadForFullMass(input: LoadForFullMassInput): number | null;
+	ceil3(input: Ceil3Input): number;
 }
 
 interface BridgeRequestFields {
@@ -266,6 +268,9 @@ export function adaptBridge(bridge: GalaxyCoreBridge): Core {
 		loadForFullMass(input: LoadForFullMassInput): number | null {
 			return bridge.loadForFullMass(input);
 		},
+		ceil3(input: Ceil3Input): number {
+			return bridge.ceil3(input);
+		},
 	};
 }
 
diff --git a/ui/frontend/static/core.wasm b/ui/frontend/static/core.wasm
index 3718e16..4e1c28e 100644
Binary files a/ui/frontend/static/core.wasm and b/ui/frontend/static/core.wasm differ
diff --git a/ui/frontend/tests/calculator-tab.test.ts b/ui/frontend/tests/calculator-tab.test.ts
index 91efa83..96a1ca4 100644
--- a/ui/frontend/tests/calculator-tab.test.ts
+++ b/ui/frontend/tests/calculator-tab.test.ts
@@ -211,4 +211,71 @@ describe("calculator-tab", () => {
 			ui.getByTestId("calculator-ships-per-turn"),
 		).not.toHaveTextContent("—");
 	});
+
+	test("zero cargo disables the load toggle", async () => {
+		const ui = mount();
+		await setBlock(ui, "drive", 10);
+		await setBlock(ui, "shields", 5);
+		await setBlock(ui, "cargo", 0);
+		expect(ui.getByTestId("calculator-load-full")).toBeDisabled();
+		expect(ui.getByTestId("calculator-load-custom")).toBeDisabled();
+	});
+
+	test("full load shows the cargo capacity", async () => {
+		const ui = mount();
+		await setBlock(ui, "drive", 10);
+		await setBlock(ui, "shields", 5);
+		await setBlock(ui, "cargo", 5);
+		// A fresh design starts with cargo 0, which pins load to empty;
+		// pick full now that there is a hold.
+		await fireEvent.click(ui.getByTestId("calculator-load-full"));
+		// capacity = cargoTech(1) * (5 + 25/20) = 6.25.
+		expect(ui.getByTestId("calculator-full-capacity")).toHaveTextContent("6.25");
+	});
+
+	test("flags a custom load above cargo capacity", async () => {
+		const ui = mount();
+		await setBlock(ui, "drive", 10);
+		await setBlock(ui, "shields", 5);
+		await setBlock(ui, "cargo", 5);
+		await fireEvent.click(ui.getByTestId("calculator-load-custom"));
+		await fireEvent.input(ui.getByTestId("calculator-custom-load"), {
+			target: { value: "100" },
+		});
+		expect(ui.getByTestId("calculator-custom-load")).toHaveAttribute(
+			"aria-invalid",
+			"true",
+		);
+	});
+
+	test("marks an invalid block value with aria-invalid", async () => {
+		const ui = mount();
+		// 0.5 is neither 0 nor ≥ 1.
+		await setBlock(ui, "drive", 0.5);
+		expect(ui.getByTestId("calculator-block-drive")).toHaveAttribute(
+			"aria-invalid",
+			"true",
+		);
+	});
+
+	test("disables the speed lock when drive is zero", async () => {
+		const ui = mount();
+		await setBlock(ui, "drive", 0);
+		await setBlock(ui, "shields", 5);
+		await setBlock(ui, "cargo", 5);
+		expect(ui.getByTestId("calculator-lock-speedEmpty")).toBeDisabled();
+	});
+
+	test("displays computed values rounded up to three decimals", async () => {
+		const ui = mount();
+		await setBlock(ui, "drive", 7);
+		await setBlock(ui, "shields", 3);
+		await setBlock(ui, "cargo", 1);
+		// empty mass = 11; max speed = 11 * driveTech... use a value that is
+		// not already 3-decimal: speedEmpty = 20*7*1.2 / 11 = 15.2727…
+		// ceil to 3 → 15.273.
+		expect(ui.getByTestId("calculator-out-speedEmpty")).toHaveTextContent(
+			"15.273",
+		);
+	});
 });
diff --git a/ui/frontend/tests/fake-core.ts b/ui/frontend/tests/fake-core.ts
index 6e33f7a..61a231d 100644
--- a/ui/frontend/tests/fake-core.ts
+++ b/ui/frontend/tests/fake-core.ts
@@ -123,6 +123,7 @@ export function makeFakeCore(overrides: Partial<Core> = {}): Core {
 			cargoTech <= 0 || targetFullMass < emptyMass
 				? null
 				: (targetFullMass - emptyMass) * cargoTech,
+		ceil3: ({ value }) => Math.ceil(Math.round(value * 1e9) / 1e6) / 1000,
 	};
 	return { ...base, ...overrides };
 }
diff --git a/ui/frontend/tests/wasm-core-calc.test.ts b/ui/frontend/tests/wasm-core-calc.test.ts
index 4d31d5d..826436c 100644
--- a/ui/frontend/tests/wasm-core-calc.test.ts
+++ b/ui/frontend/tests/wasm-core-calc.test.ts
@@ -70,4 +70,10 @@ describe("WasmCore calculator bridge (Phase 30)", () => {
 			core.driveForSpeed({ targetSpeed: 100, driveTech: 1.2, restMass: 35 }),
 		).toBeNull();
 	});
+
+	test("ceil3 rounds up to three decimals", () => {
+		expect(core.ceil3({ value: 5.0003 })).toBeCloseTo(5.001, 9);
+		expect(core.ceil3({ value: 4.2761 })).toBeCloseTo(4.277, 9);
+		expect(core.ceil3({ value: 5 })).toBeCloseTo(5, 9);
+	});
 });
diff --git a/ui/wasm/main.go b/ui/wasm/main.go
index bbecb63..e26d8b5 100644
--- a/ui/wasm/main.go
+++ b/ui/wasm/main.go
@@ -86,6 +86,7 @@ func main() {
 		"shieldsForDefence":  js.FuncOf(shieldsForDefence),
 		"cargoForEmptyMass":  js.FuncOf(cargoForEmptyMass),
 		"loadForFullMass":    js.FuncOf(loadForFullMass),
+		"ceil3":              js.FuncOf(ceil3),
 	}))
 
 	// Block forever so the Go runtime stays alive while JS keeps calling
@@ -420,6 +421,15 @@ func loadForFullMass(_ js.Value, args []js.Value) any {
 	return js.ValueOf(v)
 }
 
+// ceil3 bridges `calc.Ceil3`. Input `{ value }`, output a JS number
+// rounded up to three decimal places.
+func ceil3(_ js.Value, args []js.Value) any {
+	if len(args) != 1 {
+		return js.Null()
+	}
+	return js.ValueOf(calc.Ceil3(args[0].Get("value").Float()))
+}
+
 // copyBytesFromJS materialises a JS Uint8Array (or any indexable
 // byte-shaped value) into a Go byte slice. We avoid `js.CopyBytesToGo`
 // because TinyGo's implementation panics on values it does not