diff --git a/ui/docs/calculator-ux.md b/ui/docs/calculator-ux.md
index 7bb8ec4..5797eec 100644
--- a/ui/docs/calculator-ux.md
+++ b/ui/docs/calculator-ux.md
@@ -111,13 +111,19 @@ 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.
+Every displayed number — the derived results, the inherited tech /
+planet MAT labels, 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`) and always padded to three decimals so the
+column reads the same on integers and fractions alike (a speed of 20
+shows as `20.000`, of 5.0003 as `5.001`). Labels and inputs use the
+monospace stack from the design tokens (`--font-mono`) with
+right-aligned, tabular numerals so values line up vertically across
+rows. To match the display rule, every number input also refuses a
+fourth decimal as the user types: typing `1.2345` clamps the input to
+`1.234` on input. 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 739f47a..48e9cac 100644
--- a/ui/frontend/src/lib/calculator/ship-design-area.svelte
+++ b/ui/frontend/src/lib/calculator/ship-design-area.svelte
@@ -139,6 +139,20 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 		const floor = techFloor[key];
 		techs[key] = next < floor ? floor : next;
 	}
+	// Refuse a fourth decimal as typing happens: keeps the calculator
+	// from ever displaying a >3-decimal fraction the user could not
+	// have intended (the calculator math is `Ceil3`-rounded for display
+	// anyway). Pairs with `bind:value` — `apply` overwrites the bound
+	// state when Svelte's own bind handler has already read the
+	// over-precise number.
+	function capDecimals(event: Event, apply: (next: number) => void): void {
+		const el = event.currentTarget as HTMLInputElement;
+		const txt = el.value;
+		const dot = txt.indexOf(".");
+		if (dot < 0 || txt.length - dot - 1 <= 3) return;
+		el.value = txt.slice(0, dot + 4);
+		apply(el.valueAsNumber);
+	}
 
 	const BLOCK_ROWS: {
 		key: keyof DesignBlocksState;
@@ -196,6 +210,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 					title={blockError(row.key)}
 					data-testid={`calculator-block-${row.key}`}
 					onkeydown={(e) => onBlockKey(e, row.key, row.smartStep)}
+					oninput={(e) => capDecimals(e, (v) => (blocks[row.key] = v))}
 				/>
 			{/if}
 			{#if row.tech !== null}
@@ -213,6 +228,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 							title={techError(techKey)}
 							data-testid={`calculator-tech-${techKey}`}
 							onkeydown={(e) => bumpTech(e, techKey)}
+							oninput={(e) => capDecimals(e, (v) => (techs[techKey] = v))}
 						/>
 						<button
 							type="button"
@@ -274,7 +290,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 		font-size: 0.8rem;
 	}
 	input {
-		font: inherit;
+		font-family: var(--font-mono);
 		font-size: 0.8rem;
 		width: 100%;
 		min-width: 0;
@@ -284,6 +300,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 		border: 1px solid var(--color-border);
 		border-radius: 3px;
 		font-variant-numeric: tabular-nums;
+		text-align: right;
 	}
 	/* Hide native spinners across the design area — the row drives
 	   every numeric edit through ArrowUp/ArrowDown so the column
@@ -313,6 +330,7 @@ calculator math — so the ship-group upgrade flow can reuse it later.
 	.tech-val {
 		flex: 1;
 		min-width: 0;
+		font-family: var(--font-mono);
 		font-size: 0.8rem;
 		font-variant-numeric: tabular-nums;
 		text-align: right;
diff --git a/ui/frontend/src/lib/sidebar/calculator-tab.svelte b/ui/frontend/src/lib/sidebar/calculator-tab.svelte
index 5396a06..41edfb2 100644
--- a/ui/frontend/src/lib/sidebar/calculator-tab.svelte
+++ b/ui/frontend/src/lib/sidebar/calculator-tab.svelte
@@ -237,12 +237,35 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 
 	// Display every computed number rounded up to three decimals via the
 	// shared `Ceil3` bridge, so a value is never shown lower than it is.
+	// Always three decimals (`1` → `1.000`) for column-aligned readability,
+	// and without thousands grouping so the same string also embeds in the
+	// read-only goal-seek `<input type="number">` cell.
 	function fmt(value: number | null | undefined): string {
 		if (value === null || value === undefined) {
 			return i18n.t("game.calculator.unavailable");
 		}
 		const rounded = core !== null ? core.ceil3({ value }) : value;
-		return rounded.toLocaleString(undefined, { maximumFractionDigits: 3 });
+		return rounded.toLocaleString(undefined, {
+			minimumFractionDigits: 3,
+			maximumFractionDigits: 3,
+			useGrouping: false,
+		});
+	}
+
+	// Cap typed precision at three decimal digits. Number inputs use
+	// `step="any"`, which lets the browser accept arbitrary precision; the
+	// owner asked us to refuse a fourth decimal as typing happens so the
+	// calculator never displays a longer-than-three-digit fraction. Pairs
+	// with `bind:value`: if Svelte's bind handler has already read the
+	// over-precise number, `apply` overwrites the state with the truncated
+	// value so the next reactive flush does not undo our truncation.
+	function capDecimals(event: Event, apply: (next: number) => void): void {
+		const el = event.currentTarget as HTMLInputElement;
+		const txt = el.value;
+		const dot = txt.indexOf(".");
+		if (dot < 0 || txt.length - dot - 1 <= 3) return;
+		el.value = txt.slice(0, dot + 4);
+		apply(el.valueAsNumber);
 	}
 
 	// The goal-seek back-solved block, shown in its read-only cell, is
@@ -496,6 +519,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 				bind:value={cs.lockValue}
 				onkeydown={(e) =>
 					onStepKey(e, cs.lockValue, 0.001, 0, (v) => (cs.lockValue = v))}
+				oninput={(e) => capDecimals(e, (v) => (cs.lockValue = v))}
 				data-testid={`calculator-locked-${output}`}
 				title={result.lockFeasible ? "" : i18n.t("game.calculator.lock.infeasible")}
 			/>
@@ -632,6 +656,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 					bind:value={cs.customLoad}
 					onkeydown={(e) =>
 						onStepKey(e, cs.customLoad, 0.01, 0, (v) => (cs.customLoad = v))}
+					oninput={(e) => capDecimals(e, (v) => (cs.customLoad = v))}
 					aria-invalid={customLoadError !== "" ? "true" : "false"}
 					title={customLoadError}
 					data-testid="calculator-custom-load"
@@ -718,6 +743,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 								bind:value={cs.matValue}
 								onkeydown={(e) =>
 									onStepKey(e, cs.matValue, 0.01, 0, (v) => (cs.matValue = v))}
+								oninput={(e) => capDecimals(e, (v) => (cs.matValue = v))}
 								aria-invalid={matError !== "" ? "true" : "false"}
 								title={matError}
 								data-testid="calculator-planet-mat"
@@ -795,6 +821,8 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 									playerTech[row.key],
 									(v) => (cs.targetTech[row.key] = v),
 								)}
+							oninput={(e) =>
+								capDecimals(e, (v) => (cs.targetTech[row.key] = v))}
 							aria-invalid={targetError !== "" ? "true" : "false"}
 							title={targetError}
 							data-testid={`calculator-target-${row.key}`}
@@ -913,13 +941,15 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 	}
 	.custom-load {
 		width: 4rem;
-		font: inherit;
+		font-family: var(--font-mono);
 		font-size: 0.8rem;
+		font-variant-numeric: tabular-nums;
 		padding: 0.15rem 0.3rem;
 		background: var(--color-bg);
 		color: var(--color-text);
 		border: 1px solid var(--color-border);
 		border-radius: 3px;
+		text-align: right;
 	}
 	.results,
 	.modern {
@@ -949,6 +979,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 		justify-content: flex-end;
 	}
 	.cell .val {
+		font-family: var(--font-mono);
 		font-variant-numeric: tabular-nums;
 		font-size: 0.85rem;
 		text-align: right;
@@ -956,7 +987,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 	.cell input {
 		width: 100%;
 		min-width: 0;
-		font: inherit;
+		font-family: var(--font-mono);
 		font-size: 0.8rem;
 		padding: 0.15rem 0.3rem;
 		background: var(--color-bg);
@@ -1038,6 +1069,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 	}
 	.planet-stats dd {
 		margin: 0;
+		font-family: var(--font-mono);
 		font-variant-numeric: tabular-nums;
 		font-size: 0.85rem;
 		text-align: right;
@@ -1055,6 +1087,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 		cursor: not-allowed;
 	}
 	.full-capacity {
+		font-family: var(--font-mono);
 		font-variant-numeric: tabular-nums;
 		font-size: 0.8rem;
 		color: var(--color-accent);
@@ -1065,6 +1098,7 @@ long-lived planning tool. `ensureGame` resets it when the game changes.
 	.mat-val {
 		flex: 1;
 		min-width: 0;
+		font-family: var(--font-mono);
 		font-size: 0.85rem;
 		font-variant-numeric: tabular-nums;
 		text-align: right;
diff --git a/ui/frontend/tests/calculator-tab.test.ts b/ui/frontend/tests/calculator-tab.test.ts
index c123e0f..e45c3c3 100644
--- a/ui/frontend/tests/calculator-tab.test.ts
+++ b/ui/frontend/tests/calculator-tab.test.ts
@@ -401,15 +401,12 @@ describe("calculator-tab", () => {
 	});
 
 	test("renders unoverridden tech as a 3-decimal label (matches the report)", () => {
-		// Player drive tech 1.2 → "1.200" via the shared ceil3 formatter.
+		// Player drive tech 1.2 → "1.200" via the shared ceil3 formatter,
+		// always padded to three decimals (calculator labels are column-
+		// aligned with the report).
 		const ui = mount();
-		expect(ui.getByTestId("calculator-tech-value-drive")).toHaveTextContent(
-			"1.2",
-		);
-		// Stable column-aligned formatting (3 decimals) is what the report
-		// uses, so the tech labels read consistently.
 		const tech = ui.getByTestId("calculator-tech-value-drive");
-		expect(tech.textContent ?? "").toMatch(/^1\.20?0?$/);
+		expect((tech.textContent ?? "").trim()).toBe("1.200");
 	});
 
 	test("planet MAT label renders through the 3-decimal formatter", () => {
@@ -419,12 +416,84 @@ describe("calculator-tab", () => {
 			report: makeReport({ planets: [LOCAL_PLANET] }),
 			selection,
 		});
-		// Planet MAT is 100 → "100" through the shared formatter; the
+		// Planet MAT is 100 → "100.000" through the shared formatter; the
 		// label is monospaced + right-aligned via the existing `.mat-val`
-		// rule. Formatting check: no stray fractional digits on integers.
-		expect(
-			ui.getByTestId("calculator-planet-mat-value"),
-		).toHaveTextContent("100");
+		// rule. Integer MAT pads to three decimals like every other label.
+		const mat = ui.getByTestId("calculator-planet-mat-value");
+		expect((mat.textContent ?? "").trim()).toBe("100.000");
+	});
+
+	test("derived results pad to three decimals (integer empty mass)", async () => {
+		// Integer-valued outputs read with the same trailing zeros as
+		// fractional ones — column-aligned tabular display.
+		const ui = mount();
+		await setBlock(ui, "drive", 10);
+		await setBlock(ui, "shields", 5);
+		await setBlock(ui, "cargo", 5);
+		const mass = ui.getByTestId("calculator-out-emptyMass");
+		expect((mass.textContent ?? "").trim()).toBe("20.000");
+	});
+
+	test("number inputs refuse a fourth decimal as the user types", async () => {
+		const selection = new SelectionStore();
+		selection.selectPlanet(17);
+		const ui = mount({
+			report: makeReport({ planets: [LOCAL_PLANET] }),
+			selection,
+		});
+		// MAT input: typed "12.3456" must clamp to "12.345" on input.
+		await fireEvent.click(ui.getByTestId("calculator-mat-override"));
+		const mat = ui.getByTestId("calculator-planet-mat") as HTMLInputElement;
+		await fireEvent.input(mat, { target: { value: "12.3456" } });
+		expect(mat.value).toBe("12.345");
+		expect(mat.valueAsNumber).toBeCloseTo(12.345, 9);
+
+		// Custom-load input on a ship with a non-zero cargo: typed
+		// "1.2345" must clamp to "1.234".
+		await setBlock(ui, "drive", 10);
+		await setBlock(ui, "shields", 5);
+		await setBlock(ui, "cargo", 5);
+		await fireEvent.click(ui.getByTestId("calculator-load-custom"));
+		const load = ui.getByTestId("calculator-custom-load") as HTMLInputElement;
+		await fireEvent.input(load, { target: { value: "1.2345" } });
+		expect(load.value).toBe("1.234");
+	});
+
+	test("tech and target-tech inputs cap at three decimals", async () => {
+		const ui = mount();
+		// Tech override input.
+		await fireEvent.click(ui.getByTestId("calculator-tech-override-drive"));
+		const tech = ui.getByTestId("calculator-tech-drive") as HTMLInputElement;
+		await fireEvent.input(tech, { target: { value: "2.9999" } });
+		expect(tech.value).toBe("2.999");
+
+		// Modernization target tech input.
+		await fireEvent.click(ui.getByTestId("calculator-mode-modernization"));
+		const target = ui.getByTestId(
+			"calculator-target-drive",
+		) as HTMLInputElement;
+		await fireEvent.input(target, { target: { value: "3.1416" } });
+		expect(target.value).toBe("3.141");
+	});
+
+	test("lock value input caps at three decimals", 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-lock-attack"));
+		const lock = ui.getByTestId(
+			"calculator-locked-attack",
+		) as HTMLInputElement;
+		await fireEvent.input(lock, { target: { value: "0.1234" } });
+		expect(lock.value).toBe("0.123");
+	});
+
+	test("ship-block input caps at three decimals", async () => {
+		const ui = mount();
+		const drive = ui.getByTestId("calculator-block-drive") as HTMLInputElement;
+		await fireEvent.input(drive, { target: { value: "1.2345" } });
+		expect(drive.value).toBe("1.234");
 	});
 
 	test("lock spinner step is replaced by ArrowUp/ArrowDown (±0.001)", async () => {