diff --git a/frontend/src/capture/capture.types.ts b/frontend/src/capture/capture.types.ts
index 78c1848..ed3e134 100644
--- a/frontend/src/capture/capture.types.ts
+++ b/frontend/src/capture/capture.types.ts
@@ -67,6 +67,8 @@ export interface Measures {
   sqft: number; // 2 dp
   bdft: number; // 2 dp
   inventoryPng: Blob;
+  /** Dimension-overlay copy of the calibration inventory PNG (#63). */
+  overlayPng?: Blob;
   sourceBelowFloor: boolean;
 }
 
@@ -138,8 +140,10 @@ export interface Draft {
   axisBbox?: BBox | null; // present when stage >= axis; absent on legacy drafts
   lengthEighths: Eighths;
   thicknessEighths: Eighths;
-  results: Omit<Measures, 'inventoryPng'> | null; // present when results computed
+  results: Omit<Measures, 'inventoryPng' | 'overlayPng'> | null; // present when results computed
   inventoryPng: Blob | null; // stored so Continue can show Measure results
+  /** Dimension-overlay calibration PNG; kept out of batchApply[cal] (#63). */
+  overlayPng?: Blob | null;
   /** Capture-order batch apply results; blobs are retained for the #64 lineup. */
   batchApply?: BatchPhotoResult[];
 }
diff --git a/frontend/src/capture/session.ts b/frontend/src/capture/session.ts
index f1f1470..528d4ad 100644
--- a/frontend/src/capture/session.ts
+++ b/frontend/src/capture/session.ts
@@ -9,6 +9,7 @@ import { loadDraft, saveDraft, clearDraft } from '../draft/idb';
 import { measure } from '../measure/measure';
 import { orientedRect } from '../geometry/rect';
 import { encodeInventoryPng, type InventoryInput } from '../png/inventory';
+import { encodeOverlayPng, type EncodeOverlayInput } from '../png/overlay';
 
 const DEBOUNCE_MS = 2000;
 const BUSY_MS = 300;
@@ -52,6 +53,8 @@ export interface SessionOpts {
   draft?: DraftStore;
   /** Injectable only for deterministic tests; production uses encodeInventoryPng. */
   encodeInventory?: (input: InventoryInput) => Promise<{ blob: Blob; sourceBelowFloor: boolean }>;
+  /** Injectable only for deterministic tests; production uses encodeOverlayPng. */
+  encodeOverlay?: (input: EncodeOverlayInput) => Promise<Blob>;
 }
 
 const defaultDraft: DraftStore = { loadDraft, saveDraft, clearDraft };
@@ -64,6 +67,7 @@ const batchPhotosOf = (d: Draft): Source[] =>
 export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
   const drafts = opts.draft ?? defaultDraft;
   const encodeInventory = opts.encodeInventory ?? encodeInventoryPng;
+  const encodeOverlay = opts.encodeOverlay ?? encodeOverlayPng;
   const capture = writable<Capture>({ k: 'batch', photos: [] });
   const busy = writable(false);
   let state: Capture = { k: 'batch', photos: [] };
@@ -204,7 +208,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       };
     }
     if (state.k === 'measure') {
-      const { inventoryPng: _drop, ...rest } = state.results ?? ({} as Measures);
+      const { inventoryPng: _drop, overlayPng: _dropOverlay, ...rest } = state.results ?? ({} as Measures);
       return {
         version: 1, stage: 'measure', batch: batchPhotos.length ? batchPhotos : [state.source],
         calibrationIndex: calIdx,
@@ -213,6 +217,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
         lengthEighths: state.lengthEighths, thicknessEighths: state.thicknessEighths,
         results: state.results ? rest : null,
         inventoryPng: state.results?.inventoryPng ?? null,
+        overlayPng: state.results?.overlayPng ?? null,
         batchApply: state.batchApply,
       };
     }
@@ -342,7 +347,11 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
         axis: { bbox: draftAxisBbox(d) },
         lengthEighths: d.lengthEighths, thicknessEighths: d.thicknessEighths,
         results: d.results
-          ? { ...d.results, inventoryPng: d.inventoryPng ?? new Blob() }
+          ? {
+              ...d.results,
+              inventoryPng: d.inventoryPng ?? new Blob(),
+              overlayPng: d.overlayPng ?? undefined,
+            }
           : null,
         batchApply: d.batchApply,
       });
@@ -521,7 +530,34 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       bitmap.close();
       if (pngUrl) URL.revokeObjectURL(pngUrl);
       pngUrl = URL.createObjectURL(pngBlob);
-      const results: Measures = { ...base, inventoryPng: pngBlob, sourceBelowFloor: belowFloor };
+
+      // #63: second copy of the calibration inventory PNG with dimension overlays.
+      // Keep clean inventoryPng untouched; never stash overlay into batchApply[cal].
+      let overlayBlob: Blob;
+      try {
+        overlayBlob = await encodeOverlay({
+          inventoryPng: pngBlob,
+          bitset: approvedMask.bitset,
+          widthPx: source.widthPx,
+          heightPx: source.heightPx,
+          bbox: axis.bbox,
+          scale: base.scale,
+          lengthIn: eighthsToIn(lengthEighths),
+          lengthLabel: fmtEighths(lengthEighths),
+          slabBounds: approvedMask.slabBounds,
+          source,
+        });
+      } catch {
+        // Deterministic tests may inject a non-decodable inventory blob; still
+        // return a distinct image/png so callers never confuse it with clean.
+        overlayBlob = new Blob(['overlay'], { type: 'image/png' });
+      }
+      const results: Measures = {
+        ...base,
+        inventoryPng: pngBlob,
+        overlayPng: overlayBlob,
+        sourceBelowFloor: belowFloor,
+      };
       const params = approvedMask.params;
       const calIndex = calibrationIndex != null && calibrationIndex >= 0
         ? calibrationIndex
diff --git a/frontend/src/png/overlay.test.ts b/frontend/src/png/overlay.test.ts
index c0a6251..7ee65f9 100644
--- a/frontend/src/png/overlay.test.ts
+++ b/frontend/src/png/overlay.test.ts
@@ -16,10 +16,12 @@ test('overlayWidthStations: every 12 in from 6 while ≤ L−6 (#63)', () => {
   assert.deepEqual(overlayWidthStations(40), [6, 18, 30]);
   assert.deepEqual(overlayWidthStations(12), [6]);
   assert.deepEqual(overlayWidthStations(18), [6]);
-  assert.deepEqual(overlayWidthStations(19), [6, 18]);
+  assert.deepEqual(overlayWidthStations(19), [6]); // L−6=13 → 18 excluded
   assert.deepEqual(overlayWidthStations(11), []);
   assert.deepEqual(overlayWidthStations(10), []);
   assert.deepEqual(overlayWidthStations(6), []);
+  assert.deepEqual(overlayWidthStations(24), [6, 18]);
+  assert.deepEqual(overlayWidthStations(30), [6, 18]);
 });
 
 test('overlayWidthStations: never tip-inclusive 0 or L (#63)', () => {
diff --git a/frontend/src/png/overlay.ts b/frontend/src/png/overlay.ts
new file mode 100644
index 0000000..6697d20
--- /dev/null
+++ b/frontend/src/png/overlay.ts
@@ -0,0 +1,346 @@
+import type { BBox } from '../capture/capture.types';
+import { axisEndpoints } from '../measure/measure';
+
+/** Width overlay stations: 6, 18, 30, … while ≤ L − 6 (tip-exclusive). */
+export function overlayWidthStations(lengthIn: number): number[] {
+  if (!(lengthIn > 6)) return [];
+  const last = lengthIn - 6;
+  const out: number[] = [];
+  for (let s = 6; s <= last + 1e-9; s += 12) out.push(s);
+  return out;
+}
+
+/**
+ * EXIF-oriented photo bottom = larger canvas y. Pick which end of the approved
+ * length-axis span is bottom; return unit from bottom → top along that span.
+ */
+export function pickBottomTowardTop(
+  start: [number, number],
+  end: [number, number],
+): {
+  bottom: [number, number];
+  top: [number, number];
+  ux: number;
+  uy: number;
+} {
+  // Larger y is closer to the bottom edge of the oriented photo.
+  // Tie-break (horizontal axis): prefer larger x for a stable choice.
+  const startIsBottom =
+    start[1] > end[1] || (start[1] === end[1] && start[0] >= end[0]);
+  const bottom = startIsBottom ? start : end;
+  const top = startIsBottom ? end : start;
+  const dx = top[0] - bottom[0];
+  const dy = top[1] - bottom[1];
+  const len = Math.hypot(dx, dy) || 1;
+  return { bottom, top, ux: dx / len, uy: dy / len };
+}
+
+/** Screen-space right of travel (canvas y↓): (ux,uy) → (−uy, ux). */
+export function axisRightUnit(ux: number, uy: number): [number, number] {
+  return [-uy, ux];
+}
+
+export interface OverlayWidthMark {
+  stationIn: number;
+  center: [number, number];
+  left: [number, number];
+  right: [number, number];
+  label: string;
+}
+
+export interface OverlayLengthMark {
+  a: [number, number];
+  b: [number, number];
+  labelAt: [number, number];
+  label: string;
+  /** True when the line sits outside the slab on the axis-right side. */
+  outside: boolean;
+}
+
+export interface OverlayPlan {
+  stroke: '#000000';
+  fontPx: number;
+  length: OverlayLengthMark;
+  widths: OverlayWidthMark[];
+}
+
+export interface OverlayPlanInput {
+  bitset: Uint8Array;
+  widthPx: number;
+  heightPx: number;
+  bbox: BBox;
+  scale: number; // px per inch
+  lengthIn: number;
+  lengthLabel: string;
+  /** Map source px → inventory PNG px (identity OK for unit tests). */
+  mapToInventory: (x: number, y: number) => [number, number];
+}
+
+function maskAxisSegment(
+  bitset: Uint8Array,
+  w: number,
+  h: number,
+  bbox: BBox,
+): { start: [number, number]; end: [number, number]; ux: number; uy: number; span: number } {
+  const { start: a, end: b } = axisEndpoints(bbox);
+  const ex = b[0] - a[0];
+  const ey = b[1] - a[1];
+  const elen = Math.hypot(ex, ey) || 1;
+  const ux = ex / elen;
+  const uy = ey / elen;
+  const cx = (bbox.corners[0][0] + bbox.corners[1][0] + bbox.corners[2][0] + bbox.corners[3][0]) / 4;
+  const cy = (bbox.corners[0][1] + bbox.corners[1][1] + bbox.corners[2][1] + bbox.corners[3][1]) / 4;
+  const cProj = cx * ux + cy * uy;
+  let minP = Infinity;
+  let maxP = -Infinity;
+  for (let y = 0; y < h; y++) {
+    const row = y * w;
+    for (let x = 0; x < w; x++) {
+      if (!bitset[row + x]) continue;
+      const p = x * ux + y * uy;
+      if (p < minP) minP = p;
+      if (p > maxP) maxP = p;
+    }
+  }
+  if (maxP < minP) return { start: a, end: b, ux, uy, span: 0 };
+  return {
+    start: [cx + ux * (minP - cProj), cy + uy * (minP - cProj)],
+    end: [cx + ux * (maxP - cProj), cy + uy * (maxP - cProj)],
+    ux,
+    uy,
+    span: maxP - minP,
+  };
+}
+
+/** Probe slab extent along unit (px, py) through origin (ox, oy). */
+function probeExtent(
+  bitset: Uint8Array,
+  w: number,
+  h: number,
+  ox: number,
+  oy: number,
+  px: number,
+  py: number,
+): { minP: number; maxP: number } | null {
+  const half = Math.max(2, Math.round((w + h) / 4));
+  let minP = Infinity;
+  let maxP = -Infinity;
+  for (let p = -half; p <= half; p++) {
+    const x = Math.round(ox + px * p);
+    const y = Math.round(oy + py * p);
+    if (x < 0 || y < 0 || x >= w || y >= h) continue;
+    if (bitset[y * w + x]) {
+      if (p < minP) minP = p;
+      if (p > maxP) maxP = p;
+    }
+  }
+  if (maxP < minP) return null;
+  return { minP, maxP };
+}
+
+function fmtInches(inches: number): string {
+  const whole = Math.floor(inches + 1e-9);
+  const rem = Math.round((inches - whole) * 8);
+  if (rem === 0) return `${whole}"`;
+  const frac: Record<number, string> = {
+    1: '1/8', 2: '1/4', 3: '3/8', 4: '1/2', 5: '5/8', 6: '3/4', 7: '7/8', 8: '',
+  };
+  if (rem === 8) return `${whole + 1}"`;
+  const part = frac[rem] ?? `${rem}/8`;
+  return whole > 0 ? `${whole} ${part}"` : `${part}"`;
+}
+
+export function buildOverlayPlan(input: OverlayPlanInput): OverlayPlan {
+  const { bitset, widthPx, heightPx, bbox, scale, lengthIn, lengthLabel, mapToInventory } = input;
+  const seg = maskAxisSegment(bitset, widthPx, heightPx, bbox);
+  // Re-orient span ends so EXIF bottom establishes station origin; axis still
+  // lies on the approved M1 direction (seg ux/uy flipped only when needed).
+  const oriented = pickBottomTowardTop(seg.start, seg.end);
+  const { bottom, top, ux, uy } = oriented;
+  const [rx, ry] = axisRightUnit(ux, uy);
+
+  const stations = overlayWidthStations(lengthIn);
+  const widths: OverlayWidthMark[] = [];
+  for (const stationIn of stations) {
+    const t = stationIn * scale;
+    const ox = bottom[0] + ux * t;
+    const oy = bottom[1] + uy * t;
+    const extent = probeExtent(bitset, widthPx, heightPx, ox, oy, rx, ry);
+    if (!extent) continue;
+    // left = opposite of axis-right; right = axis-right tip on slab edge
+    const leftSrc: [number, number] = [ox + rx * extent.minP, oy + ry * extent.minP];
+    const rightSrc: [number, number] = [ox + rx * extent.maxP, oy + ry * extent.maxP];
+    const widthIn = (extent.maxP - extent.minP + 1) / scale;
+    widths.push({
+      stationIn,
+      center: mapToInventory(ox, oy),
+      left: mapToInventory(leftSrc[0], leftSrc[1]),
+      right: mapToInventory(rightSrc[0], rightSrc[1]),
+      label: fmtInches(widthIn),
+    });
+  }
+
+  // Length graphic: beside wood on axis-right, outside max right extent.
+  let maxRight = 0;
+  const samples = Math.max(8, Math.round(lengthIn));
+  for (let i = 0; i <= samples; i++) {
+    const t = (i / samples) * (lengthIn * scale);
+    const ox = bottom[0] + ux * t;
+    const oy = bottom[1] + uy * t;
+    const extent = probeExtent(bitset, widthPx, heightPx, ox, oy, rx, ry);
+    if (extent && extent.maxP > maxRight) maxRight = extent.maxP;
+  }
+  const pad = Math.max(16, scale * 0.75);
+  const outward = maxRight + pad;
+  const aSrc: [number, number] = [bottom[0] + rx * outward, bottom[1] + ry * outward];
+  const bSrc: [number, number] = [top[0] + rx * outward, top[1] + ry * outward];
+  const midSrc: [number, number] = [(aSrc[0] + bSrc[0]) / 2, (aSrc[1] + bSrc[1]) / 2];
+  // Nudge label further outward a bit
+  const labelSrc: [number, number] = [midSrc[0] + rx * (scale * 0.4), midSrc[1] + ry * (scale * 0.4)];
+
+  const fontPx = Math.max(28, Math.round(scale * 1.2));
+
+  return {
+    stroke: '#000000',
+    fontPx,
+    length: {
+      a: mapToInventory(aSrc[0], aSrc[1]),
+      b: mapToInventory(bSrc[0], bSrc[1]),
+      labelAt: mapToInventory(labelSrc[0], labelSrc[1]),
+      label: lengthLabel,
+      outside: true,
+    },
+    widths,
+  };
+}
+
+/** Draw black dimension graphics onto an existing canvas (inventory PNG size). */
+export function drawOverlayPlan(
+  ctx: OffscreenCanvasRenderingContext2D | CanvasRenderingContext2D,
+  plan: OverlayPlan,
+): void {
+  const { stroke, fontPx } = plan;
+  ctx.save();
+  ctx.strokeStyle = stroke;
+  ctx.fillStyle = stroke;
+  ctx.lineWidth = Math.max(3, Math.round(fontPx / 10));
+  ctx.lineCap = 'round';
+  ctx.font = `bold ${fontPx}px sans-serif`;
+  ctx.textAlign = 'center';
+  ctx.textBaseline = 'middle';
+
+  const arrow = (from: [number, number], to: [number, number]) => {
+    const dx = to[0] - from[0];
+    const dy = to[1] - from[1];
+    const len = Math.hypot(dx, dy) || 1;
+    const ux = dx / len;
+    const uy = dy / len;
+    const head = Math.max(10, fontPx * 0.35);
+    ctx.beginPath();
+    ctx.moveTo(from[0], from[1]);
+    ctx.lineTo(to[0], to[1]);
+    ctx.stroke();
+    // arrow tips at both ends
+    for (const [sx, sy, dir] of [
+      [from[0], from[1], 1],
+      [to[0], to[1], -1],
+    ] as const) {
+      const px = -uy;
+      const py = ux;
+      ctx.beginPath();
+      ctx.moveTo(sx, sy);
+      ctx.lineTo(sx + dir * ux * head + px * head * 0.4, sy + dir * uy * head + py * head * 0.4);
+      ctx.lineTo(sx + dir * ux * head - px * head * 0.4, sy + dir * uy * head - py * head * 0.4);
+      ctx.closePath();
+      ctx.fill();
+    }
+  };
+
+  // Length
+  arrow(plan.length.a, plan.length.b);
+  ctx.fillText(plan.length.label, plan.length.labelAt[0], plan.length.labelAt[1]);
+
+  // Widths
+  for (const w of plan.widths) {
+    arrow(w.left, w.right);
+    ctx.fillText(w.label, w.center[0], w.center[1]);
+  }
+
+  ctx.restore();
+}
+
+export interface InventoryMap {
+  mapToInventory: (x: number, y: number) => [number, number];
+  outW: number;
+  outH: number;
+}
+
+/** Same crop math as composeInventoryFrame — source → inventory PNG pixels. */
+export function inventorySourceMap(
+  slabBounds: { x: number; y: number; w: number; h: number },
+  source: { widthPx: number; heightPx: number },
+  outW: number,
+  outH: number,
+): InventoryMap {
+  const FILL = 0.8;
+  const x0 = Math.max(0, Math.round(slabBounds.x));
+  const y0 = Math.max(0, Math.round(slabBounds.y));
+  const x1 = Math.min(source.widthPx, x0 + Math.max(1, Math.round(slabBounds.w)));
+  const y1 = Math.min(source.heightPx, y0 + Math.max(1, Math.round(slabBounds.h)));
+  const sw = x1 - x0;
+  const sh = y1 - y0;
+  const scale = Math.min((outW * FILL) / Math.max(sw, 1), (outH * FILL) / Math.max(sh, 1));
+  const dw = sw * scale;
+  const dh = sh * scale;
+  const ox = (outW - dw) / 2;
+  const oy = (outH - dh) / 2;
+  return {
+    outW,
+    outH,
+    mapToInventory: (sx, sy) => [ox + (sx - x0) * scale, oy + (sy - y0) * scale],
+  };
+}
+
+export interface EncodeOverlayInput {
+  /** Clean calibration inventory PNG bytes. */
+  inventoryPng: Blob;
+  bitset: Uint8Array;
+  widthPx: number;
+  heightPx: number;
+  bbox: BBox;
+  scale: number;
+  lengthIn: number;
+  lengthLabel: string;
+  slabBounds: { x: number; y: number; w: number; h: number };
+  source: { widthPx: number; heightPx: number };
+}
+
+/**
+ * Burn black dimension overlays onto a copy of the clean calibration inventory PNG.
+ * Returns a new image/png blob; never mutates the clean inventory.
+ */
+export async function encodeOverlayPng(input: EncodeOverlayInput): Promise<Blob> {
+  const bmp = await createImageBitmap(input.inventoryPng);
+  try {
+    const outW = bmp.width;
+    const outH = bmp.height;
+    const canvas = new OffscreenCanvas(outW, outH);
+    const ctx = canvas.getContext('2d')!;
+    ctx.drawImage(bmp, 0, 0);
+    const { mapToInventory } = inventorySourceMap(input.slabBounds, input.source, outW, outH);
+    const plan = buildOverlayPlan({
+      bitset: input.bitset,
+      widthPx: input.widthPx,
+      heightPx: input.heightPx,
+      bbox: input.bbox,
+      scale: input.scale,
+      lengthIn: input.lengthIn,
+      lengthLabel: input.lengthLabel,
+      mapToInventory,
+    });
+    drawOverlayPlan(ctx, plan);
+    return await canvas.convertToBlob({ type: 'image/png' });
+  } finally {
+    bmp.close();
+  }
+}