diff --git a/src/components/DesignCanvas.tsx b/src/components/DesignCanvas.tsx
index cd37e32..7368b4a 100644
--- a/src/components/DesignCanvas.tsx
+++ b/src/components/DesignCanvas.tsx
@@ -168,6 +168,8 @@ function PrintSurface({
   onNodeReady,
   scale,
   product,
+  referenceWidthCm,
+  referenceHeightCm,
 }: {
   photo: React.ReactNode;
   printArea: PrintArea;
@@ -183,6 +185,8 @@ function PrintSurface({
   onNodeReady: () => void;
   scale: number;
   product: ProductDTO;
+  referenceWidthCm: number | null;
+  referenceHeightCm: number | null;
 }) {
   const stageW = printArea.wPct * DISPLAY;
   const stageH = printArea.hPct * DISPLAY;
@@ -204,13 +208,13 @@ function PrintSurface({
         </div>
 
         {/* Horizontal Ruler */}
-        {product.printAreaWidthMm > 0 && stageW > 0 && (
+        {referenceWidthCm && referenceWidthCm > 0 && (
         <div
           className="absolute pointer-events-none bg-paper border-b border-line text-xs font-mono"
           style={{
-            left: stageLeft,
-            top: stageTop - 20,
-            width: stageW,
+            left: 0,
+            top: -20,
+            width: DISPLAY,
             height: 18,
             display: 'flex',
             alignItems: 'flex-end',
@@ -219,12 +223,11 @@ function PrintSurface({
         >
           {(() => {
             const marks = [];
-            const cmPerStageW = product.printAreaWidthMm / 10; // total cm width of stage
-            const pxPerCm = stageW / cmPerStageW;
+            const pxPerCm = DISPLAY / referenceWidthCm;
 
-            for (let cm = 0; cm <= Math.ceil(cmPerStageW); cm += 2) {
+            for (let cm = 0; cm <= Math.ceil(referenceWidthCm); cm += 2) {
               const px = cm * pxPerCm;
-              if (px > stageW) break;
+              if (px > DISPLAY) break;
               marks.push(
                 <div
                   key={`h-${cm}`}
@@ -247,14 +250,14 @@ function PrintSurface({
         )}
 
         {/* Vertical Ruler */}
-        {product.printAreaHeightMm > 0 && stageH > 0 && (
+        {referenceHeightCm && referenceHeightCm > 0 && (
         <div
           className="absolute pointer-events-none bg-paper border-r border-line text-xs font-mono"
           style={{
-            left: stageLeft - 28,
-            top: stageTop,
+            left: -28,
+            top: 0,
             width: 26,
-            height: stageH,
+            height: DISPLAY,
             display: 'flex',
             flexDirection: 'column',
             alignItems: 'flex-end',
@@ -263,12 +266,11 @@ function PrintSurface({
         >
           {(() => {
             const marks = [];
-            const cmPerStageH = product.printAreaHeightMm / 10; // total cm height of stage
-            const pxPerCm = stageH / cmPerStageH;
+            const pxPerCm = DISPLAY / referenceHeightCm;
 
-            for (let cm = 0; cm <= Math.ceil(cmPerStageH); cm += 2) {
+            for (let cm = 0; cm <= Math.ceil(referenceHeightCm); cm += 2) {
               const px = cm * pxPerCm;
-              if (px > stageH) break;
+              if (px > DISPLAY) break;
               marks.push(
                 <div
                   key={`v-${cm}`}
@@ -404,6 +406,8 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
   const [justAdded, setJustAdded] = useState(false);
   const [showLoginPrompt, setShowLoginPrompt] = useState(false);
   const [nodeReadyTick, setNodeReadyTick] = useState(0);
+  const [referenceWidthCm, setReferenceWidthCm] = useState<number | null>(null);
+  const [referenceHeightCm, setReferenceHeightCm] = useState<number | null>(null);
 
   const stageFrontRef = useRef<Konva.Stage | null>(null);
   const stageBackRef = useRef<Konva.Stage | null>(null);
@@ -439,14 +443,12 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
   const activeStageW = activePrintArea.wPct * DISPLAY;
   const activeStageH = activePrintArea.hPct * DISPLAY;
 
-  // Calculate cm per pixel for display
-  const printAreaWidthPx = activePrintArea.wPct * DISPLAY;
-  // printAreaWidthMm is in mm, divide by 10 to get cm
-  // printAreaWidthPx is in pixels
-  // cmPerPx = cm per pixel
-  const mmPerPx = printAreaWidthPx > 0 ? product.printAreaWidthMm / printAreaWidthPx : 0;
-  const cmPerPx = mmPerPx > 0 ? mmPerPx / 10 : 0;
+  // Calculate cm per pixel for display using reference size
+  // Reference size maps to full canvas width/height (560px)
+  const cmPerPx = referenceWidthCm && referenceWidthCm > 0 ? referenceWidthCm / DISPLAY : 0;
+  const cmPerPxHeight = referenceHeightCm && referenceHeightCm > 0 ? referenceHeightCm / DISPLAY : 0;
   const pxToCm = (px: number) => {
+    if (!cmPerPx || cmPerPx <= 0) return '—';
     const cm = px * cmPerPx;
     return isNaN(cm) ? '?' : cm.toFixed(2);
   };
@@ -748,6 +750,8 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
               onNodeReady={() => setNodeReadyTick((t) => t + 1)}
               scale={canvasScale}
               product={product}
+              referenceWidthCm={referenceWidthCm}
+              referenceHeightCm={referenceHeightCm}
             />
 
             {hasBack && (
@@ -766,6 +770,8 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
                 onNodeReady={() => setNodeReadyTick((t) => t + 1)}
                 scale={canvasScale}
                 product={product}
+                referenceWidthCm={referenceWidthCm}
+                referenceHeightCm={referenceHeightCm}
               />
             )}
           </>
@@ -855,6 +861,38 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
           </div>
         )}
 
+        {canPersonalize && (
+        <div>
+          <p className="tag-label mb-3">Reference Size (cm)</p>
+          <div className="flex gap-3">
+            <div className="flex-1">
+              <label className="text-xs text-muted">Width</label>
+              <input
+                type="number"
+                min="1"
+                step="0.1"
+                value={referenceWidthCm ?? ''}
+                onChange={(e) => setReferenceWidthCm(e.target.value ? Number(e.target.value) : null)}
+                placeholder="e.g. 40"
+                className="w-full border border-line bg-paper px-3 py-2 text-sm placeholder:text-muted"
+              />
+            </div>
+            <div className="flex-1">
+              <label className="text-xs text-muted">Height</label>
+              <input
+                type="number"
+                min="1"
+                step="0.1"
+                value={referenceHeightCm ?? ''}
+                onChange={(e) => setReferenceHeightCm(e.target.value ? Number(e.target.value) : null)}
+                placeholder="e.g. 60"
+                className="w-full border border-line bg-paper px-3 py-2 text-sm placeholder:text-muted"
+              />
+            </div>
+          </div>
+        </div>
+        )}
+
         {canPersonalize && (
         <div>
           <p className="tag-label mb-3">
@@ -1002,9 +1040,9 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
             </div>
             <button
               onClick={handleAddToBag}
-              disabled={elementsFront.length === 0 && elementsBack.length === 0}
+              disabled={!canPersonalize || (elementsFront.length === 0 && elementsBack.length === 0) || !referenceWidthCm || !referenceHeightCm}
               className={`flex-1 px-6 py-3 text-sm font-medium transition-colors ${
-                elementsFront.length === 0 && elementsBack.length === 0
+                !canPersonalize || (elementsFront.length === 0 && elementsBack.length === 0) || !referenceWidthCm || !referenceHeightCm
                   ? 'bg-muted text-muted/50 cursor-not-allowed'
                   : 'bg-clay text-paper hover:bg-clay-dark'
               }`}
@@ -1012,7 +1050,10 @@ export default function DesignCanvas({ product }: { product: ProductDTO }) {
               Add to bag — <Price cents={((elementsFront.length > 0 || elementsBack.length > 0) ? effectivePriceForPersonalised : product.effectivePrice) * quantity} />
             </button>
           </div>
-          {elementsFront.length === 0 && elementsBack.length === 0 && (
+          {canPersonalize && (!referenceWidthCm || !referenceHeightCm) && (
+            <p className="text-xs text-splash-pink">Please enter the reference size (width and height) before adding to bag</p>
+          )}
+          {canPersonalize && (elementsFront.length === 0 && elementsBack.length === 0) && (referenceWidthCm && referenceHeightCm) && (
             <p className="text-xs text-splash-pink">Add text or images to personalize your design before adding to bag</p>
           )}
           {justAdded && <p className="text-sm text-pine">Added to your bag.</p>}