diff --git a/src/app/calibration-ui.test.ts b/src/app/calibration-ui.test.ts
new file mode 100644
index 0000000..0f42a38
--- /dev/null
+++ b/src/app/calibration-ui.test.ts
@@ -0,0 +1,22 @@
+import { describe, it, expect } from 'vitest';
+import { residuals } from './calibration-ui';
+import { estimateHomography } from '../geometry/homography';
+import type { Vec2 } from '../types';
+
+describe('calibration residuals', () => {
+  it('reports near-zero error for a clean fit', () => {
+    const machine: Vec2[] = [[0, 0], [100, 0], [100, 100], [0, 100]];
+    const image: Vec2[] = [[10, 10], [210, 12], [208, 210], [8, 208]];
+    const H = estimateHomography(machine, image);
+    const errs = residuals(H, machine, image);
+    expect(Math.max(...errs)).toBeLessThan(1e-6);
+  });
+
+  it('flags a mismatched point with a large residual', () => {
+    const machine: Vec2[] = [[0, 0], [100, 0], [100, 100], [0, 100], [50, 50]];
+    const image: Vec2[] = [[0, 0], [100, 0], [100, 100], [0, 100], [80, 20]]; // last is wrong
+    const H = estimateHomography(machine, image);
+    const errs = residuals(H, machine, image);
+    expect(errs[4]!).toBeGreaterThan(5);
+  });
+});
diff --git a/src/app/calibration-ui.ts b/src/app/calibration-ui.ts
new file mode 100644
index 0000000..b78cf07
--- /dev/null
+++ b/src/app/calibration-ui.ts
@@ -0,0 +1,106 @@
+import type { Vec2, Mat3 } from '../types';
+import { estimateHomography, applyHomography } from '../geometry/homography';
+
+/** Per-point reprojection error in pixels: |H·machine − image|. */
+export function residuals(H: Mat3, machine: Vec2[], image: Vec2[]): number[] {
+  return machine.map((m, i) => {
+    const p = applyHomography(H, m);
+    return Math.hypot(p[0] - image[i]![0], p[1] - image[i]![1]);
+  });
+}
+
+interface CalibPoint {
+  machine: Vec2;
+  image: Vec2;
+}
+
+export interface CalibrationDeps {
+  panel: HTMLElement;
+  overlay: HTMLCanvasElement;
+  onHomography: (H: Mat3, points: CalibPoint[]) => void;
+}
+
+/**
+ * Renders the calibration panel. Flow: enter machine X/Y → click "Click point in video" →
+ * click the spindle tip in the video → repeat ≥4× → "Compute".
+ */
+export function mountCalibration(deps: CalibrationDeps): void {
+  const { panel, overlay } = deps;
+  const points: CalibPoint[] = [];
+  let pendingMachine: Vec2 | null = null;
+
+  panel.innerHTML = `
+    <h2>Calibration</h2>
+    <div class="row"><small>Jog the spindle to a known X/Y, enter it, then click the tip in the video.</small></div>
+    <div class="row">
+      X <input id="cal-x" type="number" step="0.1" />
+      Y <input id="cal-y" type="number" step="0.1" />
+      <button id="cal-arm">Click point in video</button>
+    </div>
+    <ul id="cal-list"></ul>
+    <div class="row">
+      <button id="cal-compute">Compute homography</button>
+      <button id="cal-clear">Clear</button>
+    </div>
+    <textarea id="cal-json" rows="6" style="width:100%" readonly placeholder="Calibration JSON appears here"></textarea>
+  `;
+
+  const xEl = panel.querySelector('#cal-x') as HTMLInputElement;
+  const yEl = panel.querySelector('#cal-y') as HTMLInputElement;
+  const armBtn = panel.querySelector('#cal-arm') as HTMLButtonElement;
+  const list = panel.querySelector('#cal-list') as HTMLUListElement;
+  const jsonEl = panel.querySelector('#cal-json') as HTMLTextAreaElement;
+
+  const renderList = (errs?: number[]) => {
+    list.innerHTML = points
+      .map((p, i) => `<li>(${p.machine[0]}, ${p.machine[1]}) → px(${p.image[0].toFixed(0)}, ${p.image[1].toFixed(0)})${errs ? ` <small>err ${errs[i]!.toFixed(1)}px</small>` : ''}</li>`)
+      .join('');
+  };
+
+  armBtn.addEventListener('click', () => {
+    pendingMachine = [parseFloat(xEl.value), parseFloat(yEl.value)];
+    if (Number.isNaN(pendingMachine[0]) || Number.isNaN(pendingMachine[1])) {
+      pendingMachine = null;
+      return;
+    }
+    overlay.classList.add('interactive');
+  });
+
+  overlay.addEventListener('click', (ev) => {
+    if (!pendingMachine || !overlay.classList.contains('interactive')) return;
+    const rect = overlay.getBoundingClientRect();
+    // Map click to native image pixels: canvas backing store == displayed media box.
+    const px: Vec2 = [
+      ((ev.clientX - rect.left) / rect.width) * overlay.width,
+      ((ev.clientY - rect.top) / rect.height) * overlay.height,
+    ];
+    points.push({ machine: pendingMachine, image: px });
+    pendingMachine = null;
+    overlay.classList.remove('interactive');
+    renderList();
+  });
+
+  (panel.querySelector('#cal-clear') as HTMLButtonElement).addEventListener('click', () => {
+    points.length = 0;
+    jsonEl.value = '';
+    renderList();
+  });
+
+  (panel.querySelector('#cal-compute') as HTMLButtonElement).addEventListener('click', () => {
+    if (points.length < 4) {
+      jsonEl.value = 'Need at least 4 points.';
+      return;
+    }
+    const machine = points.map((p) => p.machine);
+    const image = points.map((p) => p.image);
+    const H = estimateHomography(machine, image);
+    const errs = residuals(H, machine, image);
+    renderList(errs);
+    jsonEl.value = JSON.stringify(
+      { imagePoints: image, machinePoints: machine, homography: H },
+      null,
+      2,
+    );
+    deps.onHomography(H, [...points]);
+  });
+}
diff --git a/src/main.ts b/src/main.ts
index 9e09683..f3fadbb 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -3,6 +3,7 @@ import { loadConfig } from './config';
 import { createState } from './app/state';
 import { computeOverlayRect, syncOverlay } from './app/layout';
 import { drawOverlay, clearCanvas } from './render/renderer';
+import { mountCalibration } from './app/calibration-ui';
 import type { RenderStyle } from './types';
 
 const stage = document.getElementById('stage') as HTMLDivElement;
@@ -42,6 +43,16 @@ fileInput.addEventListener('change', async () => {
 // expose for the UI modules added in later tasks
 export const app = { state, render, resize, overlay, stage, stream };
 
+mountCalibration({
+  panel: document.getElementById('calib-panel') as HTMLElement,
+  overlay,
+  onHomography: (H) => {
+    state.setHomography(H);
+    statusEl.textContent = 'Calibrated (unsaved — paste JSON into config.json to persist).';
+    render();
+  },
+});
+
 async function boot(): Promise<void> {
   try {
     const cfg = await loadConfig('config.json');