Chris Collins

Tools · sensors

Measure from a photo

Photograph something next to a bank card or a sheet of paper, drag four handles onto the reference, and measure lengths, rectangles, and areas on that surface — perspective-corrected, with a magnifier, pinch-zoom, undo, and an annotated PNG to save.

Added

How to use it

  1. Get a photo. Lay a reference flat on the surface you want to measure — a bank or ID card, a sheet of US Letter or A4 paper, a US banknote, or anything rectangular whose size you know — and photograph it together with the thing to measure. Take photo opens the phone camera, Live camera shows a preview with a Capture button, and Open image (or drag and drop, or paste) loads an existing picture. Sample loads a synthetic picture to practise on.

  2. Mark the reference. With the Reference tool, drag the four orange handles onto the four corners of the card or sheet; the order does not matter. Pick its size from the list, or choose Custom size and type it (100 mm, 4-1/2 in, 29.7cm). The width and height are matched to the longer and shorter sides automatically; Swap width/height fixes the rare wrong guess. Show plane grid draws a grid on that surface — if the grid lines run along the edges of floorboards or tiles, the reference is placed well.

  3. Measure. Line — drag from one end to the other. Rectangle — drag corner to corner; the rectangle lines up with the reference’s edges, so place the reference square to what you measure. Polygon — tap each corner, then tap the first point or Finish shape to get the area and perimeter. Every handle can be dragged afterwards, and the results update when you change the reference or the units.

  4. Read and save. Measurements are listed with a ± figure and drawn on the photo. Name them, copy them as text, or Download annotated PNG at the photo’s full resolution.

Choose millimetres, centimetres, metres, inches, or feet; for inches and feet you can show fractions down to 1/64".

On a phone, a magnifier appears in a corner while you drag a handle, so your finger does not hide the point. Pinch to zoom and drag with two fingers to pan; with a mouse, scroll to zoom and drag on empty space with the Pan or Reference tool. Undo (Ctrl/⌘ Z) steps back through every change.

Line (scale only) reference mode takes a single line of known length instead of a rectangle. It is just a scale — millimetres per pixel — so it is only right for things at the same distance from the camera as that line, on a surface square-on to the lens. Use a rectangle whenever you can.

How it works

A flat surface seen through a camera is related to the picture by a homography: a 3 × 3 projective transform. Four points with known positions fix it. Here the corners of the reference are mapped to a rectangle of the reference’s size in millimetres by the direct linear transform (with Hartley normalisation), and every point you mark is sent through it to millimetres on the table. Lengths, rectangles, and areas are then ordinary geometry on that plane, so foreshortening is corrected: a line far from the camera measures the same as an equal line close to it.

The ± figure estimates error from where points can be placed: a pixel at each end of the measurement, converted with the local scale, plus 1.5 pixels at each reference corner as a fraction of the reference’s size in the picture. It does not include any of the effects below.

AR tape measure

On phones whose browser supports WebXR augmented reality with hit testing (Chrome on most ARCore Android phones), an AR tape measure button appears. Point the phone at a surface until the ring appears, tap to drop a start point, tap again for the end; repeat for more measurements, then Done. The lengths are added to the list. AR relies on the phone’s motion tracking, and is typically within a centimetre or two over a metre. Where AR is not supported (iPhone, desktop), the button is hidden.

Limits

  • Only on the reference’s plane. Points must lie on the same flat surface as the reference. Anything standing up from it — the height of a box, a thing raised on legs, the thickness of a board — is measured wrongly, often badly. Move the reference onto the face you measure.

  • Lens distortion is not corrected. Wide and ultra-wide phone lenses bend straight lines, most near the edges of the frame. Use the main (1×) camera, keep the reference and the object near the middle of the picture, and step back and zoom rather than getting close with a wide lens.

  • Small references magnify errors. A card 200 pixels wide marked a pixel and a half off at each corner makes every measurement about 1% off, and it gets worse the further you measure from the card. Get the reference big in the frame, zoom in when placing the handles, and use a sheet of paper for bigger things.

  • Real objects are not ideal. Cards have rounded corners (place the handles where the straight edges would meet), paper may be trimmed or curled, and banknotes vary slightly.

  • Steep angles hurt. The flatter you look along the surface, the more a pixel covers far away. Shoot from as close to straight above as you can.

  • Rectangles are aligned with the reference’s edges; for anything at an angle to it, use Polygon.

  • The photo stays in your browser and is not saved; reloading the page clears it. Only the reference size, mode, and units are remembered.

Sharing

Share link copies a URL that opens the tool with your reference size, mode, and units — not the photo or the measurements. To write a link by hand, use ref (card, letter, a4, dollar, custom), mode (rect, line), unit (mm, cm, m, in, ft), frac (0, 2, 4 … 64), w and h for a custom size, and len for a reference line — for example /tools/measure/?ref=custom&w=300mm&h=200mm&unit=in&frac=16.

Runs entirely in your browser; nothing you type leaves this page.