Measure Things in a Photo
Put a bank card in the shot and measure anything else in the picture.
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About the Measure Things in a Photo converter
Put something of a known size in the shot — a bank card is ideal — and everything else in the picture can be measured from it. Distances, areas and angles, in millimetres, centimetres or inches.
It is the answer to a small, frequent problem: something needs measuring and you are not there, or the tape measure is not long enough, or you have already taken the photograph and gone home.
Why four corners rather than one length
The obvious approach is to mark a known length, work out how many millimetres a pixel is worth, and scale everything by that number. It works — and only when the camera was exactly square on to the subject.
Photographs almost never are. In a picture taken at even a slight angle, the far side of a table is genuinely smaller in the image than the near side, so a single ratio cannot describe the whole picture. Scaling by one length will confidently report a wrong answer, and the error grows the further you measure from the reference.
Marking the four corners of a known rectangle solves for the projective transform between the photograph and the real surface instead. Once that is known, anything lying on that surface measures correctly however oblique the shot — which is the difference between a novelty and a tool.
The one thing to watch
The correction applies to a single flat surface. If your reference is lying on the floor, then the floor measures correctly — but a bookcase standing up off it is on a different plane, and will read wrong.
The rule is simple: put the reference on the same surface as the thing you are measuring. Measure a wall with a reference on that wall, a table with one on the table.
The page also refuses to pretend when the four marked corners are nearly in a line or crossed over, since a nearly-flat quadrilateral produces a transform that is arithmetically valid and practically useless.
What to use as a reference
A bank card is the best thing to hand. Everybody has one, it is rigid so it will not curl in the shot, and its dimensions are fixed by an international standard rather than by whoever printed it — 85.6 by 53.98 millimetres, the world over.
A sheet of A4 or US Letter works well for larger scenes, and a coin will do for small objects, though a round reference can only give a single length rather than a full plane correction.
What it is actually for
Buying second-hand furniture from a listing photograph and wanting to know whether it fits. Sending a joiner or a fitter the size of something without a site visit. Recording damage for an insurance claim with the dimensions marked on the picture. Working out how much tile or paper a wall needs from a photograph rather than a ladder.
The marked-up photograph downloads as a picture with the measurements drawn on it, which is what makes it worth sending to somebody rather than just reading off the screen.
It stays on your device
The photo is read into a canvas in your own browser and the marked-up copy is produced there too. Nothing is uploaded at any point.
That matters more here than for most tools, because the photographs people measure are overwhelmingly of their own homes, rooms and possessions.
Frequently asked questions
What do I need in the photo?
Something of a known size lying on the same surface as whatever you want to measure. A bank card is ideal. A sheet of A4 suits larger scenes, and a coin will do for small objects.
Does the photo have to be taken straight on?
No, and that is the point. Mark the four corners of your reference and the page works out the perspective, so a photo taken at an angle measures correctly. There is also a simpler two-point mode for pictures that genuinely are square on.
Why is my measurement wrong?
Almost always because the thing being measured is not on the same flat surface as the reference. The correction applies to one plane at a time, so a reference on the floor cannot measure the height of something standing on it.
How accurate is it?
Within a percent or two for a reasonable photo, dominated by how precisely you click the corners. Click carefully, zoom in if you can, and use the largest reference available — a sheet of paper beats a coin.
Can I measure an area?
Yes. Click round the shape and it reports the enclosed area, switching to square metres once the numbers get large. Angles are available too, which is useful for checking whether a corner is truly square.
Is my photo uploaded?
No. It is read into a canvas in your browser, all the work happens on your device, and the marked-up image is produced there. Nothing is sent anywhere.