What it does
Finds photographed with a scale bar can be measured later, at the desk, without handling them again. This tool turns pixels into millimetres from the scale bar in the picture and gives you:
- Lengths between two taps (length, width, thickness, a scar or a hole), with a ± from how precisely you can place a tap.
- Angles at a corner from three taps, such as the edge angle of a scraper or the tip angle of a point. Angles do not need the scale.
- Areas of an outline you tap round, with its perimeter, for example the area of a sherd, a flake or a stain.
- A perspective check: mark a second scale bar (the other arm of an L-scale, or a second ruler) and the tool compares the two. If they disagree by more than 2 %, the photo was taken at a slant.
The photo is opened inside your browser. It is never uploaded, and nothing is kept when you close the page.
How to use it
- Open a photo (or drop it on the button). Photograph the object square-on, with the scale bar lying on the same level as the surface you want to measure.
- In Scale bar mode, tap one end of the bar, then the other, and type its length (mm, cm or inch). Use Zoom (2× or 4×) to tap precisely.
- Pick Length, Angle or Area and tap on the object. For an area, tap round the edge and tap the first point again or press Finish shape.
- Optional: Check scale with a second bar at right angles to the first.
- Download the annotated photo (PNG with every measurement drawn on) and the CSV (values, ±, notes and the pixel coordinates of every tap).
Click accuracy is how far off, in pixels, you think each tap is (1 px at a good zoom). The ± for a length combines the taps at both ends of the line and of the scale bar. Notes warn when a measurement is more than three times as long as the scale bar, lies far from it, or when an outline crosses itself.
Worked example
Press Use the example photo. It is a made-up leaf-shaped point with an L-shaped scale: 50 mm along the bottom and 25 mm up the side, both 8 px per mm. The tool shows 8 px per mm (one pixel is 0.125 mm) and the two bars agree to within 0.0 %, 90° apart. The three example measurements are:
- L1, the length from tip to base: 500 px, so 62.5 mm ± 0.3.
- A1, the tip angle: 41.7°.
- P1, the outline: 52,600 px², so 822 mm² ± 6, perimeter 135.1 mm.
Now type 30 in the second bar’s length: the bars disagree by 18.2 %, and the tool warns that the photo looks slanted.
FAQ
How accurate is it? With a sharp photo, the bar in the same plane and a square-on camera, about the ± shown. Tilt and wide-angle lens distortion add errors the tool cannot see, except through the second-bar check.
Can I measure a curved length? Use Area and read the perimeter, or add short straight lengths.
Is my photo uploaded? No. Everything runs in your browser and works offline.
Where the numbers come from
- Geometry only: distance, the angle between two vectors (atan2 of the cross and dot products) and the shoelace formula for polygon area. Uncertainty from click placement follows the usual propagation of independent errors (JCGM 100:2008, the GUM).
- Checked against numpy: areas of the example outline and three random polygons, the example angle and perimeter.
- The example photo (a leaf-shaped point and an L-shaped scale) is drawn by the page and made up for this tool.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.