Photo Measure — Add Measurements to a Photo
Drop in a photo, drag along something whose real size you already know, and type that length. From then on every line and box you draw is labelled in centimetres, inches or feet instead of pixels. Export the annotated image as a PNG when you are done. Your photo is read straight from disk into this tab and never uploaded.
Use Cases
How to add measurements to a photo
Drop a JPG, PNG, WebP or HEIC onto the page. Large photos are scaled to fit your screen, but every measurement is taken in the original pixels, so the fit does not cost you accuracy.
With Set reference selected, drag along something in the shot whose real length you know — a ruler, a tile, a door, a coin — then type that length and choose the unit.
Switch to Measure line for distances or Measure box for a width, height and area. Each annotation is labelled with the real-world figure as you draw it, in the reference unit.
Click any annotation to select it and press Delete to remove it, or clear the lot. Download annotated PNG writes the photo and every label out at full original resolution.
Frequently Asked Questions
- How accurate is measuring from a photo?
- As accurate as your reference and your camera angle allow. Anything in the same plane as the reference and shot straight on is usually within a couple of percent. Perspective is what kills accuracy: an object nearer the lens than the reference measures too big, and a photo taken at an angle compresses one axis. Shoot square-on, put the reference in the same plane as the thing you are measuring, and stand back rather than zooming in.
- What makes a good reference object?
- Something rigid, flat-on to the camera, in the same plane as your subject, and as long as you can manage — error in the reference scales up into every other measurement, so a 30 cm ruler beats a 2 cm coin. A tape measure laid across the scene is the gold standard.
- Does zooming or resizing the window change my measurements?
- No. Annotations are stored in the original image's pixel coordinates and the on-screen fit is display only, so a photo shown at 20% measures exactly the same as one shown at 100%.
- Is my photo uploaded?
- No. The file is read into an object URL inside this tab, drawn to a canvas, and exported locally. No part of the image is sent to a server, which matters when the photo is of your house, an insurance claim or a job site.
- Can I measure area?
- Yes — draw a box and it is labelled with its width, height and, once a reference is set, its area in square units. Boxes are axis-aligned rectangles, so for an irregular shape measure the bounding box or take a few lines instead.
- What if I have not set a reference yet?
- Lines and boxes are labelled in pixels and the reference panel warns you. Set a reference at any point and every existing annotation is relabelled in real units immediately — you do not have to redraw them.
Getting an accurate measurement out of a photo
A photo is a projection, so the scale you calibrate is only strictly true in the plane your reference sits in. These are the things that move the number, in rough order of how much damage they do.
| What affects accuracy | Why it matters | What to do about it |
|---|---|---|
| Camera angle | Shooting off-axis foreshortens one direction, so a square reads as a rectangle | Stand square to the surface and keep the camera level with the middle of the subject |
| Depth difference | Anything closer to the lens than the reference measures larger, and vice versa | Put the reference in the same plane as what you are measuring, touching it if you can |
| Reference length | Error in the reference multiplies into every reading taken from it | Use the longest known object available — a tape measure or ruler beats a coin |
| Lens distortion | Wide-angle and phone ultra-wide lenses bow straight lines near the edges | Keep the subject near the centre of the frame and avoid the ultra-wide camera |
| Zoom and crop | Neither changes the ratio between the reference and the subject | Harmless — digital zoom and cropping do not affect a measurement calibrated in the same image |
| Screen size | None — measurements are taken in the original pixels | Harmless; the fit-to-screen percentage is display only |
- Calibrate close to the scale you care about. A reference roughly as long as the thing you are measuring keeps the relative error small; measuring a 4 m wall from a 2 cm coin multiplies every reading error by 200.
- For a flat surface photographed head-on — a wall, a floor, a tabletop, a document — a single reference is genuinely accurate, and this is where the tool earns its keep. For anything with real depth, a photo cannot give you a metric answer no matter which tool you use; that needs two cameras or a depth sensor.
- Exported PNGs keep the original pixel dimensions, so a 12-megapixel phone photo comes back out at 12 megapixels with the labels burned in at a readable size.
Private by design. Your photo is read straight from disk into this browser tab, measured and annotated with canvas, and exported locally — it is never uploaded to a server.