Size from a photo

Built by Nitin Raja · Last updated October 24, 2026

How to measure your size from a photo

A photo on its own cannot tell how big anything is: a foot photographed from close up looks as big as a building from far away. The fix is to put something of a known size in the same picture. A sheet of A4 or US Letter paper and a bank card are perfect, because they are exactly the same size everywhere. Once you mark the four corners, the page knows how many pixels make a millimetre at every point on that flat surface, even if the photo was taken at a slight angle, and it can measure your foot, a ring or your wrist from it.

Shoe size from a photo of your foot

1. Put a sheet of paper on a hard floor, short edge against a wall.
2. Stand on it barefoot (or in the socks you will wear) with your heel against the wall, weight on that foot.
3. Ask someone to take a photo from straight above, with all four corners of the paper in the picture. Your toes must be on the paper.
4. Upload it, drag the orange dots to the paper's corners and the blue dots to the back of your heel and the tip of your longest toe.

The page shows your foot length and the matching US men's, US women's, UK and EU sizes, plus the Mondopoint size (foot length in millimetres) that ski boots and many work boots use. Measure both feet and use the longer one. Sizes differ between brands, so check the brand's own size chart for the final choice; the foot length in centimetres is the most reliable number to compare.

Ring size from a photo

Take a ring that already fits the finger you are buying for, lay it flat on a table next to a bank or ID card, and take a photo from straight above. Mark the card's corners and the inside edges of the ring across its widest point. The inner diameter gives the US size (each size adds about 0.81 mm), the UK letter size and the EU size, which is simply the inner circumference in millimetres. If the person's knuckle is large, go a quarter or half size up. No card handy for a photo, or prefer to size on screen? Use the online ring sizer, which also has a full ring size chart.

Which watch size fits your wrist

Hold a bank card flat on top of your wrist (or lay your wrist next to it on a table) and photograph it from above. Mark the card and the two edges of your wrist. The tool gives the width of the top of your wrist, which is what decides whether a watch looks right: the lugs should not hang over the edges. It shows the longest lug-to-lug length that fits, the case sizes that usually suit that width, and draws 36 to 46 mm watches on your own photo at true size, so you can compare a 40 mm and a 44 mm on your actual wrist before you buy.

For a strap or bracelet length you need the wrist's circumference instead: wrap a strip of paper around the wrist bone, mark where it overlaps and measure it flat.

Tips for accurate results

• Shoot from straight above and keep the object flat on the reference's surface. Things that stick up above the paper or card (a thick ring standing up) measure larger.
• Use a sharp, well-lit photo and zoom in with the magnifier when placing dots.
• Measure twice. If two measurements agree within a couple of millimetres, you can trust them.

Why we built this size-from-photo tool

Before building CheckMyHeight, I tested a lot of height and sizing websites to see what people actually need from them and what gets in the way. A photo on its own has no scale: a foot photographed close up and a boot photographed from across the room can fill the same number of pixels. Any tool that names a size from a bare photo is guessing.

Things of a known size fix that. Every bank and ID card is 85.60 x 53.98 mm (ISO/IEC 7810 ID-1), an A4 sheet is 210 x 297 mm (ISO 216) and a US Letter sheet is 8.5 x 11 inches. With one of them in the picture, the page can work out real millimetres on that flat surface, even when the photo is taken at a slight angle. We built it for the three sizes people most often buy online without trying on: shoes, rings and watches.

What this tool measures

You place four orange dots on the reference's corners and two blue dots on the points to measure. The code then:

• Corrects perspective: it solves a homography from the four corners to the reference's real size in millimetres (portrait or landscape, whichever fits the marked shape), and measures the distance between the two blue dots on that plane.
• Shoe size (foot length 90 to 360 mm): US men = 3 x length in inches - 22, US women = 3L - 21, UK = 3L - 23, EU = 1.5 x (length in cm + 1.5), all rounded to half sizes; Mondopoint = the foot length rounded to 5 mm.
• Ring size (inner diameter 12 to 25 mm): US/Canada from inside diameter = 11.63 mm + 0.8128 mm per size, UK letters per BS 6820, EU per ISO 8653 (inside circumference in mm). This is the same ring code as the online ring sizer, covered by automated tests.
• Watch fit (wrist width 35 to 90 mm): the longest lug-to-lug length that fits equals your wrist width, case sizes from a rule of thumb in the code (under 50 mm wide: 36 to 40 mm cases, up to 42 to 46 mm for wrists over 62 mm), and watch outlines from 36 to 46 mm drawn on your photo at true scale.

Last checked 24 October 2026. What it does not measure: foot width, volume or arch, a knuckle bigger than the finger base, wrist circumference for a strap, or anything that is not lying flat on the reference's surface. Brand sizing differs, so compare the length in cm with the brand's chart.

Why this tool is different

• A real reference, not a guess. Sizes come from millimetres measured against a card or a sheet of paper.
• Perspective correction, so a photo taken slightly off straight still measures on the flat plane, plus a magnifier while you drag the dots.
• Your photo never leaves your device. It is opened and measured inside your browser and is not uploaded or stored.
• Nothing until you add your own photo: no sample result, no account, no app.
• Formulas and standards named on this page, so you can check any size by hand.

Measuring more of yourself? Try pupillary distance for glasses, or measure your height with the camera.