How to measure without a tape measure

If you don't have a tape measure, you still have objects with a known size. A bank card is 85.6 mm wide, a sheet of A4 paper is 297 mm long, a US dollar bill is 156 mm, and your phone's camera can measure anything longer than that. Here is what to use for each job.

Things in your pocket with a known size

Lay the object end over end along what you are measuring and count. Three card lengths is about 257 mm. For anything under 10 cm, a card or a coin beats any phone app, because you read it directly against the edge.

Your own body, measured once

Measure your hand span (thumb tip to little-finger tip, spread wide) and your foot length once, against a card or a sheet of paper, and write both numbers in your phone. A hand span is usually between 17 and 23 cm, a shoe about 24 to 30 cm. From then on you can pace off a room or span a shelf and land within a few centimeters.

For gaps, walls and furniture, use the camera

Anything longer than a sheet of paper is where a known object gets slow, and the error adds up with every flip. This is the job an AR measuring app does well: point the camera, tap the two ends, read the length.

1. Open the app and point the camera at the floor or wall you want to measure along. Move the phone slowly until it says it has the surface.
2. Get close, about half a meter to a meter from the first point, and tap it.
3. Move to the second point and tap. The length shows as you move, and the point snaps to a corner if there is one.
4. Save the photo. The dimensions are drawn on it, so there is nothing to write down.

AR Tape Measure does this for distance, area, rectangles and circles, and it refuses to place a point when the reading would be wrong, for example when the camera picks up the floor behind an object instead of the object's edge.

Where the camera is not the right tool

AR measurement is at its best on bright, textured surfaces: wood floors, tiles, patterned walls, furniture edges. On a blank white wall, in a dim room or on a very small object the error widens, and the app will ask you to move the phone slowly. For anything where millimeters matter, use the card or the paper, or a real tape measure. The camera is for deciding whether the fridge fits in the gap, not for cutting a shelf to size.

FAQ

Can I measure with just my phone, with no app?

Yes, as a known-size object. Look up your model's exact dimensions and use the phone itself like a ruler. Past about 30 cm that gets slow, and an AR app is faster and keeps a photo.

How accurate is measuring with a phone camera?

Accurate enough to decide whether something fits, on a well-lit textured surface and with the phone close to the points you tap. Not accurate enough for joinery or anything that has to be right to the millimeter.