# How to measure something without a ruler, using what is in your wallet

Source: https://spolvero.design/articles/measure-without-a-ruler/
Updated: 2026-09-23
Tool: https://spolvero.design/tools/measure/ruler/

> Use something made to a standard size. Every bank card is 85.6 mm by 53.98 mm, an A4 sheet is 210 by 297 mm, a US Letter sheet is 8.5 by 11 inches, and a ₹10 coin is 27 mm across. For anything finer, calibrate an on-screen ruler by matching its outline to a bank card: after that the screen measures to about a quarter of a millimetre.

Most homes have a ruler somewhere, which is not the same as having one now. Fortunately you are carrying several objects made to a published standard, accurate to a fraction of a millimetre.

## Things of a known size

| Object | Size | Good for |
| --- | --- | --- |
| Bank, credit or ID card | 85.6 × 53.98 mm | Small things, and calibrating a screen |
| A4 sheet | 210 × 297 mm | Anything up to 30 cm |
| US Letter sheet | 215.9 × 279.4 mm (8.5 × 11 in) | The same, in inches |
| ₹10 coin | 27 mm across | Quick checks of small parts |
| US quarter | 24.26 mm across | The same |
| €1 coin | 23.25 mm across | The same |

Cards are the best of these. The size is an international standard (ISO/IEC 7810 ID-1), which is why any card fits any machine in the world. Indian ₹1, ₹2 and ₹5 coins have each been minted at more than one diameter, so they are left out; the ₹10 has been 27 mm in every series.

## Paper as a tape measure

An A4 sheet gives you 297 mm in one go. For a finer scale, **fold it**: each fold along the long side halves the length — 148.5, 74.25, 37.1 mm. Mark the folds with a pencil and you have a rough ruler in about a minute.

For something longer than a sheet, mark the end on the object in pencil, move the sheet along and add up.

## Turn your screen into a ruler

A screen can be a genuinely accurate ruler — but **never out of the box**. Web pages assume 96 pixels per inch and almost no real screen matches that, so an on-screen ruler that skips calibration is simply wrong, sometimes by a third.

The fix takes seconds:

1. Open the [online ruler](/tools/measure/ruler/).
2. Hold a bank card against the screen and press + or − until the outline matches its edges.
3. Save. The ruler is now true to size on that screen, and stays that way.

After that, hold the object against the glass and drag the two markers to its ends. The page tells you what one pixel is worth on your screen — typically 0.15 to 0.3 mm — which is the honest limit of the reading.

Two things undo a calibration: **browser zoom** and moving the window to a different monitor. The page notices both and asks again.

## How far to trust each

- **Card or paper edge against the object:** good to about a millimetre, limited by your eye.
- **Calibrated screen:** good to a quarter of a millimetre or so. Look from directly above — the glass has thickness, and from an angle the ruler and the object shift against each other.
- **Uncalibrated screen, or a printed ruler you did not check:** do not trust it at all. If you print a ruler, print at 100% and check it against a card before using it.

For the one case where a third of a millimetre decides the answer — **ring size** — there is a [sizer built on the same calibration](/tools/measure/ring-size/).

## Questions

### How big is a credit card in cm?

8.56 cm by 5.398 cm — 85.6 mm by 53.98 mm, or 3.37 by 2.125 inches. The size is fixed by the ISO/IEC 7810 ID-1 standard, so every bank card, and most ID cards and driving licences, are identical.

### Are online rulers accurate?

Only after calibration. A browser assumes 96 pixels per inch and real screens range from about 90 to over 150 in CSS pixels, so an uncalibrated ruler can be out by a third. Once matched to a bank card, it is good to about one pixel — 0.2 to 0.3 mm.

### How can I measure 1 cm without a ruler?

The short edge of a bank card is 5.4 cm, so a fifth of it is close to 1 cm. A standard A4 sheet folded in half three times along its short edge gives 2.6 cm sections. For anything that needs to be right, use a calibrated on-screen ruler.

### How long is an A4 sheet?

297 mm long and 210 mm wide — 29.7 by 21 cm. Folded in half it is A5, 210 by 148 mm.