Everyday objects with known sizes
No ruler to hand? Some things in your wallet or printer tray are made to a published size. Here is what they measure, and how far to trust each one as a calibration reference.
A real ruler or tape measure is still the best reference. Use these when you don't have one, and prefer the longest edge that fits on your screen: a 1 mm placement error is 1% of a 100 mm edge but 5% of a 20 mm coin.
Bank, credit and ID cards
Almost every payment card, driver licence and transport card uses the ID-1 format from the international standard ISO/IEC 7810. It is the most consistent object most people carry.
| Dimension | Millimetres | Inches |
|---|---|---|
| Width | 85.60 mm | 3.370" |
| Height | 53.98 mm | 2.125" |
| Corner radius | about 3.18 mm | 1/8" |
Screen Ruler has a Credit Card Outline in Calibrate drawn to these dimensions. Lay the card flat on the glass, line its left edge up with the outline, and adjust the scale until the right edge matches too. Then rotate the card and check the 53.98 mm height. Embossed numbers and worn edges don't change the outline, but a card that has been trimmed, or a loyalty card printed on cheaper stock, might not be ID-1 at all.
Business cards are not standardised in the same way. Common sizes include 90 × 55 mm in Australia, 85 × 55 mm in much of Europe and 3.5 × 2 inches (88.9 × 50.8 mm) in the United States, and printers vary.
Paper
Paper sizes are standardised, which makes a fresh sheet a good reference for a longer span if you are using a tablet or a desktop monitor.
| Size | Millimetres | Inches | Used in |
|---|---|---|---|
| A4 | 210 × 297 | 8.27 × 11.69 | Australia, Europe, most of the world |
| A5 | 148 × 210 | 5.83 × 8.27 | Notebooks, flyers |
| A6 | 105 × 148 | 4.13 × 5.83 | Postcards, pocket notebooks |
| US Letter | 215.9 × 279.4 | 8.5 × 11 | United States, Canada |
Sheets are cut with a small manufacturing tolerance, so trust paper to about a millimetre, not a tenth of one. Use an unfolded sheet straight from the ream; a page torn out of a notebook or trimmed by a guillotine is not a reference.
Coins
Mints publish a diameter for each coin. Coins are small, so a placement error is a larger share of the reading, and a worn or rimmed edge is harder to line up. Use a coin to sanity-check a scale rather than to set one.
| Coin | Diameter |
|---|---|
| Australia 5c | 19.41 mm |
| Australia 10c | 23.60 mm |
| Australia 20c | 28.65 mm |
| Australia $1 | 25.00 mm |
| Australia $2 | 20.50 mm |
| United States 1¢ | 19.05 mm |
| United States 5¢ | 21.21 mm |
| United States 10¢ | 17.91 mm |
| United States 25¢ | 24.26 mm |
| Euro 10c | 19.75 mm |
| Euro 50c | 24.25 mm |
| Euro €1 | 23.25 mm |
| Euro €2 | 25.75 mm |
| United Kingdom 1p | 20.30 mm |
| United Kingdom 2p | 25.90 mm |
| United Kingdom £2 | 28.40 mm |
The table leaves out coins that are not round, such as the Australian 50c and the UK 20p, 50p and £1. Their size depends on whether you measure across the flats or across the corners, and that is easy to get wrong on a screen.
To measure a coin, set the green caliper on one edge and the amber caliper on the opposite edge, through the centre. Turn the coin a quarter turn and measure again; the two readings should agree. If they differ by more than a few tenths of a millimetre, your placement is the problem, not the coin.
What not to use
- A picture of a ruler. Its size on screen depends on the same scale you are trying to check.
- Phone dimensions from a spec sheet. They describe the body, not the display, and cases change them.
- Batteries, bottle caps and plugs. Their dimensions are allowed to vary within a range, sometimes by a millimetre or more.
- Thick objects held above the glass. Anything that doesn't sit flat is subject to parallax: look from a slight angle and the edge appears to move against the ruler.
Calibrating with a reference
- Open the ruler, open the settings cog and choose Calibrate.
- Lay the object flat, with its longest straight edge along the ruler.
- Adjust Ruler Scale until the edge matches the published length, looking straight down at the screen.
- Check a second object, or the same object along the other axis. If both agree, your scale is set.
The accuracy guide explains how to estimate how far off a reading might be.