The idea behind it
A ruler is just an object of known length, marked in units. Plenty of things in a typical wallet or kitchen drawer are objects of known length that simply are not marked that way, because a bank card, a coin or a sheet of printer paper is manufactured to a published standard far tighter than most people ever measure by hand. Laid beside something unknown, they turn a guess into a comparison, which is a far more reliable way to arrive at a number.
The most useful reference objects
Not all reference objects are equally trustworthy, and it is worth knowing which ones are genuinely standardised rather than merely common.
- A bank card. The single best option, because ISO/IEC 7810 fixes every payment card worldwide at 85.6 × 53.98 mm, regardless of issuer or country. It is also usually within reach.
- A sheet of A4 or US Letter paper. A4 is fixed at exactly 210 × 297 mm by ISO 216; US Letter is 215.9 × 279.4 mm. Either gives a long, straight reference edge, useful for anything longer than a card.
- Coins. Reliable within a country, since mints hold tight tolerances, but sizes vary between currencies and even between denominations, so check the actual figure rather than assuming. A US quarter is 24.26 mm across; a two-euro coin is 25.75 mm; a UK 50p is 27.3 mm but is not circular, so measure it corner to corner rather than edge to edge.
- A door. Interior doors are commonly built to a small number of standard widths (in the US, 762 mm or 30 inches is typical for an internal door), making one a rough but usable reference for anything room-sized.
How to use one to measure something else
Lay the reference object directly against what you are measuring, edge to edge, on a flat surface, and count how many of it fit along the length in question, including any fraction. A book that spans one and a half bank cards laid end to end is roughly 128 mm long (1.5 × 85.6 mm). The method is only as accurate as the alignment, so keep both objects flat, keep the edges square rather than at an angle, and take the reading from directly above rather than at a sideways angle, which introduces parallax error.
For anything too large to lay a card against directly, a photograph works almost as well. Photograph the object with the reference item placed beside it in the same plane, then compare the two in an image editor or even by eye: the ratio between their lengths in the photo is the same as the ratio in reality, as long as both are the same distance from the camera and photographed straight on rather than at an angle.
What ruins the accuracy
Three mistakes account for most of the error in this kind of measuring. Photographing at an angle introduces perspective distortion, so the reference object and the target need to be in the same plane and photographed straight on. Using a worn or bent coin, or a bank card that has been through a wallet for years, introduces small but real error, since edges wear unevenly. And skipping the fraction, rounding "two and a bit cards" down to exactly two, throws away most of the precision the method can offer; estimate the fraction as carefully as the whole units.
When to use a screen instead
A physical reference object is the right tool when nothing digital is at hand. When a screen is available, a calibrated on-screen ruler is usually faster and finer-grained, since it reads in exact millimetres rather than fractions of a card width, and it remembers its calibration for next time. The ruler on this site calibrates the same way this guide describes, against a bank card, so the same 85.6mm reference either method relies on does the calibrating either way.