Field note 05

One inch is exactly 25.4 mm and one centimeter is 10 mm. Few everyday things match those lengths exactly, but several come close enough to picture them. Sizes below are official or standardized; where items vary, we say so.

About 1 inch (25.4 mm)

• Coins: check your own currency first, as many countries publish exact diameters. Two that are close: the 2-euro coin (25.75 mm) and, a little smaller, the US quarter (24.26 mm).
• The width of an adult thumb at the knuckle is often around an inch, but it varies a lot from person to person. Measure yours once.
• A golf ball is at least 42.67 mm across, roughly 1.7 in; half a golf ball is a decent rough inch.

About 1 centimeter (10 mm)

• A nano-SIM card is 12.3 × 8.8 mm, so its short side is just under 1 cm.
• The plug of a USB-A cable is 12 mm wide.
• An AA battery is 14.5 mm in diameter: one and a half centimeters.
• A fingernail on a small finger is often close to 1 cm wide, but again, measure your own.

Better than guessing

For anything that fits on a phone or laptop, the online ruler calibrated with a bank card gives you real centimeters and inches to about half a millimeter. To switch between the two, use inches to cm or cm to inches.