This image has currency in it to indicate scale.

Avoid using objects such as coins for scale, as they will require people unfamiliar with them to look up the dimensions or guess, both of which defeat the purpose of the object in the first place. Coins are particularly bad as they can reinforce a geographical bias, and some coins' designs are copyrighted.

Ideally, a photograph should include a ruler with the subject (as seen in this photo) or add a scale marking via an image editor instead (as seen in this photo). SI ("metric") units are the most commonly used worldwide (see metre, 1 E-2 m).

Examples of bad practice: here.

• U.S. dollar:
  • 1¢: 19.05 mm , 0.75 in
  • 5¢: 21.21 mm , 0.79 in
  • 10¢: 17.91 mm , 0.71 in
  • 25¢: 24.26 mm , 0.94 in
  • 50¢: 30.61 mm , 1.22 in
  • $1: 26.5 mm , 1.02 in

• Canadian dollar:
  • 1¢: 19.05 mm , 0.75 in
  • 5¢: 21.2 mm , 0.83 in
  • 10¢: 18.03 mm , 0.71 in
  • 25¢: 23.88 mm , 0.94 in
  • 50¢: 27.13 mm , 1.07 in
  • $1: 26.5 mm , 1.02 in
  • $2: 28 mm , 1.1 in

• Pound sterling
  • 1p: 20.32 mm , 0.8 in
  • 2p: 25.91 mm , 1.02 in
  • 5p: 18 mm , 0.71 in
  • 10p: 24.5 mm , 0.96 in
  • 20p: 21.4 mm , 0.84 in
  • 50p: 27.3 mm , 1.07 in
  • £1: 22.5 mm , 0.89 in
  • £2: 28.4 mm , 1.12 in

• Euro
  • 1c: 16.25 mm , 0.64 in
  • 2c: 18.75 mm , 0.74 in
  • 5c: 21.25 mm , 0.84 in
  • 10c: 19.75 mm , 0.78 in
  • 20c: 22.25 mm , 0.88 in
  • 50c: 24.25 mm , 0.95 in
  • €1: 23.25 mm , 0.92 in
  • €2: 25.75 mm , 1.01 in