One inch is exactly 2.54 centimetres, fixed by international agreement in 1959. Every disagreement after that is about rounding or about what is being measured.
The exact factor and the safe shortcut
Inches = centimetres / 2.54. Centimetres = inches x 2.54. Both are exact. For a rough figure, divide centimetres by 10 and multiply by 4: 30 cm gives 12 inches against an exact 11.81.
Keep one decimal place for anything you will cut, and two for anything that has to fit into a frame or a slot.
Screens are measured diagonally, not across
A 27-inch monitor is 68.6 cm corner to corner, not 68.6 cm wide. At a 16:9 ratio, that screen is about 59.8 cm wide and 33.6 cm tall - so desk clearance has to be checked against the width, never the advertised size.
Two screens with the same diagonal and different aspect ratios have different widths and different total areas, which is why an ultrawide feels much larger than its diagonal suggests.
Print and photo sizes do not line up neatly
A4 is 21.0 x 29.7 cm, or 8.27 x 11.69 inches. US Letter is 8.5 x 11 inches, or 21.6 x 27.9 cm. They are close enough to load in the same printer and different enough to clip a design that assumed the other one.
Standard photo prints are quoted in inches - 4x6, 5x7, 8x10 - so a design laid out in centimetres needs the conversion before the crop, not after.
Furniture and clothing ordered from abroad
Convert the opening, not the object: a doorway of 76 cm is 29.9 inches, so a sofa listed at 30 inches deep will not turn the corner. Always convert both and leave a margin.
Clothing sizes are not a pure conversion. A waist measurement converts cleanly, but sizing labels do not - use the retailer's own measurement chart in centimetres where they publish one.
Last reviewed 2026-09-13.