Inches to Centimeters

Multiply the number of inches by 2.54: 10 in × 2.54 = 25.4 cm. To go the other way, divide by 2.54. The relation is exact by definition, not a rounded laboratory result. The 1959 international yard and pound agreement — signed by the US, the UK, Canada, Australia, New Zealand and South Africa — fixed the yard at exactly 0.9144 m, which makes the inch exactly 2.54 cm and the foot exactly 30.48 cm. It is one of the very few cross-system conversions that loses no precision in either direction. One loose end was tied off recently. The US had kept a second, very slightly different foot for land surveying, and on December 31, 2022 NIST and the National Geodetic Survey retired the US survey foot, leaving the international foot as the only one. The difference was about two parts per million — invisible on a tape measure, but enough to move a property corner over the length of a state.

This is the single biggest source of confusion for anyone raised metric: the inch is not subdivided into tenths. It is subdivided into binary fractions — 1/2, 1/4, 1/8, 1/16, 1/32. That is why an American tape measure shows tick marks of unequal length instead of ten identical ones, and why a wrench comes labeled 3/8" or 9/16". In centimeters: 1/8" = 0.3175 cm; 1/4" = 0.635 cm; 3/8" = 0.9525 cm; 1/2" = 1.27 cm; 5/8" = 1.5875 cm; 3/4" = 1.905 cm. The classic mistake is reading 3/8" as 3.8 inches, or treating 1/2" as "0.2". A fraction is a division: 3 ÷ 8 = 0.375 inch, which is 0.95 cm. Convert the fraction to a decimal first, then multiply — 5 3/8 in = 5.375 × 2.54 ≈ 13.65 cm.

Televisions and monitors are measured on the diagonal, so a 50" TV has a 127 cm diagonal — but nobody hangs a diagonal on a wall. What matters for the furniture is the width, and that depends on the aspect ratio. At the standard 16:9 ratio, a 50" screen is about 110.7 cm wide by 62.3 cm tall, bezel excluded. Other widths: 43" ≈ 95.2 cm; 55" ≈ 121.8 cm; 65" ≈ 143.9 cm; 75" ≈ 166 cm. There is a second trap when comparing sizes: doubling the inches does not double the screen. A 65" TV is not twice the picture of a 32" — it is about four times the area, because area grows with the square of the diagonal. The same math explains why stepping from 50" to 55" feels smaller than the numbers suggest: it is only about 21% more area.

In plumbing and construction, the inch often appears as a nominal size — a trade name rather than a physical dimension. A "1/2 inch" pipe measures 1.27 cm nowhere on it: the number identifies a size within a standard table, and the actual outside diameter is larger, around 0.84 in for that designation. The reason is historical. The naming grew out of the approximate inside diameter of pipes with much thicker walls than modern ones, and the label stuck after wall thicknesses changed. So fittings are bought by the stated nominal size, never by measuring the tube with calipers and converting. Lumber does the same thing, and more aggressively: a US "2x4" is actually about 1.5 in × 3.5 in (3.8 cm × 8.9 cm) once it has been dried and planed. Wheel rims follow the same convention — a "15-inch wheel" is 15 inches across the rim, not including the tire — as do screws, threads and bicycle chains.

The values that answer most questions: 1" = 2.54 cm; 2" = 5.08 cm; 4" = 10.16 cm; 6" = 15.24 cm; 12" = 30.48 cm (one foot); 24" = 60.96 cm; 36" = 91.44 cm (one yard). Going the other way: 10 cm ≈ 3.94"; 20 cm ≈ 7.87"; 30 cm ≈ 11.81"; 50 cm ≈ 19.69"; 100 cm ≈ 39.37". For a rough mental estimate, an inch is a little more than the width of an adult thumb — which is not a coincidence, since that is where the unit and its name came from in the first place. The shortcut of using 2.5 instead of 2.54 is within 2%, fine for a sanity check but off by about 1.4 cm over a yard, so do not cut material with it.