Since 1959 the inch has been defined as exactly 0.0254 metres by international agreement. That makes this one of the cleanest conversions there is: the factor is exact, not measured, so any imprecision comes from rounding the answer rather than from the conversion itself.
The reciprocal is 1 cm = 0.3937007874 in, which is why you often see 0.3937 quoted. For mental arithmetic, dividing by 2.5 gets you within about 2% — close enough to sanity-check a result, not close enough to cut material to.
| Centimetres | Inches | Nearest 16th |
|---|---|---|
| 1 cm | 0.39 in | 0 in 6/16 |
| 2 cm | 0.79 in | 0 in 13/16 |
| 5 cm | 1.97 in | 1 in 15/16 |
| 10 cm | 3.94 in | 3 in 15/16 |
| 15 cm | 5.91 in | 5 in 14/16 |
| 20 cm | 7.87 in | 7 in 14/16 |
| 25 cm | 9.84 in | 9 in 13/16 |
| 30 cm | 11.81 in | 11 in 13/16 |
| 40 cm | 15.75 in | 15 in 12/16 |
| 50 cm | 19.69 in | 19 in 11/16 |
| 75 cm | 29.53 in | 29 in 8/16 |
| 100 cm | 39.37 in | 39 in 6/16 |
| 150 cm | 59.06 in | 59 in 1/16 |
| 180 cm | 70.87 in | 70 in 14/16 |
| 200 cm | 78.74 in | 78 in 12/16 |
Fractional inches, which is what most trades use
Rulers, timber, pipe and fabric in the US are marked in binary fractions — halves, quarters, eighths, sixteenths — not decimals. Converting 30 cm to "11.81 inches" is arithmetically right and practically useless at a workbench.
To get there: take the decimal part, multiply by 16, and round to the nearest whole number. 30 cm is 11.811 in; 0.811 × 16 = 12.98, so 30 cm ≈ 11 and 13/16 inches. If the fraction lands on an even number, simplify — 8/16 is a half, 12/16 is three-quarters.
Rounding to the nearest sixteenth introduces up to about 0.8 mm of error. Fine for a bookshelf, not fine for a machined part. If tolerance matters, work in the original unit system and convert only the final dimension — or better, don't convert at all.
Heights, the most common real use
| Centimetres | Feet and inches |
|---|---|
| 150 cm | 4′ 11″ |
| 155 cm | 5′ 1″ |
| 160 cm | 5′ 3″ |
| 165 cm | 5′ 5″ |
| 170 cm | 5′ 7″ |
| 175 cm | 5′ 9″ |
| 180 cm | 5′ 11″ |
| 185 cm | 6′ 1″ |
| 190 cm | 6′ 3″ |
| 195 cm | 6′ 5″ |
| 200 cm | 6′ 7″ |
To do it yourself: divide by 2.54 for total inches, then divide by 12 for feet with the remainder as inches. 175 cm gives 68.9 inches, which is 5 feet and 8.9 inches — so 5′9″ rounded.
Paper sizes, where the two systems diverge visibly
A4 is 210 × 297 mm, which converts to 8.27 × 11.69 in. US Letter is 8.5 × 11 in. A4 is narrower and taller. That is why printing an A4 PDF on Letter without scaling clips the sides, and why "fit to page" exists.
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