66 categories · 788 units · exact arithmetic

The unit converter that doesn't round early

Most converters turn every factor into a floating-point number before you have typed anything, and quietly lose digits. This one keeps conversion factors as exact fractions and rounds once, at the end — so you can ask for fifty significant figures and get fifty correct ones.

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0.393 700 787 402in

≈ Rounded to 12 s.f. 1 cm ≈ 0.393 700 787 402 in

As a fraction: 50/127

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Significant digits 12 Round to Notation 1 cm = 0.393700787402 in

Show this value in all 50 length units

Gigameters Gm 1e-11
Megameters Mm 1e-8
Kilometers km 0.000 01
Hectometers hm 0.0001
Decameters dam 0.001
Meters m 0.01
Decimeters dm 0.1
Centimeters cm 1
Millimeters mm 10
Micrometers µm 10 000
Nanometers nm 10 000 000
Picometers pm 10 000 000 000
Femtometers fm 10 000 000 000 000
Inches in 0.393 700 787 402
Feet ft 0.032 808 398 950 1
Yards yd 0.010 936 132 983 4
Miles mi 0.000 006 213 711 922 37
Nautical miles nmi 0.000 005 399 568 034 56
Thous thou 393.700 787 402
Hands hh 0.098 425 196 850 4
Links li 0.049 709 695 379
Spans span 0.043 744 531 933 5
Cubits cubit 0.021 872 265 966 8
Ells ell 0.008 748 906 386 7
Fathoms ftm 0.005 468 066 491 69
Rods rd 0.001 988 387 815 16
Chains ch 0.000 497 096 953 79
Furlongs fur 0.000 049 709 695 379
Leagues lea 0.000 002 071 237 307 46
Barleycorns barleycorn 1.181 102 362 2
US survey feet ft (US) 0.032 808 333 333 3
US survey miles mi (US) 0.000 006 213 699 494 95
Ångströms Å 100 000 000
Microns µ 10 000
Bohr radiuss a₀ 188 972 612.591
Planck lengths ℓₚ 6.18714249917e+32
Astronomical units au 6.68458712227e-14
Light-years ly 1.05700083402e-18
Light-seconds ls 3.33564095198e-11
Light-minutes lmin 5.55940158664e-13
Parsecs pc 3.24077928944e-19
Kiloparsecs kpc 3.24077928944e-22
Megaparsecs Mpc 3.24077928944e-25
Solar radiuss R☉ 1.43740117867e-11
Earth radiuss R🜨 1.56961230576e-9
Point (DTP)s pt 28.346 456 692 9
Picas pc 2.362 204 724 41
Scandinavian miles mil 0.000 001
Versts вер 0.000 009 373 828 271 47
Lis 里 0.000 02

Every row is Centimeter (cm) converted through the meter, in exact rational arithmetic.

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Why exact arithmetic matters

Nearly every unit in daily use is defined as an exact fraction of an SI unit. An inch is exactly 25.4 millimeters. A pound is exactly 0.453 592 37 kilograms. A pound per square inch is exactly 6894.757 293 168 361 336 7… pascals — a repeating decimal, because the definition works out to a fraction with 127² in the denominator.

A converter built on 64-bit floats has about 15–17 usable digits and accumulates a small error at every step. For a recipe that never matters. For a tolerance stack, a calibration record, or a physics problem set, it eventually does. This site stores every factor as a ratio of arbitrary-precision integers and does the whole conversion in exact rational arithmetic, so the only rounding happens when the number is printed.

Where a unit genuinely is irrational — anything involving π, or a logarithmic unit like the decibel — the site says so, and falls back to a 50-digit computation instead of pretending. How the maths works →

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Light and dark themes that follow your system, text size and high-contrast controls, a search palette on Ctrl + K that answers "12 kg to lb" without leaving the page, expressions in the value box, per-line units in the batch converter, CSV and Markdown export everywhere, and offline support once a page has loaded. All of it is in the shortcut reference, and none of it sends anything anywhere.