How to use it
Pick a category, type a value, choose the units, and read the answer, along with the same value in every other unit of that category. Swap flips the direction. Change the number of significant digits if you need more or fewer.
Exact by definition
US customary units are defined from metric ones: the inch is exactly 2.54 centimeters and the US gallon exactly 231 cubic inches, as NIST notes, and the avoirdupois pound is exactly 0.45359237 kilograms. NIST’s wallet card gives rounded factors such as 3.79 liters per gallon for quick use and points to Special Publication 811, Appendix B for exact ones. This converter builds every unit from those definitions (a mile is 5,280 feet of 0.3048 m, a fluid ounce is 1/128 gallon) and keeps each factor as an exact fraction.
What we tested
On October 14, 2026 we checked the page’s own converter against 33 reference values (exact factors as published from NIST SP 811 and the unit definitions), shown at 12 significant digits: 33 / 33 matched. For comparison, a plain floating-point converter (x × factor ÷ factor) failed to give back the typed number for 5.6% to 12.9% of 10,000 two-decimal values per pair, for example 1,000 of 10,000 for inches to centimeters and back. All 2,472 there-and-back conversions between every pair of units with awkward values came back exactly. In Chromium: 1 mi = 1.609344 km; 98.6 F = 37 C; 1000 GB = 931.322574615 GiB; 150 lb = 68.0388555 kg, and swapping gave back 68.0388555 kg = 150 lb; the page fitted a 390 px phone screen with no errors
| Conversion | Result | Reference |
|---|---|---|
| 1 mi → km | 1.609344 | match |
| 1 in → cm | 2.54 | match |
| 1 ft → m | 0.3048 | match |
| 1 yd → m | 0.9144 | match |
| 1 nmi → m | 1852 | match |
| 1 lb → kg | 0.45359237 | match |
| 1 oz → g | 28.349523125 | match |
| 1 st → kg | 6.35029318 | match |
| 1 ston → kg | 907.18474 | match |
| 1 gal → l | 3.785411784 | match |
| 1 floz → ml | 29.5735295625 | match |
| 1 qt → l | 0.946352946 | match |
| 1 cup → ml | 236.5882365 | match |
| 1 ft3 → l | 28.316846592 | match |
| 1 in3 → ml | 16.387064 | match |
| 1 ac → m2 | 4046.8564224 | match |
Sources
Checked on .
Frequently Asked Questions
Built by Derek Giordano · Part of Ultimate Design Tools