How We Calculate

Every number on this site comes from the tables below. Nothing is measured, estimated or copied from another conversion site.

One Definition per Unit

Each category has a base unit, and every other unit in that category is defined as an exact multiple of it. A conversion between two units is therefore a single multiplication and a single division, with no chain of rounded intermediate values.

Temperature is the exception. Fahrenheit, Celsius and Kelvin have different zero points as well as different degree sizes, so they are converted with an offset as well as a scale rather than a single factor.

Length: base unit is the meter

Source: NIST Special Publication 811, Appendix B

Weight: base unit is the kilogram

Source: NIST Special Publication 811, Appendix B

Volume: base unit is the liter

Source: NIST Handbook 44, Appendix C

Temperature: base unit is the kelvin

Source: The SI Brochure, BIPM

Temperature Formulas

• Celsius to kelvin: add 273.15
• Fahrenheit to Celsius: subtract 32, then multiply by 5/9
• Celsius to Fahrenheit: multiply by 9/5, then add 32
• Fahrenheit to kelvin: add 459.67, then multiply by 5/9

Rounding

Results are shown with up to six significant digits, with trailing zeros removed. Where a value is exact we show it exactly instead of rounding: one foot is 0.3048 meters, not 0.304800, and one pound is 0.45359237 kilograms rather than 0.453592.

Conversion results are also snapped to thirteen significant digits before display. That removes the floating-point dust computers produce — 32 °F converted to Celsius arrives as 0.0000000000000284 rather than 0 — without touching any digit a reader would ever see.

Where a rounded figure is more useful than a precise one, the quick answer adds it explicitly, as in "about 2.2 pounds". We never silently substitute the rounded value for the exact one.

The One Exception: Cooking by Weight

Everything above is a definition. The cups to grams pages are not, and they say so on every page. A cup measures volume and grams measure mass; what connects them is density, which varies with the ingredient, how finely it is milled, how humid the kitchen is and — most of all — how the cook filled the cup. Scooping flour straight from the bag can yield 145 grams where spooning and levelling gives 120.

Those pages therefore use "≈" rather than "=", publish the realistic range alongside the headline figure, and follow the King Arthur Baking ingredient weight chart, which is the reference most American recipes are written against. Butter is the exception within the exception: it is sold by weight, so a cup really is 227 grams.

Units We Deliberately Keep Apart

• Fluid ounces and ounces. One measures volume, the other weight. They are separate units in our data and cannot be converted into each other.
• US and UK volumes. A US gallon is 3.785411784 liters; a UK imperial gallon is 4.54609 liters. Only US customary volumes are on the site today, and every page says so.
• Cups. Recipes mean the US customary cup of 236.5882365 mL. The "legal cup" on nutrition labels is exactly 240 mL and the metric cup is 250 mL. We use the customary cup and flag the difference where it matters.
• The US survey foot. Retired by NIST at the end of 2022. All length values use the international foot of exactly 0.3048 meters.

Verification

The conversion code is covered by an automated test suite that runs before every deploy. It asserts known benchmarks, checks that every unit round-trips to its original value within floating-point tolerance, and validates that the structured data on each page is well formed. Found something wrong anyway? Tell us.