How we calculate

Every unit on the site is defined once, in a single file, as an exact value in the SI base unit: 1 inch = 0.0254 m, 1 pound = 0.45359237 kg, 1 US gallon = 231 cubic inches. The pair pages, the category converters and the charts are all generated from that file, so a factor cannot differ from one page to the next.

Exact arithmetic

The converters do not use rounded multipliers or floating-point shortcuts. A factor such as 1/2.54 is kept as the fraction 50/127, the calculation is done on exact fractions, and the result is rounded once, to the precision you choose.

Where the factors come from

From the definitions published by the U.S. National Institute of Standards and Technology (Special Publication 811 and Handbook 44) and the International Bureau of Weights and Measures (the SI Brochure), plus the UK Weights and Measures Act 1985 for imperial units. Each page links the sources its units rely on, and automated tests compare the exact values with the rounded factors NIST prints.

Fractions of an inch

A result in inches can be rounded to the nearest 1/16, 1/32 or 1/64. The converter shows how far the fraction is from the exact value, so you can judge whether the nearest mark on a tape measure is close enough.

Cooking conversions

Cups to grams depends on the ingredient. Weight-to-volume conversions are only offered for ingredients with a weight we could check in a named source, and the page shows the line the figure was taken from and the date it was checked. If we have no verified weight for an ingredient, the converter does not guess.

US and imperial units

Gallons, quarts, pints and fluid ounces have different sizes in the United States and the United Kingdom, and “ton” has three meanings. Pages for those units start with the US unit and let you switch.

Where the calculation happens

In your browser. The page downloads the tool once and does the work on your device, so results appear as you type and your inputs are not sent to us.

What a tool page shows

• The date the page was last updated.
• How the result is produced.
• On the tools rebuilt on our current template: the formula or rule, a worked example you can check by hand, and the sources the numbers come from, linked. Older tools are being brought up to that standard.

Sources we rely on

• NIST Special Publication 811: Guide for the Use of the International System of Units, Appendix B
• NIST Handbook 44, Appendix C: General Tables of Units of Measurement
• BIPM: The International System of Units (SI Brochure), 9th edition
• NIST: Prefixes for binary multiples
• Weights and Measures Act 1985 (UK), Schedule 1
• King Arthur Baking: Ingredient Weight Chart — ingredient weights for the cooking converters
• U.S. Geological Survey: Water Density

Limits

UnitCheck is a reference tool. Unit factors are exact, but ingredient weights vary with brand and how a cup is filled, and nothing here is medical or dosing advice. For medicines, follow the label or a pharmacist. For anything safety-critical, check the result against the original specification.