Methodology
Where the numbers come from, how they are rounded, and what they cannot tell you.
Where the factors come from
Every factor used on this site is defined, not measured by us. The metric units follow the International System of Units as published by the Bureau International des Poids et Mesures. The relation between the two systems follows the international yard and pound agreement of 1959, which fixes one inch at exactly 25.4 millimetres and one pound at exactly 0.45359237 kilograms. Every length and weight conversion on this site is derived from those two definitions rather than from a rounded factor.
Temperature is a scale conversion rather than a ratio: degrees Fahrenheit and degrees Celsius share no zero, which is why the formula contains an offset of 32 and a factor of 9/5. Kelvin shares the Celsius interval but starts at absolute zero, 273.15 degrees below the freezing point of water.
Cooking measurements are the exception
Volume in the kitchen is not standardised worldwide. A US customary cup is 236.588 millilitres, a US legal cup 240, a metric cup 250, and an imperial cup 284.131. A US tablespoon is 14.787 millilitres against 20 in Australia. We state which cup a page uses, because a recipe that silently mixes two of them fails.
Weight to volume is worse: it depends on the ingredient and on how it was packed. One cup of flour weighs between 120 and 145 grams depending on whether it was scooped or spooned and levelled. Where a page converts an ingredient by weight, it names the density used, and a kitchen scale remains more reliable than any cup.
Rounding
The calculators compute at full precision and round only for display. A result is shown to the precision that the input justifies: a value entered to the nearest unit is not given back to four decimals, because that would suggest an accuracy the input never had. Where a factor is exact by definition, such as 25.4 millimetres to the inch, no rounding error is introduced at all.
What the converters do not do
They do not convert between quantities of different kinds. A volume becomes a mass only through a density, and a density belongs to a substance at a temperature, not to a unit. They do not handle dosage, structural load or any figure where an error has a physical consequence: those belong to the standard or the professional that governs them.
Nothing here is professional, medical or engineering advice. A measurement that matters should be checked against the document that defines it, and where a figure on this site and a standard disagree, the standard prevails.
Updates and corrections
The tables are reviewed when a standard changes, which is rare, and when a reader reports an error, which is not. Each page carries the date of its last real change rather than today's date, so that an unchanged page does not pass for a fresh one.
If a figure looks wrong, write to contact@howmanyof.com naming the page and, where possible, the standard you believe it contradicts. We check it against the source, correct it if the error is confirmed, and carry the correction across to every page that used the same factor.
Official sources
Unit definitions: Bureau International des Poids et Mesures. US customary units and their metric equivalents: NIST Office of Weights and Measures. Nutrition and ingredient densities: USDA FoodData Central.