Why conversion factors are exact - and why some are not
Unit conversions split into two families. The first family is defined by international agreement and is therefore exact: one international pound is defined as 0.45359237 kilograms, a value fixed by the 1959 international yard and pound agreement among English-speaking nations, and one ounce is exactly 28.349523125 grams. These definitions are recorded in the references published by the US National Institute of Standards and Technology. When a calculator converts between these units, there is no rounding in the factor itself - the only rounding is in how many decimal places the result displays.
The second family is conventional and approximate: a "cup" is 236.588 mL only in US customary volume, a "tablespoon" is about 14.8 mL by US convention but 15 mL in metric-defined kitchens and 20 mL in Australia, and the gram weight of a cup of any given food depends on that food's density. Conversions in this family carry real uncertainty, and honest calculators label them as approximate.
The exact family: kilograms, stones, pounds, ounces
The chain runs: 1 stone = 14 pounds exactly; 1 pound = 16 ounces exactly; 1 pound = 0.45359237 kg exactly; 1 ounce = 28.349523125 g exactly. Everything else follows by multiplication. A 70 kg person weighs 70 ÷ 0.45359237 = 154.32 lb, which is 154.32 ÷ 14 = 11.02 stone, or 11 stone and 0.3 pounds once the fractional stone is converted back. Our kg to stone conversion calculator carries the full precision and displays the pounds remainder separately, because "11.02 stone" is a less useful format on a form than "11 st 0.3 lb".
The same exactness applies to mass-per-volume food measures once the weight is in grams: an ounce of cheese is 28.35 g regardless of the cheese, so per-ounce food calculators can scale any per-100 g database value with a single multiplication, 28.35 ÷ 100.
The approximate family: cups, tablespoons, handfuls
Volume measures behave differently. One US cup is 236.588 mL, but a cup of shredded cabbage weighs far less than a cup of water because cabbage is mostly air. Published gram equivalents for "1 cup" of a food are averages measured for that food, and they vary by how the food was cut or packed. Our cabbage calories per cup calculator therefore uses the USDA's gram weight for a cup of shredded cabbage rather than treating the cup as 236.6 g of anything.
Handfuls, spoons, and slices are even looser: a "tablespoon" of nut butter can be a level 16 g or a heaped 25 g depending on the pour. Where a tool accepts such units, the page states the gram weight it assumes, so you can see the assumption rather than guess at it.
Why our nutrition tools default to grams
FoodData Central reports nutrients per 100 g. Converting your portion to grams first means the rest of the calculation is exact scaling with no second conversion in the chain. Every intermediate step that stays in grams removes one source of error. The unit converters on this site exist for the opposite direction - translating a recipe written in cups and ounces into the gram weights the nutrition calculators want.
A worked conversion chain
A recipe calls for 8 oz of walnuts. Step one: 8 × 28.349523125 = 226.8 g. Step two: FoodData Central lists about 654 kcal per 100 g of walnuts, so 226.8 × 654 ÷ 100 = 1,483 kcal for the batch. Both steps are exact arithmetic once the gram weight is established; the kg to stone converter and the per-gram food calculators implement each half. Try the same chain with our calories per ounce calculator to check the result.
Summary
Agreement-defined units (kg, lb, oz, g, stone) convert exactly; volume and household measures (cup, tablespoon, handful) convert through food-specific gram weights that are averages. Our tools state which family each input belongs to and which conversion factor is in force. Data sources are documented on the methodology page, and the disclaimer covers the standing limits of these estimates.
Temperature: exact, but offset
Temperature is the one exact family that is not a pure multiplication. Celsius to Fahrenheit is °F = °C × 9/5 + 32, and the reverse is °C = (°F − 32) × 5/9. The 9/5 factor is exact; the 32 offset exists because the two scales chose different zero points, not because the units differ in size by a percentage. Forgetting the offset is the classic error, and it is why conversion forms show both the formula and the result rather than only the result. Display precision follows the same principle across all our tools: exact factors are shown with the digits that matter, approximate household conversions are shown with one decimal place at most, and every page states its assumption so a reader can recompute by hand.