Defined units versus measured units
Since the 1959 international yard and pound agreement — documented in NIST SP 811 — the inch is exactly 2.54 centimeters and the avoirdupois pound is exactly 0.45359237 kilograms. These are definitions, not measurements — they have no uncertainty. The meter itself is defined through the speed of light, and the kilogram through the Planck constant, both fixed values agreed by the BIPM.
Measured quantities are different. The density of flour, the weight of a cup of chopped vegetables, and the volume of a “large” egg all vary by sample and method. A conversion involving a measured density carries that uncertainty with it, no matter how many digits the website prints.
Why two sites show different digits
Websites round at different steps. One may multiply by 0.621371 and display three decimals; another may multiply by 0.62 and display the same number of decimals. Both are “correct” to their own precision, but they diverge in the third digit. Some sites also reuse rounded tables from other sites rather than computing from the definition.
A good conversion page states its factor and its rounding rule. Ours computes from the full-precision definition and rounds only the displayed result, which is why the pre-calculated table and the live converter always agree.
When the trailing digits matter
For everyday cooking, travel, and shopping, two or three significant digits are plenty. A difference in the fourth digit corresponds to less than a gram in a kilogram or a millimeter in a meter. Spending time on it is usually wasted effort.
Precision matters in engineering, medicine, aviation, and any workflow where small errors accumulate over many operations. In those cases, the right move is to use the defined factor and carry full precision through the calculation, then round at the very end.
The special case of ingredient density
Cooking conversions often look exact because the numbers are precise, but the underlying density is a kitchen convention. A cup of flour is 120 g under one measuring method and 150 g under another. USDA publishes reference weights, but they describe a specific method. When a recipe and a table disagree, the recipe's stated convention wins for that recipe.
This is why our ingredient pages name the measuring convention next to every factor. Stating the assumption is more useful than pretending there is a single true number.
How to check a conversion you do not trust
Find the official definition — NIST publishes SI conversion factors, and BIPM publishes the SI Brochure. Multiply by hand or with a calculator at full precision. Then compare with the site's stated factor and rounding rule. If the site does not state either, treat the result as an estimate.
For cooking, the more useful check is whether the page states its cup size and measuring method. A precise number built on an unstated assumption is less trustworthy than a rounded number with clearly stated conditions.
A worked example: kilograms to pounds
Take 70 kg. The exact factor is 2.20462262 pounds per kilogram, so the multiplication gives 154.3235834 lb. Rounded to two decimals, that is 154.32 lb; rounded to a whole number, 154 lb. The popular shortcut of multiplying by 2.2 gives 154 lb — a difference of about 0.3 lb, or 0.2 percent.
That gap is harmless for body weight and meaningful in shipping or dosing. The lesson is not that the shortcut is wrong, but that the precision you need should decide the method. An exact factor carried to the end and rounded once is the cleanest approach for anything that matters.