Keep working precision through the calculation and round the final display. Rounding every intermediate value can create a total that differs from the same calculation performed directly. More digits should still not imply that the source nutrition is exact.
A repeating-decimal example
Imagine a fictional 900-calorie batch split into seven equal portions. The calculated value is about 128.571 calories each. Rounding each to 129 and then multiplying by seven gives 903. That does not mean the batch gained three calories.
Separate display from arithmetic
The interface can display approximately 129 calories per portion while retaining the unrounded division internally. If it reports the whole batch, it should use the original 900 rather than adding rounded display values. The same principle applies to protein, price ratios and unit conversions.
Know the input’s limits
The source label may already be rounded, and the recipe may have uneven nutrient distribution. Carrying more digits through arithmetic prevents an additional calculation error; it does not remove those uncertainties. A sensible display can use “about” where a rounded estimate is being presented.
The FDA serving and label guidance supplies the input context. The seven-portion example is invented arithmetic. When two displayed totals do not reconcile exactly, first check intermediate rounding and serving definitions before assuming the source is wrong. A reliable calculator preserves the difference between a clean display and the quantities used to calculate it.
Sources and example boundaries
- FDA: using a Nutrition Facts label
Label quantities refer to the stated serving; nutrients are not interchangeable.
All food figures and prices in the worked example are invented to explain the method. They are not restaurant nutrition, tested recipes, shopping prices or personal dietary advice. Use the current label for the product you actually have.