Formula
Portion factor = amount eaten ÷ label serving weight. The calculator multiplies calories, protein, carbohydrate, and fat by that factor.
Example
A 30g serving has 150 calories. If you eat 45g, the factor is 1.5 and the portion has 225 calories. Read the serving-size calculation guide for the full example, whole-package method, and common mistakes.
Use it for the same food, different amount
This works for a packaged food or mixed meal when the original and eaten amounts have the same proportions. It is especially useful when the label lists a serving in both pieces and grams, but your pieces weigh more or less than the stated amount.
If your scale and label use different mass units, convert the weight first with the ounces to grams food calculator. If your label lists values per 100g instead of per serving, use the nutrition per 100g calculator. If the same weighed meal will be reused, the weighed repeat meal calculator can save the per-gram math as an editable Mori note. If the food is coming from a jar or bottle and you have before-and-after container weights, the reverse weighing calculator combines the subtraction with this serving-size math. If you are unsure which label column belongs in either calculator, start with the nutrition-label reading guide.
When the serving is "2 biscuits" and yours are not that size
This is the case the tool handles best, and the one people most often eyeball instead. A pack says a serving is 2 biscuits, 30g. Nothing guarantees that the two you took weigh 30g. Biscuits, slices of bread, chicken thighs and handfuls of nuts all vary within the same pack.
Weigh what you took and use the gram figure, not the count. If the label offers both a count and a weight, the weight is the reliable one; the count is a convenience. Where a label gives only a count and no weight, the piece count is all you have, and the result is rougher than it looks.
Scaling from a whole package
When you eat some of a package rather than a labelled serving, work from the pack total instead. Divide what you ate by the total net weight to get the fraction, then apply it to the per-pack figures.
Two things trip this up. The net weight on the front is the food, not the food plus packaging, so weigh the contents rather than the sealed pack. And for anything packed in liquid, the drained weight and the net weight are different numbers, and the panel does not always say which one its figures describe.
If the label is per 100g rather than per serving or per pack, the nutrition per 100g calculator is the more direct route.
What usually goes wrong
- Scaling calories and leaving protein, carbohydrate and fat at their label values. Every number takes the same factor.
- Weighing with the plate, bowl or wrapper still on the scale.
- Using the serving count when the pieces are visibly not label-sized.
- Applying a per-serving factor to a per-100g column, or the reverse. Take every value from one column.
- Scaling a raw label against a cooked weight. That is a different problem, and the raw to cooked weight calculator handles it.
- Forgetting oil, butter or sauce added after weighing. None of it was on the label of the thing you scaled.
For a portion you repeat often, scale it once and save the result rather than recalculating. The weighed repeat meal calculator keeps it as an editable entry. To compare two foods at equal weight rather than scale one, the calorie density guide explains the comparison.
Do not scale across different food states
A dry serving weight cannot be compared directly with a cooked portion weight. Water gained or lost during cooking changes weight without adding or removing the same proportion of nutrients. Match dry with dry or cooked with cooked.
One factor can also mislead when the ingredient mix changed, such as taking extra rice but less sauce. Edit the ingredient that changed or use the recipe macro calculator.