After scaling a label, work backward once. If the displayed result cannot recover the original value under the same assumptions, check the serving basis, units and rounding. This is a calculation audit, not independent verification of the label.
A forward calculation
Imagine a fictional product with 180 calories per 60 g. You enter a 90 g portion. The multiplier is 1.5, so the expected result is 270 calories. Reverse-check by dividing 270 by 1.5: it returns 180.
Find a common mistake
If the tool result is 162, someone may have multiplied by 90 ÷ 100 instead of 90 ÷ 60. The 100 g basis was never in the source. A familiar-looking per-100 figure is not a substitute for reading the actual panel.
Allow for display precision
A result rounded to whole calories may not reverse exactly. Inspect the unrounded operation rather than rejecting a sound method over a tiny display difference. Conversely, a large difference should not be dismissed as rounding.
FDA’s serving source supports using the stated label amount. All figures here are invented. The inverse check verifies that a chosen transformation is internally consistent; it cannot prove that the original product label is current or that a mixed portion was uniform. Save the source basis alongside the output so the calculation remains auditable.
Sources and example boundaries
- FDA: serving information
Serving sizes and household/metric units on U.S. labels.
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.