A scale showing whole grams cannot resolve every tiny food portion
Separate the displayed step from accuracy, and see why a rounding-only uncertainty matters more for a tiny portion.
A kitchen scale displays 2 g of a food ingredient. Writing 2.000 g in your notebook would add digits the scale never supplied. Before using a tiny portion in a nutrition calculation, record the unit and the smallest step the display actually shows.
A display step is not an accuracy certificate
NIST's discussion of scale applications distinguishes the displayed scale division, d, from the verification division, e. It also explains why weighing light loads can produce relatively large errors. The article concerns commercial Class II scales; its legal tolerances and recommended minimum loads do not establish how a reader's household scale performs.
For your instrument, look for its own instructions and specifications. Display resolution, repeatability and calibration are different pieces of evidence. A stable number on the screen does not settle all three.
Work through an ideal rounding example
Suppose an ideal instrument rounds to the nearest whole gram, with exact half values rounded upward. Assume no other error for this mathematical exercise. A display of 2 g could then represent a quantity from 1.5 g up to, but not including, 2.5 g.
| Displayed amount | Half of the 1 g display step | Half-step relative to display |
|---|---|---|
| 2 g | 0.5 g | 0.5 ÷ 2 × 100 = 25% |
| 100 g | 0.5 g | 0.5 ÷ 100 × 100 = 0.5% |
The same display step occupies a much larger share of the tiny reading. These are rounding-only illustrations, not error limits certified for a real scale. Different rounding behavior, calibration error, movement or other effects can change the actual uncertainty.
Subtraction does not create missing precision
If you subtract two readings that were each rounded independently, their rounding contributions can act in opposite directions. Under this ideal model, the difference can have a rounding contribution bounded by 1 g, rather than the 0.5 g associated with one reading. Do not report a small difference to extra decimal places merely because subtraction produced a neat result.
Record the displayed values and the method, keeping any modeled interval in a separate field. Repeating an unchanged reading is not proof of accuracy. If a task needs better resolution, use an appropriate measurement method and its documented limitations rather than inventing hidden digits.
This guide concerns ordinary food notes, not medication dosing or a commercial weighing inspection. Once you have a defensible portion record, try food-label reading with Ingredi AI through the official Apple listing or official Google Play listing. The app links do not imply scale calibration or more precise nutrient measurements than the underlying evidence supports.