CarbTable practical guides
What does a portion really mean?
A portion is an amount of a particular food. A cup of one food does not necessarily weigh the same as a cup of another.
Start with the food and its preparation
Choose the record that matches what you are eating. Cooked rice, dry rice and a mixed rice dish describe different foods. The original USDA description on every CarbTable page helps you check the match.
The weight makes a measure useful
A household measure becomes usable for calculation when it has a documented weight for that food. CarbTable only offers household measures found in the selected source record. A reference portion is a measurement, not a recommended amount to eat.
Use your own amount
Enter a number and a measure in the portion calculator. If you have a kitchen scale, use the edible weight in grams. We calculate each nutrient as its value per 100 g multiplied by your weight and divided by 100.
Compare on a common basis
Use 100 g to compare the same mass of different foods. Use the custom-portion mode to compare the amounts you plan to eat. Both are useful, but they answer different questions.
A worked example from the catalogue
Rice, white, cooked reports 28.2 g of carbohydrate per 100 g. Its documented 1 cup weighs 158 g.
Calculation: 28.2 × 158 ÷ 100 = 44.6 g of carbohydrate, rounded for display. A weighed 80 g portion uses the same profile: 28.2 × 80 ÷ 100 = 22.6 g.
Check the original USDA record. Values here come from the same catalogue records used in the calculators.
Check the source
This guide explains how to use the reference data. Read the USDA documentation and CarbTable’s methodology.
Prepared by CarbTable, October 26, 2026. Editorial policy and review status. General information, not individualized dietary or medical advice.