Serving Size vs 100 g: How to Compare Food Nutrients
Published by Renal Diet · · Food-data explanation, not clinical review
To calculate a nutrient amount for a portion, multiply its value per 100 g by the portion's weight in grams, then divide by 100. Compare the same basis before deciding that one food has more or less of a nutrient.
The calculation
Nutrient in a portion = nutrient per 100 g × portion grams ÷ 100. A 50 g portion contains half the reference amount per 100 g. A 150 g portion contains one and a half times that amount. These are arithmetic examples, not recommended portions.
A real record you can check
Our Banana, raw record uses the USDA portion “1 medium (7" to 7-7/8" long)”, weighing 118 g. The table below puts its per-serving and per-100-g potassium side by side. The source link identifies the exact record.
Scroll the table sideways to see every nutrient and its unit.
| Food | Serving (USDA) | Potassium (mg) | Potassium per 100 g (mg) | USDA FDC id |
|---|---|---|---|---|
| Banana, raw | 1 medium (7" to 7-7/8" long) (118 g) | 422 | 358 | 173944 |
Open the food page for phosphorus, sodium, protein and the other available household portions. Use the gram weight of the portion you actually measure. A cup, piece or slice is not a universal weight across foods.
Why rounded values may not multiply exactly
This site calculates from the source value before rounding for display. Milligrams and calories display as whole numbers; protein displays to one decimal place. Multiplying an already rounded display value can therefore differ slightly from calculating directly from the underlying data. Keep the original precision during the calculation and round at the end.
A short comparison checklist
- Match the preparation: raw, cooked, drained or another description.
- Check the unit: milligrams and grams are different units.
- Choose a basis: the same gram weight for composition comparisons, or the actual portion for the amount you plan to log.
- Read the FDC identifier so you can return to the same record.
- Check a packaged food's label instead of assuming a generic entry exactly matches the product.
The potassium comparison table uses the displayed USDA portion, so two rows may have different gram weights. Its per-100-g column helps separate portion size from concentration. Neither comparison sets your personal food allowance.
Source and scope
The values come from USDA FoodData Central, a public-domain reference database. The arithmetic and display rules above describe this website. Personal nutrient targets belong with your nephrologist or dietitian.