Use grams with grams and milliliters with milliliters. If one label uses weight and the other uses volume, you need a product-specific conversion before comparing them. Changing the unit name in a calculator does not convert the food.
A gram measures mass; a milliliter measures volume. NIST’s mass definitions and volume guide explain the distinction. Foods need not have the same mass for the same volume.
Choose the comparison you can support
| Label A | Label B | What to do |
|---|---|---|
| Per 100 g | Per 40 g | Scale both to your chosen weight. |
| Per 100 mL | Per 250 mL | Scale both to your chosen volume. |
| Per 100 g | Per 100 mL | Find a weight-to-volume conversion for the actual product, or compare separately measured portions. |
| Per tablespoon (15 g) | Per 15 mL | Do not assume the two portions weigh the same. |
A dressing example
These invented figures illustrate the arithmetic, not the nutrition of a real dressing. Suppose a product label explicitly gives 30 mL as 36 g and lists 120 calories for that serving. You want to use 20 g of that same product.
20 ÷ 36 × 120 = about 67 calories. The label’s own weight is enough to scale the portion. You do not need to turn 20 g into mL first. If you did need the volume, this example gives 20 ÷ 36 × 30 = about 16.7 mL.
That conversion belongs to this hypothetical product. Do not reuse it for another dressing, oil or yogurt. If a label does not provide both units, measure the portion in the unit it uses or ask the manufacturer.
Keep the unit in your notes
Write “20 g dressing” or “20 mL dressing,” rather than just “20 dressing.” The food comparison tool uses grams. Use it when both label serving weights are available in grams. For mL-only labels, do the volume calculation above or get a product-specific gram conversion; never put a milliliter figure into a field labeled grams. Comparing equal weights answers a different question from comparing your two chosen portions.
Compare two food labels once the units match.
Sources and scope
These sources support the factual explanations above. Worked label examples are illustrative where stated. Restaurant comparisons use the listed U.S. portions; availability and local recipes may differ.