A food weight in ounces can be compared with a weight in grams once you use the same mass unit. A fluid ounce is a volume measurement and needs a different treatment. Look for “fl oz” before converting anything.
Put both foods on one basis
For ordinary avoirdupois mass, one ounce is about 28.35 g. Imagine Food A lists 7 g protein per 1 oz, while Food B lists 20 g per 100 g. Food A’s approximate amount per 100 g is 7 × 100 ÷ 28.35 = 24.7 g. That is higher than Food B on an equal-weight basis, even though its printed serving number is smaller.
Keep the original portion useful
Equal weight is useful for comparing concentration. It is not necessarily the amount you will eat. If you eat one ounce of A and 100 g of B, the actual portions give 7 g and 20 g protein respectively. Keep both answers in your notes rather than letting normalization erase the order size.
Do not borrow the calculation for a drink
A bottle listed in fluid ounces describes volume. You cannot turn that into grams with the 28.35 conversion. Use the product’s own metric volume or an explicit product-specific mass measurement. A unit abbreviation can change the entire calculation.
Keep a sensible number of decimal places. The converted protein density is an estimate based on rounded printed inputs, not a laboratory result to four decimal places. NIST’s unit references support the distinction between mass and volume. This worked comparison is invented so it can illustrate the two competing questions clearly.
Sources and example boundaries
- NIST: units of mass
Gram, milligram and kilogram relationships.
- NIST: units of volume
Volume quantities and their distinction from mass.
- NIST: unit conversion factors
Avoirdupois ounce and pound conversion factors and thermochemical calorie-to-joule conversion; rounded factors are identified as approximations.
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.