Find the serving basis first: per serving, per package, per 100g, or per 100ml. Compare that basis with the amount you ate. If the amounts differ, calculate a multiplier and apply it to calories, protein, carbohydrate, and fat from the same column.
Recent tracking discussions show why this order matters. People often know the package values but get stuck between the label's reference amount and the portion an app asks them to enter. A May 2026 r/bevelhealth discussion focused on exactly that problem for European labels stated per 100g. Another recent r/nutrition thread started with macro values that could not possibly fit the listed calories, raising the possibility of a copied value or serving-size error.
Those discussions identify workflow problems, not nutrition rules. This guide uses current guidance from the U.S. Food and Drug Administration, the NHS, and Food Standards Australia New Zealand for the label facts.
Use this reading order
- Identify the column. Is the nutrition stated per serving, per package, per 100g, or per 100ml?
- Read the reference amount. Note the household measure and metric amount, such as 2 crackers (30g).
- Record the amount eaten. Use the same unit and the same food state as the label.
- Calculate the portion factor. Divide the amount eaten by the label reference amount.
- Scale one complete set of values. Multiply calories, protein, carbohydrate, and fat from the same column by the factor.
- Check the result before saving. Look for a unit mix-up, an impossible macro value, or a database entry that does not match the package.
This sequence is intentionally mechanical. It prevents a common mistake: copying the calories from a per-serving column and the macros from a per-100g column because the numbers appear next to each other.
Serving size and portion eaten are different
On a U.S. Nutrition Facts label, the serving size reflects the amount people typically consume. FDA states that it is not a recommendation of how much to eat. Nutrition is usually shown for one serving, though some products also display a second column for the whole package.
This page is about reading one label. If you are holding two products and deciding between them, comparing two nutrition labels covers why per-serving figures cannot be compared across brands.
Your portion is the amount you actually ate. It may be smaller, larger, or exactly the same as the label serving. If a label shows 30g and you eat 45g, your portion is 1.5 label servings. That factor applies to every printed nutrient in that column.
Read the number of servings per container too. A small bag, bottle, or tray can contain more than one serving even when it looks like a single item. If the package has a dual-column label, choose either the per-serving or per-package column and stay with it for the whole calculation.
Per serving and per 100g labels answer different questions
A per-serving column describes the serving printed on that package. It is convenient when your portion matches the stated amount or when the product comes in repeatable pieces.
A per-100g or per-100ml column gives a standard comparison basis. Current NHS guidance says UK nutrition information is provided per 100g or 100ml and may also be shown per portion. Food Standards Australia New Zealand requires a standard panel with average quantities per serving and per 100g, or per 100ml for liquids.
- Use per serving when you know how many labeled servings you ate.
- Use per 100g when you weighed a solid portion or want to compare products on the same basis.
- Use per 100ml when the label and your measured liquid amount are both in millilitres.
- Use per package only when the column explicitly describes the full package.
Do not convert grams to millilitres by assuming they are equal. Water is close to 1g per ml under ordinary conditions, but foods and drinks have different densities. Use a conversion supplied for that product or keep the printed unit.
Two examples cover most packaged foods
Example 1: a U.S. per-serving label
The label serving is 55g and lists 230 calories, 3g protein, 37g carbohydrate, and 8g fat. You eat 82.5g.
Portion factor = 82.5 ÷ 55 = 1.5. The portion has 345 calories, 4.5g protein, 55.5g carbohydrate, and 12g fat before normal display rounding. The serving size and portion calculator performs this calculation without changing the label basis.
Example 2: a UK, Australian, or European per-100g label
The label lists 240 calories, 12g protein, 30g carbohydrate, and 8g fat per 100g. You eat 65g.
Multiplier = 65 ÷ 100 = 0.65. The portion has 156 calories, 7.8g protein, 19.5g carbohydrate, and 5.2g fat. Use the nutrition per 100g calculator when this is the column you have.
Keep more decimal places during the calculation than you plan to display. Rounding the multiplier too early can create avoidable differences, especially across several nutrients.
Calories, macros, and Daily Value are separate fields
For macro tracking, copy the gram amounts for protein, total carbohydrate, and total fat. The percent Daily Value beside a nutrient is not the number of grams in the food and is not a personalized macro target. On U.S. labels, FDA uses a 2,000-calorie diet as a general reference for Daily Values.
Total carbohydrate can include dietary fiber and sugars. Do not add those subcomponents to total carbohydrate again. If fiber is the number you want to follow, read the fiber tracking guide and compare the two general methods with the daily fiber intake calculator. If you calculate net carbs, the formula depends on the label format and the carbohydrate ingredients involved; use the separate net-carb guide instead of subtracting every line automatically.
Calories may not equal protein × 4 + carbohydrate × 4 + fat × 9 exactly. Displayed values can be rounded separately, and food-label energy calculations can use factors that the simple check does not reproduce. The calorie and macro mismatch guide explains how to distinguish ordinary differences from an obvious serving or data-entry problem.
Use the package for the exact packaged product
If you have the current package in hand, its label is the direct reference for that product. A food-tracking database can contain older labels, a different regional recipe, a user-created entry, or a serving unit that was copied incorrectly.
Match the full product name, flavor, serving basis, metric weight, calories, and macros before reusing an entry. If the normalized values still disagree, use the food database entry checker and review the database checking workflow.
For food without a package label, use a documented reference that matches the food and preparation state. Raw, cooked, drained, and prepared values are not interchangeable. Natural foods also vary, so the result remains a practical journal estimate.
Choose the calculator from the label basis
- Serving size and portion calculator: the label gives nutrition for one gram-weight serving and you ate a different weight.
- Nutrition per 100g calculator: the label gives values per 100g and you know the grams eaten.
- kJ to Calories calculator: the label gives energy in kilojoules and you want kilocalories. The kilojoules to Calories guide covers the 4.184 factor, the kcal naming convention, and why a label's two energy figures rarely divide out exactly.
- Macro calorie calculator: you want to check the simple 4/4/9 energy estimate from macro grams without replacing the printed label value.
- Food database entry checker: an app entry and a package label disagree after you put them on the same basis.
If you want an app to read the label with the camera, the nutrition label scanner app guide explains what to check before trusting OCR results.
If the calculation is for a homemade dish rather than one unchanged packaged food, total the ingredients and divide the finished recipe. A single serving multiplier cannot account for taking more of one ingredient and less of another.
Common nutrition-label questions
Is serving size the amount I should eat?
No. On U.S. labels it reflects an amount customarily consumed, not a recommendation for an individual.
Should I enter per-serving or per-100g values in my tracker?
Either can work if the tracker understands the same basis. Use one complete label column and record the portion in the matching unit.
What if I eat the whole package?
Use the full-package column when the label provides one. Otherwise multiply the per-serving values by the number of servings you ate.
Why does the barcode entry differ from my package?
The database entry may describe an older or regional label, a different product variation, or an incorrect serving. For the exact packaged product you have, compare the app entry with the current package.
Can I use grams when the label is per 100ml?
Only with a reliable density conversion for that specific food or drink. Grams and millilitres measure different things.
This guide uses the FDA's serving-size guidance, the NHS food-label guide, and Food Standards Australia New Zealand's nutrition information panel guidance. Community discussions informed the workflow examples but are not used as nutrition authorities.