A standard comparison basis. It is the whole packet only when the packet contains exactly that amount.
Packaged food guide
Per 100g vs per serving: how to log packaged food correctly.
A nutrition label may show values per 100g, per serving, and sometimes per pack. The useful number for a food log is the value for the amount that was actually eaten—not whichever number happens to be largest on the label.
Last updated: August 26, 2026
The quick formula
When a label gives calories per 100g, scale that number to the weight eaten:
Calories eaten = calories per 100g × grams eaten ÷ 100
If a product contains 240 kcal per 100g and the portion weighs 35g, the calculation is 240 × 35 ÷ 100 = 84 kcal. If the whole packet weighs 180g, the full packet is 240 × 180 ÷ 100 = 432 kcal.
The same method works for protein, carbohydrate, fat, fibre, and other nutrients listed per 100g. For drinks labelled per 100ml, use millilitres instead of grams.
Read the reference amount before the nutrition number
The manufacturer's stated serving size. Check its grams, millilitres, slices, biscuits, or other stated quantity.
The full retail pack, bottle, tub, or box. Confirm whether the pack contains one serving or several.
The portion that belongs in the log. It may be smaller than a serving, more than one serving, or shared across profiles.
Current UK guidance says mandatory nutrition values are given per 100g or 100ml, while packaged food generally shows a net quantity in grams, kilograms, millilitres, or litres. See the official GOV.UK packaging and nutrition-labelling guidance and net quantity guidance.
Do not mix “as sold” and “as prepared” values
Some labels describe the food as it comes out of the packet. Others state values for the product after it has been prepared according to the instructions. Read that line before combining a dry pack weight with a prepared-food nutrition value.
Adding plain water changes weight or volume but does not add calories. Adding milk, butter, oil, sauce, or another ingredient can change both the final weight and nutrition. When the label supplies a clear per-serving or per-pack prepared value, that can be easier to use than rebuilding the preparation from mismatched bases.
The practical check: use one basis consistently—package as sold, stated serving, or stated prepared amount—and record any added ingredients separately when they are not included in the label value.
The whole packet is not automatically one serving
Look for the net quantity and the number of servings. A 42g wrapper may be one retail unit. A 180g box may contain several portions. A 500ml bottle may list values per 100ml and also show a separate per-bottle figure.
If the packet is shared, start with the reviewed total and assign the amount each person actually ate. Equal portions are useful only when the packet really was divided equally.
- Log the whole pack only when the whole pack was eaten.
- Log half a pack when half was eaten, even if the label calls something else one serving.
- Keep an unopened or uneaten remainder out of today's log.
- If several household profiles shared it, make the portions reconcile with the amount consumed.
A safer package-photo and barcode workflow
- Capture the identity. Include the product name and exact variant, not just a generic food category.
- Capture the quantity. Keep the net weight or volume visible, including multipack counts such as 4 × 25g.
- Capture the nutrition basis. Check whether the values are per 100g, per 100ml, per serving, per pack, as sold, or as prepared.
- Review the draft. Compare any scan or database result with the label in front of you. Product sizes and variants can differ.
- Set the portion eaten. Adjust the weight, serving description, calories, and nutrients before saving.
A readable barcode can help identify a product, but it does not replace the visible wrapper. A barcode database may hold a different pack size or older label. When a result cannot be verified, use the visible label and editable fields rather than inventing precision.
Where FamilyMacro helps
FamilyMacro can use visible package, barcode, or nutrition-label evidence to create an editable packaged-food draft. For a single visible retail package, the workflow aims to scale readable per-100g or per-100ml values to the visible package quantity instead of treating the reference value as the final portion.
You still review the product name, serving description, estimated weight, calories, macros, and household profile portions before anything is saved. If the product match is not verified, FamilyMacro can ask for a barcode or nutrition-label photo and keeps a manual wrapper-entry fallback.
For a broader explanation of meal images, see AI photo food tracking for families. For cooked dishes shared across several people, use the shared family meal guide instead.
FamilyMacro provides reviewable nutrition estimates for everyday tracking. It does not guarantee perfect label recognition or nutrition accuracy and is not medical advice, diagnosis, treatment, nutrition therapy, or a child diet plan.
Common packaged-food logging mistakes
- Entering the per-100g number as the full pack when the pack is larger or smaller than 100g.
- Using a suggested serving when the amount eaten was different.
- Mixing a dry pack weight with nutrition stated for the prepared product.
- Using the right product name but the wrong flavour, pack size, or formulation.
- Trusting a barcode or photo result without checking the visible label.
- Logging the whole multipack when only one individually wrapped item was eaten.
Packaged food label FAQ
Does per 100g mean the whole packet?
Only when the whole packet contains exactly 100g. Otherwise multiply the per-100g value by the amount eaten in grams and divide by 100.
What is the difference between per serving and per 100g?
Per 100g is a standard reference amount. Per serving uses the serving size stated on that label. Neither automatically means the amount you actually ate.
How do I calculate calories for 35g from a per-100g label?
Multiply the per-100g calories by 35 and divide by 100. For example, 240 kcal per 100g becomes 84 kcal for 35g.
Should I trust a scan more than the package label?
No. Treat a barcode or photo result as an editable draft and compare it with the visible label, product variant, reference basis, and net quantity before saving.
What if several family members share one packet?
Start with the reviewed packet total, then assign the portion each selected profile actually ate. The profile portions should reconcile with the amount consumed rather than assuming an equal split.
Try packaged-food review in FamilyMacro
Install FamilyMacro on Android, capture the product and label evidence, then review the package quantity and household portions before saving.
Google Play shows the exact local price and trial terms. Eligible new subscribers can start a 14-day trial.