A nutrition label is a regulated declaration with permitted tolerances, not a measurement. Both halves of that matter.
What rounding is allowed to do
Small quantities may be declared as zero. Values may be rounded to convenient increments. And calorie figures are frequently computed from those already-rounded macro values rather than measured on the finished product.
An item declaring 0 g fat may contain up to ~0.5 g
At 9 kcal/g: ~4.5 kcal undeclared per item
Eight packaged items a day: ~36 kcal
Trivial on its own. But rounding of this kind is not symmetric in practice — it favours the flattering direction — so it accumulates rather than averaging out.
Thirty-six calories a day is about a kilogram a year. Not a crisis, and not nothing, and worth knowing about before you conclude your maintenance calculation is broken.
The bigger version of the same problem
Reformulation.
A manufacturer changes a recipe and updates the label. A crowd-sourced app entry created three years ago does not update. The barcode still matches, the product name still matches, and the figures no longer do.
This was the most common failure we found scanning packets into apps — more common than “no match”, and much harder to notice, because the app returns a confident answer with the right name.
The two-second check
Compare the per-100 g figure on the packet with what the app returned. Not the serving figure — the per-100 g one, which is standardised and hard to misread.
If they disagree, you have caught a stale or wrong entry. We started doing this halfway through testing and it caught every remaining mismatch.
Questions we get asked
Are nutrition labels accurate?
They are accurate within a permitted tolerance, which is not the same as exact. Regulations allow rounding — small amounts of fat or carbohydrate can be declared as zero, and calorie figures may be computed from rounded macro values rather than measured directly. For a single item this is trivial. Across a day built mostly from packaged food it accumulates in one direction.
Should I trust the label or the app?
The label, for packaged food, every time. It is the manufacturer's own declaration about that specific product. An app entry for the same product is a copy of that label made by somebody else at some point, and on crowd-sourced databases it may be a copy of an older formulation. Check the per-100g figure against what the app returned.
Tessa Lindqvist
Contributing writer, nutrition · Stronger by Math
Writes about intake, adherence and what the self-monitoring literature actually supports. Weighs her own food, which is why she is sceptical of estimates — including the good ones.
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