Calorie calculator scanner: getting the portion right

The per-100 g problem

Food databases store everything per 100 g. It is the only figure that survives a portion change and the only basis on which two products can be compared directly.

It is also not a portion anybody eats. So every scan involves a conversion, and the conversion is where things go wrong:

calories in your portion = (calories per 100 g ÷ 100) × grams you are eating

Cereal at 380 kcal per 100 g, in a 60 g bowl: (380 ÷ 100) × 60 = 228 kcal. The box's declared 30 g serving would have told you 114 — exactly half the truth.

The scanner does this for you and shows the weight it assumed, which is the part that matters. A figure with a hidden portion behind it cannot be checked.

Portion sizes people get wrong

Label servings are chosen to make the per-serving figures look reasonable. They are not predictions of your behaviour.

Raw versus cooked, which catches everyone

Foods absorbing or losing water change weight dramatically in cooking, while their calories do not change at all.

• Pasta and rice roughly double to triple in weight. 100 g dry pasta is around 250 g cooked — same 350 calories, spread across 2.5 times the weight.
• Meat loses roughly 25% of its weight to water. 150 g raw chicken is about 110 g cooked, with the same calories.
• Vegetables lose water when roasted, concentrating the figure per gram.

The rule is simply to match states: weigh raw and use a raw entry, or weigh cooked and use a cooked entry. Mixing them is a large error in a predictable direction — weighing cooked pasta against a dry entry roughly doubles your logged calories.

Calculating a recipe

For something you cook regularly, doing this once pays off indefinitely:

1. Scan or search every ingredient at its raw weight, cooking oil included.
2. Add them up for the whole dish.
3. Weigh the finished dish, minus the pan. This is the step people skip and it is what makes the rest accurate.
4. Divide by portions, or better, weigh your serving and take that fraction of the total.

Weighing your serving out of the known total is considerably more accurate than "serves four", because portions are rarely equal and you are not necessarily eating a quarter.

The calculations the scanner does for you

• Any portion weight — type a gram figure and everything rescales live.
• Multiple servings — two portions, half a portion, 1.5 portions.
• Whole pack — for the times it was, in fact, the whole pack.
• kJ and kcal together — for labels that declare only one.
• Salt to sodium — converted at 2.5 where a record carries only salt.

Estimating weight without scales

Scales are better. Failing that, your hand travels with you and scales roughly with your body size:

• Palm — a protein portion, about 100–120 g of cooked meat
• Cupped hand — a carbohydrate portion, about 40 g dry rice or oats
• Fist — a vegetable portion, about 100 g
• Thumb — a fat portion, about 15 g of butter, oil or nut butter

Rough, and much better than guessing. A week of weighing alongside these calibrates them properly, after which you can leave the scales in the cupboard.

Scan something free and change the portion to see how much it moves.