Recipe and Portion Calculation

Foundation · 13 min

The four-step method

Calculating a home-cooked portion takes four steps:

1. Weigh each ingredient raw or dry and find its energy value.
2. Add up the energy of all the ingredients: this is the pot's total energy.
3. After cooking, weigh the total (remember to subtract the weight of the pot).
4. Weigh your portion and set up the ratio: portion energy = total energy × (portion grams ÷ total grams).

Whether water evaporates or is added during cooking, the method still works, because the total energy comes from the ingredients, not from the water.

The first calculation takes about ten to fifteen minutes; but that time is spent once. As long as you cook the same recipe in similar proportions, the result is reusable; if a single ingredient changes, updating only its share is enough.

A worked example: lentil soup

Say the pot received: 250 grams of dry red lentils (about 850 kcal), 30 grams of olive oil (about 270 kcal), and 150 grams of onion and carrot (about 60 kcal). Total: about 1180 kcal. After cooking, the soup weighed 2000 grams with its water. If your bowl holds 300 grams of soup: 1180 × (300 ÷ 2000) = 177, so roughly 175-180 kcal. Notice that the amount of water did not change the total energy; it only changed how many grams the energy is spread across.

The same maths is even easier in solid recipes like cakes: evaporation is small, and the share falls out quickly from the slice's weight.

Where the calculation drifts

This method is an estimate, and it can drift in three main places. First, ingredient values are averages: the sugar in a tomato or the fat share of a cut of meat varies from sample to sample. Second, in methods like frying, food absorbs part of the cooking oil; the oil left in the pan should be subtracted from the total, but how much was absorbed cannot be known precisely. Third, weighing errors accumulate. That is why it is more realistic to treat the result as a range rather than a single number (160-200 kcal in our example, say).

The most effective way to shrink the drift is to be careful with the ingredients that carry most of the energy: weigh the oil, the grain and the legumes; eyeballing the parsley and the spices changes the result almost not at all.

When is it worth the effort

Calculating every meal with this rigour is neither necessary nor sustainable for most people. The method's real value is calculating a few dishes you cook often, just once, and building your own reference list: one ladle of my soup is about this much, one slice of my cake about that much. Once built, the list is used effortlessly, without scales. The aim is informed estimation, not obsessive tracking; if you notice distress around your eating, it is wiser to drop these calculations and consult a professional.

The reference list works outside the home too: someone who knows their own ladle no longer makes blind guesses when they see a similar dish at another table, but uses a calibrated eye.