The formula
kcal of an ingredient = grams × kcal per 100 g ÷ 100
Where it comes from
There is no food table here, and that is deliberate. Nutrition tables vary by country, by variety and by year, and an unsourced figure on a page people use to decide what to eat is worse than no page at all. The per-100-gram figures are printed on the packet of what you actually bought, and they are that product: brown rice and white rice do not match, and two olive oils need not either.
How to work it out by hand
- Weigh each ingredient raw, before cooking it
- Copy the calories per hundred grams off the packet
- Multiply the grams by the calories and divide by a hundred
- Add it all up and divide by the servings it makes
What is worth knowing
Two details change the answer. First: ingredients are weighed raw, because the per-100-gram figure on the packet is for the raw product; cooked rice weighs almost three times as much and its calories per hundred grams are a third. Second: what weighs most is almost never what contributes most. Thirty grams of oil hold more calories than three hundred of courgette, which is why energy density — calories per hundred grams of the finished dish — tells you more about a dish than its total.