Look up calories in common foods.
Food composition tables publish energy per 100 grams, so any portion is a simple proportion of that reference figure. The grams-per-100-kcal figure inverts the same number and is the more useful one when you are choosing between foods: 113 grams of boiled potato and 11 grams of olive oil carry the same energy, which is the entire argument for judging foods by energy density rather than by whether they feel healthy. Cooked weights are used where relevant because rice and pasta roughly triple in weight with water.
Portion energy
Portion kcal = kcal per 100 g x portion grams / 100
Reference portion
Grams per 100 kcal = 100 / kcal per 100 g x 100
These figures are population-average estimates for general planning only and are not medical or dietetic advice. Individual energy and nutrient needs vary widely with body composition, activity, medication and health status. Consult a qualified health professional or registered dietitian before making significant changes to what you eat, and especially if you are pregnant or breastfeeding, are under 18, or have a medical condition or a history of disordered eating.
Cooked, for the items where it matters — chicken, rice, potato and salmon are given as prepared. Rice and pasta absorb water and roughly triple in weight, so using a raw figure against a cooked portion would understate the energy by about two thirds.
Cut, fat trim, variety, ripeness and cooking method all move the number, and different national tables use slightly different Atwater factors. Treat any composition figure as accurate to within about 10 percent.