Calorie Deficit Calculator

The daily deficit needed to reach a target weight.

American food labels allow a tolerance of up to 20% either way on the calorie figure for a packaged item, and restaurant portions routinely exceed the serving size the label describes. The activity multiplier is the largest single source of error in any daily-calorie estimate.

Calorie guidance in America is built from resting metabolic rate — the Mifflin-St Jeor equation is the current standard — multiplied by an activity factor to reach total daily expenditure.

Dietary reference intakes from the National Academies set the protein, fat and carbohydrate ranges most plans work back from, and a pound of body fat is conventionally treated as roughly 3,500 calories.

A deficit is the gap between energy spent and energy eaten, and both sides are estimates rather than measurements. Loss slows as body mass falls because a smaller body spends less, so a fixed deficit produces a curve rather than a straight line.

That 3,500-calorie rule assumes a straight line where the body actually adapts, which is why real weight loss slows as weight comes off.

Energy balance works in both directions. A surplus of a few hundred calories a day compounds into weight gain across a year, and a deficit of the same size produces loss, but sleep, protein intake and training decide how much of that loss is fat rather than muscle.

Calorie Deficit Calculator sits with the rest of the library's health work, and it rests on the same energy-balance arithmetic the way a clinical reference table does. Its answer is only as good as the assumptions standing behind it.

The quantities it works from — current weight, target weight, weeks to reach it and maintenance calories — are the ones a real energy-balance question actually turns on, and each carries a convention of its own, set out below.

Health figures in the United States are read against reference tables published by federal agencies and professional bodies, and those tables describe populations rather than individuals. One measurement is a data point; a series taken the same way over months is information.

Clinical guidance also shifts as evidence accumulates, which is why a range that was standard a decade ago may no longer be quoted.