Calorie Deficit Calculator

Imagine your body as a factory running 24 hours a day. Even when nothing is happening on the floor — no lifting, no moving, no thinking hard — the lights stay on, the machines idle, and the heating runs. That baseline energy bill is your Basal Metabolic Rate. Add in all the movement you actually do in a day, and you get your total energy expenditure. Eat exactly that much and your weight stays flat. Eat less, and the factory draws from its reserves — body fat.

This calculator uses the Mifflin-St Jeor equation, which estimates your BMR from your weight, height, age, and sex, then multiplies it by an activity factor to get your TDEE. The formula has been shown to outperform older equations like Harris-Benedict in accuracy for most non-athlete adults. Your daily calorie target is then your TDEE minus the deficit required to lose at your chosen rate.

The 3,500-calories-per-pound figure is a planning estimate, not a biological law. It holds well for the first several weeks of a deficit, but as you lose weight your TDEE drops slightly — you are carrying less mass, so you burn fewer calories. This is why the time-to-goal estimate is a projection rather than a guarantee. Recalculate every four to six weeks as your weight changes to keep your target accurate.

Use this calculator when you have a specific goal weight and want a number to anchor your daily food choices. It works best for generally healthy adults who want to lose between five and sixty pounds, are not already at a very low body weight, and do not have metabolic conditions like hypothyroidism or PCOS that significantly affect energy expenditure.

It is also useful as a sanity check before starting a commercial diet program. If a program tells you to eat 1,000 calories per day and this calculator shows your TDEE is 2,200, you can see immediately that the program is pushing a 1,200-calorie deficit — far beyond what is considered safe or sustainable without clinical supervision.

This tool is not appropriate for athletes in periodized training whose caloric needs vary dramatically day to day, for people recovering from eating disorders, for anyone with a medically supervised diet plan, or for individuals who are already at or below a healthy body weight. It also should not be used to set calorie targets for children or adolescents — their nutritional needs are different and still developing.

The most common mistake is selecting too high an activity level. People include their gym sessions but forget that the other 23 hours of a desk worker's day are nearly motionless. Overestimating activity inflates your TDEE, which makes the calorie target look more generous than it really is. If you stall after two weeks, try dropping one activity tier and recalculating.

The second mistake is treating the daily target as a floor rather than a target. Some people eat well below the calculated number on good days to compensate for expected splurges. This triggers hunger cycles, promotes binge eating on the hard days, and makes the whole effort unsustainable. The number the calculator gives you is the right amount to eat, not the maximum you are allowed.

The third mistake specific to this tool is not updating your inputs as you lose weight. A 200-pound person burns more calories than a 170-pound person doing the same activities. If you set a target at 200 lbs and never recalculate, your effective deficit shrinks as you get lighter — progress slows and you may not understand why. Recalculate every four to six weeks or whenever you lose ten or more pounds.

The Mifflin-St Jeor BMR formulas are:

Male: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) + 5 Female: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) - 161

TDEE = BMR multiplied by your activity factor: 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active, 1.725 for very active, 1.9 for extra active.

Daily deficit = weekly loss target in pounds multiplied by 3,500, divided by 7. Daily calorie target = TDEE minus daily deficit. Time to goal = total pounds to lose multiplied by 3,500, divided by daily deficit.

Weight conversions: 1 lb = 0.453592 kg. Height conversion: 1 inch = 2.54 cm. The calculator handles these conversions internally — you enter pounds, feet, and inches.

The Mifflin-St Jeor equation assumes a fixed ratio of lean mass to fat mass implicit in the relationship between weight and BMR. This breaks down at the extremes: very muscular individuals will see their BMR underestimated because muscle is metabolically expensive and dense, while individuals with a high body fat percentage may have their BMR overestimated. For those populations, body composition-adjusted formulas like Katch-McArdle give more accurate results — but they require knowing lean body mass, which needs a DEXA scan or accurate skinfold measurement. The 3,500-calorie-per-pound rule also assumes purely fat loss, but in a real deficit you lose some muscle alongside fat, which means the actual caloric cost per pound of scale weight lost is often lower than 3,500 — which is why progress can sometimes outpace the model in the first few weeks.