Calorie Deficit Calculator
Maintenance is BMR times an activity factor. A deficit is whatever you eat below it — 500 kcal a day is roughly 450 g of fat a week.
Two numbers, one decision
Your BMR is what the body spends at complete rest. Your TDEE — total daily energy expenditure — is BMR multiplied by an activity factor. Maintenance is TDEE: eat that and weight is broadly stable. A deficit is any intake below it.
BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + swhere s is +5 for men and −161 for women. This is the Mifflin-St Jeor equation, published in 1990 and still the usual clinical default because it is more accurate than the older Harris-Benedict formula for most people.
Activity factors
| Level | Factor | What it means in practice |
|---|---|---|
| Sedentary | 1.200 | Desk job, under 5,000 steps, no training |
| Light | 1.375 | Exercise 1–3 days a week |
| Moderate | 1.550 | Exercise 3–5 days a week |
| Very active | 1.725 | Exercise 6–7 days a week |
| Extra active | 1.900 | Physical job, or two sessions a day |
The factor is where most error enters. People routinely overestimate their activity level, and the gap between "light" and "very active" is 25% of daily calories — about 500 kcal for a typical adult.
Worked example
A 35-year-old man, 80 kg, 180 cm, exercising 3–5 days a week:
- BMR = 800 + 1,125 − 175 + 5 = 1,755 kcal
- TDEE = 1,755 × 1.55 = 2,720 kcal
- A 500 kcal deficit gives 2,220 kcal a day
At 7,700 kcal per kilogram of body fat, 500 kcal a day is 3,500 kcal a week — about 450 g of fat. In practice the scale moves faster at first (water and glycogen) and then slows, because a lighter body burns less and adherence drifts.
Choosing a deficit
- 250 kcal/day — slow, easy to hold, roughly 230 g a week. Good default if you are already lean or training hard.
- 500 kcal/day — the standard figure, roughly 450 g a week. Most people can sustain this for weeks.
- 750 kcal/day — faster, but hunger, sleep and training quality usually suffer. Short blocks only, and not without supervision.
Floors and warnings. Very low intake risks muscle loss, nutrient gaps and gallstones. Common guidance is not to go below about 1,500 kcal a day for men or 1,200 for women without medical supervision. Do not run a deficit if you are pregnant, breastfeeding, under 18, recovering from illness, or have a history of disordered eating. This page is arithmetic, not medical advice; a dietitian or doctor can adjust it to your situation.
Why the estimate drifts
- Adaptive thermogenesis. A sustained deficit lowers spontaneous movement and slightly lowers BMR, so the same intake stops producing the same loss.
- Body composition. Mifflin-St Jeor does not know whether the weight is muscle or fat; very muscular people are underestimated, very high body-fat people overestimated.
- Logging error. Self-reported intake is under-reported by 20–30% on average in controlled studies.
- Activity labels. "Moderate" means different things to different people; the table above is the only definition that matters.
Questions
Should I eat back exercise calories?
The activity factor already includes your training. Eating back tracked exercise on top of it double-counts, which is a common reason a calculated deficit does not produce loss.
How often should I recalculate?
Every 4–5 kg of weight change, or every couple of months. BMR falls as weight falls, so a deficit that worked at 90 kg is smaller at 80 kg.
Is the TDEE number exact?
No. It is a population equation, typically within 10% for individuals, and the activity factor adds more error. Treat it as a starting point and adjust from two to three weeks of real weight data.