Calorie Deficit Calculator
See how long a daily calorie deficit takes to reach a target weight, using the 7,700 kcal-per-kilogram (3,500 kcal-per-pound) rule.
How many weeks a daily deficit takes to reach your target weight.
How the calorie deficit calculator works
The classic rule is that a kilogram of body fat stores about 7,700 kcal (3,500 kcal per pound). Divide the energy in the weight you want to lose by your daily deficit and you have the number of days. The calculator shows the deficit as weekly loss too, so you can judge whether the pace is realistic.
Real loss is not perfectly linear: as weight falls, maintenance calories fall with it, so the same food intake produces a shrinking deficit and progress slows. Treat the timeline as a first estimate and re-run it every month with your current weight.
Formula: days = (start − target) × 7,700 kcal/kg ÷ daily deficit
Worked examples
| Inputs | Time to reach target | Note |
|---|---|---|
| 85 → 78 kg at 500 kcal/day | 15.4 weeks | 7 kg × 7,700 = 53,900 kcal |
| 85 → 78 kg at 750 kcal/day | 10.3 weeks | a third faster |
| 187 → 172 lb at 500 kcal/day | 15 weeks | 15 lb × 3,500 = 52,500 kcal |
FAQFrequently asked questions
Where does 7,700 kcal per kilogram come from?
Adipose tissue is roughly 87% fat and fat holds about 9 kcal per gram, giving about 7,700 kcal per kilogram (3,500 per pound). It is a useful average; the real figure varies with how much lean tissue is lost alongside fat.
Why is real weight loss slower than the estimate?
Because your maintenance calories fall as you get lighter, so a fixed intake produces a smaller deficit over time. Re-run the calculator with your current weight each month.
What deficit is sensible?
Most guidelines suggest 500–750 kcal a day, which loses roughly 0.5–0.7 kg a week. The calculator warns above 1,000 kcal a day.
Does the calculator account for exercize?
Only through the deficit you enter. A deficit can come from eating less, moving more or both — the arithmetic is the same.
Where these figures come from
- Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 — the BMR equation used here
- World Health Organization — Waist circumference and waist–hip ratio (2008 expert consultation) — waist-to-hip cut-offs
- Tanaka H, Monahan KD, Seals DR, "Age-predicted maximal heart rate revisited", J Am Coll Cardiol 2001 — 208 − 0.7 × age
- Nathan DM et al., "Translating the A1C assay into estimated average glucose values" (ADAG), Diabetes Care 2008 — eAG = 28.7 × A1c − 46.7
- Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987 — BSA = √(height × weight / 3600)
- Centers for Disease Control and Prevention — national health guidance
Last checked: September 2026. The equations are fixed by their published papers and do not change with tax years or regulations; the thresholds quoted in verdicts name the body that set them.