Calorie Deficit Calculator
Estimate how long weight loss takes at a given daily calorie deficit — or how large a deficit you need to reach a goal by a target date.
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Measurements convert when you switch units
Result
The formula
A widely used energy rule is that losing about 1 kg of body fat requires roughly 7,700 kcal of cumulative deficit (Hall, 2008). Daily deficit × 7 days × weeks ≈ kcal needed; divide by 7,700 for kg lost. Real loss includes water and lean tissue, so treat output as planning guidance.
Worked example
- Goal: lose 5 kg in 10 weeks
- Energy needed ≈ 5 × 7,700 = 38,500 kcal
- Daily deficit = 38,500 ÷ (10 × 7) ≈ 550 kcal/day
Result: ≈ 550 kcal/day deficit for 10 weeks
How deficit timelines are estimated
Spreadsheets are simpler than human metabolism — use trends, not single days.
7,700 kcal per kg
Research syntheses often cite ~7,700 kcal per kg of fat mass change. Individual variation, water flux, and lean-mass loss mean actual scale movement differs week to week.
Safe deficit sizes
Many adults use 300–750 kcal/day deficits for gradual loss. Very large deficits can increase hunger, muscle loss, and adherence failure — medical supervision matters for aggressive plans.
TDEE context
Your total daily energy expenditure (TDEE) minus deficit = target intake. Use the calorie calculator to estimate TDEE first, then subtract the deficit shown here.
Interesting facts
Not linear on the scale
Water, glycogen, and menstrual cycles cause daily noise. Judge progress over 2–4 week trends.
7,700 kcal rule
Based on fat energy density (~9 kcal/g) and typical adipose composition — a planning heuristic, not a law of physics for every person.
500 kcal/day ≈ 0.5 kg/week
A 500 kcal daily deficit yields roughly 3,500 kcal/week — often quoted as ~0.5 kg/week, close to the 7,700 model.
Protein preserves lean mass
Higher protein and resistance training during a deficit help retain muscle — especially important on longer timelines.
Adaptation happens
Metabolic adaptation and NEAT changes can slow loss over time. Recalculate after large weight changes.
Frequently asked questions
Roughly 7,700 kcal total deficit spread over your chosen timeframe. At 550 kcal/day, that is about 2 weeks per kg.
Depends on your size, activity, and health. Many smaller people do better with 300–500 kcal. Ask a clinician if unsure.
Water weight, muscle, and diet composition change scale readings. Fat loss follows energy balance over time.
Estimates vary. Many people use a fixed deficit from TDEE rather than eating back burned calories to avoid overestimating.
This calculator uses the metric planning figure of ~7,700 kcal per kg of fat, which converts to roughly 3,500 kcal per pound — the classic (if imprecise) rule of thumb behind many weight-loss timelines.
Early weight loss often includes water and glycogen changes on top of fat loss, especially with reduced carbohydrate intake. The rate typically slows and stabilizes closer to the energy-balance estimate after the first week or two.
This calculator still works for planning: use it to estimate a realistic timeline, then apply the deficit through portion control, food swaps, or activity increases rather than strict daily logging.
References
- What is the required energy deficit per unit weight loss? Basis for ~7,700 kcal per kg planning rule.
- Dietary Guidelines for Americans Population context for energy balance.