Calorie Deficit Calculator
How this is calculated
Three steps. Your BMR from the Mifflin-St Jeor equation, multiplied by an activity factor to give maintenance calories, minus the deficit needed for your chosen rate of loss.
The conversion uses the standard figure of roughly 7,700 calories per kilogram of body fat:
| Rate of loss | Daily deficit | Weekly deficit |
|---|---|---|
| 0.25 kg/week | ~275 cal | ~1,925 cal |
| 0.5 kg/week | ~550 cal | ~3,850 cal |
| 0.75 kg/week | ~825 cal | ~5,775 cal |
| 1 kg/week | ~1,100 cal | ~7,700 cal |
The calculator floors the result at 1,200 calories for women and 1,500 for men. If your chosen rate would push you below that, it caps the number and says so rather than returning a target nobody should eat.
Why faster usually isn't better
A larger deficit does produce faster scale movement. It also produces more of that loss from muscle rather than fat, a sharper drop in metabolic rate, worse adherence, and stronger appetite compensation — all of which make the weight harder to keep off.
0.5 to 0.75 kg per week suits most people. A useful rule of thumb is roughly 0.5–1% of body weight per week — someone at 120 kg can comfortably lose faster than someone at 65 kg. If you're already lean and trying to get leaner, err toward the gentle end.
The 7,700 figure is a simplification. It assumes the loss is pure fat, and it doesn't account for your maintenance calories falling as you get lighter. In practice, weight loss decelerates on a fixed intake — which is the single most common reason people conclude their diet "stopped working".
What the first two weeks will look like
Expect a fast initial drop that isn't fat. Reducing carbohydrate intake depletes glycogen, and each gram of glycogen holds roughly three grams of water, so 1–3 kg can disappear in the first week. It'll appear to stall afterwards, which feels like failure and isn't — that's when real fat loss starts showing.
Judge progress on a weekly average, not individual days. Daily weight swings of 1–2 kg from water, sodium, fibre and menstrual cycle phase are normal and tell you nothing.
Protect your muscle while you do it
A deficit costs you lean mass unless you deliberately defend it. Two things do most of the work:
- Protein at 1.6–2.0 g per kg — see the protein calculator for your number.
- Resistance training two to three times a week, which signals the body to keep the muscle it has.
Without those, a meaningful share of what you lose is muscle — which lowers your maintenance calories further and leaves you worse positioned than when you started.
When it stops working
It will, and the instinct to cut further is usually wrong. A lighter body burns less, non-exercise movement drops unconsciously, and logging accuracy drifts. Work through the causes in order rather than reaching for a bigger deficit — we've laid that out in why your weight loss stopped.
This is a general estimate for healthy adults. It isn't appropriate during pregnancy or breastfeeding, for children or adolescents, or if you have a history of disordered eating. If tracking calories drives anxiety, that matters more than any number here — speak to your GP.
Frequently asked questions
How big a calorie deficit do I need to lose weight?
Why shouldn't I just eat as little as possible?
Why did I lose a lot in week one and then stall?
How do I avoid losing muscle in a deficit?
References
- Hall KD, et al. (2011). Quantification of the effect of energy imbalance on bodyweight. The Lancet, 378(9793), 826-837.
- Mifflin MD, et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 51(2), 241-247.
Related reading
A number is a starting point, not a plan
Our AI nutritionist takes your targets and turns them into meals you'll actually eat — and adjusts as your goals, training and life change.
General nutrition information only — not medical advice, diagnosis or treatment. See our health disclaimer.