CALCULATOR
Calorie Deficit Calculator
Your maintenance calories, a sustainable deficit, and the protein target that decides whether you lose fat or muscle. Set the Goal to Cut below.
Daily targets
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Estimate based on the Mifflin-St Jeor equation. Adjust over 2-3 weeks based on real-world results. General guidance, not medical advice.
How a calorie deficit actually works
A calorie deficit means eating less energy than your body uses. That is the entire mechanism behind fat loss: no food, macro split or supplement changes it. What varies between approaches is how easy the deficit is to sustain and how much of the lost weight is fat rather than muscle.
This calculator works out your maintenance calories first, then applies a deficit. The default is 20 percent, which for most people produces a loss of roughly 0.5 to 0.75 kg per week. It also sets a protein target, because protein is what determines whether the weight you lose is fat or lean tissue.
Bigger is not better. Very aggressive deficits increase muscle loss, degrade training quality and become progressively harder to hold. The deficit you can maintain for four months beats the one you abandon in three weeks.
Setting the deficit
target = maintenance x (1 - deficit percentage)
1 kg of body fat is about 7,700 kcal
weekly loss = (daily deficit x 7) / 7700
Deficit size and expected rate
| Deficit | Daily reduction at 2,600 kcal | Weekly loss | Suits |
|---|---|---|---|
| 10% | 260 kcal | 0.24 kg | Already lean, protecting muscle |
| 15% | 390 kcal | 0.35 kg | Slow and very sustainable |
| 20% | 520 kcal | 0.47 kg | Most people, most situations |
| 25% | 650 kcal | 0.59 kg | Higher starting bodyweight |
| 30% | 780 kcal | 0.71 kg | Short phases only |
The 1 percent rule
A widely used ceiling is losing no more than about 1 percent of bodyweight per week. Above that, a larger proportion of the loss comes from lean tissue rather than fat, hunger becomes difficult to manage, and training performance falls. Heavier people can sustain the upper end comfortably; leaner people should sit well below it.
| Bodyweight | 1% per week | Sensible weekly target |
|---|---|---|
| 60 kg | 0.6 kg | 0.3-0.45 kg |
| 80 kg | 0.8 kg | 0.4-0.6 kg |
| 100 kg | 1.0 kg | 0.5-0.8 kg |
| 120 kg | 1.2 kg | 0.6-1.0 kg |
Protecting muscle inside the deficit
- Protein at 1.6 to 2.2 g per kg. The single most effective dietary lever for keeping lean mass while losing fat.
- Resistance training at least twice a week. Protein supplies the material; lifting supplies the reason to keep it.
- Do not cut fat below about 0.6 g per kg. Very low fat intakes affect hormone production and vitamin absorption.
- Sleep. Short sleep measurably increases the share of weight lost as muscle rather than fat.
- Take diet breaks. A week or two at maintenance after twelve weeks helps adherence and training quality.
Why progress is never linear
Bodyweight fluctuates by a kilogram or more day to day from water, glycogen, sodium and gut contents. Early weeks drop faster than the maths predicts as glycogen and its associated water fall. Later weeks slow because a lighter body burns fewer calories at the same activity level. None of this means the deficit has stopped working; it means the scale measures more than fat.
Weigh under consistent conditions, use a weekly average, and recalculate your maintenance every 5 to 7 kg of change.
Frequently asked questions
How big should my calorie deficit be?
Is a 500 calorie deficit enough?
Why am I in a deficit but not losing weight?
Do I need to eat back exercise calories?
Sources
- Hall KD. What is the required energy deficit per unit weight loss? International Journal of Obesity, 2008.
- Helms ER et al. Dietary protein during caloric restriction in resistance trained lean athletes. IJSNEM, 2014.
- Garthe I et al. Effect of two different weight-loss rates on body composition and strength. IJSNEM, 2011.
- National Institutes of Health, Office of Dietary Supplements. Nutrient fact sheets. ods.od.nih.gov
- NHS. Nutrition and health guidance. nhs.uk
- US Departments of Agriculture and Health and Human Services. Dietary Guidelines for Americans, 2025-2030. dietaryguidelines.gov
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General information, not medical advice. Consult a qualified professional before changing your diet, especially with a medical condition.