Calorie Deficit Calculator — Find Your Fat-Loss Target

What is a calorie deficit — and does it actually work?

A calorie deficit is the controlled gap between the energy you consume and the energy your body expends each day. When that gap is maintained consistently, your body draws on stored fat to make up the shortfall. The physics are straightforward; the execution is where most people run into trouble.

The critical variable is the size of the deficit. Research consistently shows that deficits above 750–1,000 kcal/day produce disproportionate lean-muscle loss alongside fat, trigger metabolic adaptation that slows weight loss over time, and are unsustainable for most people beyond a few weeks. The 300–500 kcal/day range produces roughly 0.3–0.5 kg of fat loss per week — slow by impatient standards, but the rate at which the body maintains the most muscle, keeps hunger manageable, and allows consistent training performance.

How this calculator works

This tool uses the Mifflin-St Jeor equation, the most widely validated formula for estimating resting metabolic rate (RMR) in non-athletic adults. It then applies your activity multiplier to get your Total Daily Energy Expenditure (TDEE), and subtracts a daily deficit corresponding to your selected weekly fat-loss goal.

The protein recommendation (1.8 g per kg of body weight) sits in the middle of the range supported by ISSN research for individuals in a calorie deficit who train regularly. Higher-end estimates (2.2–2.4 g/kg) are appropriate for very lean individuals or those doing high-volume strength training.

Safety floors: No target produced by this calculator will drop below 1,200 kcal/day for women or 1,500 kcal/day for men. Below those thresholds, micronutrient adequacy becomes difficult to maintain and muscle-sparing protein intake becomes impractical.

How to read your results

Resting metabolic rate (RMR) is the energy your body burns at complete rest — the calories required to breathe, pump blood, and maintain organ function. It represents roughly 60–70% of total daily expenditure for a moderately active person.

Total daily energy expenditure (TDEE) is your RMR multiplied by your activity factor. This is what you would need to eat to maintain your current weight. A 10–15% estimation error is normal — TDEE calculators are a calibrated starting point, not a precise measurement.

Calorie target is your TDEE minus the deficit corresponding to your weekly goal. This is the number to use as your daily intake ceiling when logging food.

Protein target is expressed in grams per day. Prioritise hitting this number before adjusting carbohydrate or fat intake. Adequate protein is the single most important dietary variable for maintaining lean mass during a calorie deficit.

Adjusting when progress stalls

After two to three weeks at your calculated target, weigh yourself under consistent conditions (same time of day, post-void, pre-food) and calculate a weekly average. If the trend is not downward:

• First, verify tracking accuracy. Weigh food on a scale for two weeks — visual portion estimation is reliably off by 20–30% for energy-dense foods.
• If tracking is accurate and progress has genuinely stalled for three or more weeks, reduce your daily target by 100–150 kcal and reassess after another two weeks.
• Avoid large sudden cuts. Each reduction takes several weeks for the body to respond to.

Frequently asked questions

What is a calorie deficit?

A calorie deficit occurs when you consume fewer calories than your body expends each day. Over time, your body draws on stored fat to make up the shortfall, producing fat loss.

How many calories should I cut per day?

A deficit of 300–500 kcal/day is considered the sustainable sweet spot. It produces roughly 0.3–0.5 kg of fat loss per week while minimising muscle loss and metabolic adaptation.

Is the Mifflin-St Jeor formula accurate?

Mifflin-St Jeor is the most validated formula for estimating resting metabolic rate (RMR) in adults without unusual body composition. Studies show it estimates RMR within 10% for the majority of people.

What is TDEE?

TDEE stands for Total Daily Energy Expenditure — the total number of calories your body burns each day including basal metabolism, physical activity, and the thermic effect of food.

How do I adjust if I stop losing weight?

First verify your tracking is accurate — weigh food and log honestly for two weeks. If the scale hasn't moved, reduce your target by 100–150 kcal and reassess after 2–3 weeks. Avoid large sudden cuts.

Sources

1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO (1990). A new predictive equation for resting energy expenditure in healthy individuals. The American Journal of Clinical Nutrition. PMID 2305711 DOI
2. Thomas DM, Gonzalez MC, Pereira AZ, et al. (2014). Time to correctly predict the amount of weight loss with dieting. Journal of the Academy of Nutrition and Dietetics. PMID 24613454 DOI
3. Stiegler P, Cunliffe A (2006). The role of diet and exercise for the maintenance of fat-free mass and resting metabolic rate during weight loss. Sports Medicine. PMID 16526835 DOI
4. Helms ER, Aragon AA, Fitschen PJ (2014). Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Journal of the International Society of Sports Nutrition. PMID 24864135 DOI