How do I calculate my calorie deficit?

Estimate maintenance using Mifflin-St Jeor: RMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5 (men) or −161 (women), then multiply by an activity factor (1.2–1.9). Subtract the deficit you choose from that total and check the result against the intake floors. No review sets one correct percentage; Helms et al. (2014) express the target as a loss of about 0.5–1% of body weight per week instead. The equation has a standard error of about 10%, so treat the result as a range, not a fixed number.

Is a 500 calorie deficit safe?

A 500 kcal deficit is a common starting point, but it is not universally safe. For a woman with a maintenance of 1,600 kcal, a 500 kcal deficit yields 1,100 kcal, below the 1,200 kcal floor. The floor exists because meeting micronutrient needs below it is difficult. A 500 kcal deficit may be appropriate for someone with a higher maintenance, but it should be checked against the floors.

What is the minimum calorie intake for weight loss?

The commonly cited floors are 1,200 kcal per day for women and 1,500 kcal per day for men. These are not targets; they are lower bounds below which it is difficult to obtain adequate micronutrients. Intakes below these levels should only be undertaken with clinical supervision. This is not medical advice.

How big a calorie deficit to lose weight without losing muscle?

A 2014 review by Helms, Aragon and Fitschen in the Journal of the International Society of Sports Nutrition, written for natural bodybuilders, gives no percentage below maintenance. It recommends losing about 0.5–1% of body weight per week, keeping the deficit modest, and eating 2.3–3.1 g of protein per kg of lean body mass per day. Resistance training also influences lean mass retention.

Does the calorie deficit need to change as I lose weight?

Yes. As body mass falls, maintenance falls, so the same intake produces a smaller deficit. To keep the same percentage deficit, recalculate maintenance using the new weight and adjust intake accordingly. Otherwise, the deficit shrinks over time and weight loss slows. This is arithmetic, not a recommendation to diet indefinitely.

Calorie Deficit Size: Evidence and Arithmetic

A calorie deficit is the difference between the energy the body expends and the energy it takes in. The size of that difference determines how quickly mass changes, but it also changes the composition of the loss and the likelihood that a plan can be followed. This page derives the figures from the equations that produce them, gives the band rather than a single number, and shows where the arithmetic stops being useful.

This is not medical advice. Anyone with a medical condition, who is pregnant or breastfeeding, or who has a history of an eating disorder should consult a clinician before changing intake. In the United States, the ANAD eating disorders helpline is 1-888-375-7767 (Monday to Friday), and the 988 Suicide & Crisis Lifeline takes calls and texts at 988, 24 hours a day. In the UK, Beat’s helpline is 0808 801 0677.

Maintenance first, then the deficit

A deficit is a subtraction from maintenance. Maintenance is the energy required to keep body mass stable. The most common estimate for adults is the Mifflin-St Jeor equation, published in 1990. It predicts resting metabolic rate (RMR) from weight, height, age, and sex. A physical activity factor is then applied to estimate total daily energy expenditure (TDEE).

The equation is:

Men: RMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
Women: RMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161

TDEE is RMR multiplied by an activity factor. Common factors range from 1.2 for sedentary to 1.9 for very active. These factors are approximations; individual variation in non-exercise activity thermogenesis can be substantial.

Mifflin-St Jeor has a standard error of about 10% when compared with indirect calorimetry. That means a predicted maintenance of 2,300 kcal has a plausible range of roughly 2,070 to 2,530 kcal. Any deficit is subtracted from a number that is itself uncertain.

How big a deficit?

The evidence does not support a single optimal deficit expressed as a percentage of maintenance. A review by Helms, Aragon and Fitschen (2014) in the Journal of the International Society of Sports Nutrition, written for natural bodybuilders preparing for competition, frames the target as a rate of loss instead: about 0.5–1% of body weight per week, with the deficit kept modest to limit lean mass loss and protein at 2.3–3.1 g per kg of lean body mass per day (Helms et al., 2014). The review does not give a percentage deficit. Its population is lean, resistance-trained athletes, so the figure is a reference point, not a rule for everyone.

For a 70 kg person, 0.5–1% of body weight is 0.35–0.7 kg per week. At the rough conversion of 7,700 kcal per kg, that corresponds to an average deficit of about 385–770 kcal per day. The conversion overstates what is needed early on, because early losses include water and glycogen.

A 2014 review by Trexler, Smith-Ryan and Norton in the same journal describes metabolic adaptation: energy restriction is accompanied by changes in circulating hormones, mitochondrial efficiency and energy expenditure that shrink the deficit and slow loss. It suggests the smallest deficit that still produces appreciable loss, adjusted in steps when progress stalls (Trexler et al., 2014). It does not provide a single rate that applies to everyone.

A deficit of 500 kcal per day is often cited because 500 × 7 = 3,500 kcal, and 3,500 kcal is roughly the energy in one pound of adipose tissue. That arithmetic is a simplification. The 3,500 kcal per pound figure (or 7,700 kcal per kilogram) describes the energy content of adipose tissue, not the energy cost of losing body weight. Early weight loss includes water and glycogen, which are not adipose tissue. Later, as mass falls, maintenance falls, so the same intake produces a smaller deficit. The 500 kcal figure is not wrong as a starting point, but it is not a constant.

Worked examples at 10%, 20%, and 30%

Consider a 40-year-old woman, 165 cm tall, weighing 70 kg, with a sedentary activity factor of 1.2. Her estimated RMR and TDEE are calculated below. The same steps can be repeated with any numbers.

Step 1: RMR = 10 × 70 + 6.25 × 165 − 5 × 40 − 161
Step 2: RMR = 700 + 1031.25 − 200 − 161
Step 3: RMR = 1370.25 kcal/day
Step 4: TDEE = RMR × 1.2 = 1370.25 × 1.2 = 1644.3 kcal/day
Step 5: 10% deficit = 1644.3 × 0.10 = 164.4 kcal; intake = 1644.3 − 164.4 = 1479.9 kcal/day
Step 6: 20% deficit = 1644.3 × 0.20 = 328.9 kcal; intake = 1644.3 − 328.9 = 1315.4 kcal/day
Step 7: 30% deficit = 1644.3 × 0.30 = 493.3 kcal; intake = 1644.3 − 493.3 = 1151.0 kcal/day

The 30% deficit produces an intake of 1,151 kcal/day, which is below the 1,200 kcal floor for women. The 20% deficit produces 1,315 kcal/day, above the floor. The 10% deficit produces 1,480 kcal/day. These figures are estimates; the 10% standard error on maintenance means the true maintenance could be 1,480 to 1,809 kcal/day, and the true deficit at a given intake could be larger or smaller.

For men, the floor is 1,500 kcal/day. A man with a TDEE of 2,500 kcal would reach the floor at a 40% deficit (2,500 × 0.60 = 1,500). A 20% deficit would be 2,000 kcal/day.

The table: when the figure changes

The appropriate deficit depends on the condition. The table below lists common conditions and the corresponding deficit or intake floor. These are not prescriptions; they are reference points.

Condition	Reference point	Notes
General weight loss	No single percentage	The deficit is an arithmetic choice; check the resulting intake against the floors below.
Resistance-trained, high protein	About 0.5–1% of body weight per week	Helms et al. (2014), written for natural bodybuilders.
Larger deficit	Faster loss	Helms et al. advise a modest deficit to avoid lean mass losses.
Stalled progress	Re-check maintenance	Trexler et al. (2014) describe adaptation shrinking the deficit over time.
Women, minimum intake	—	1,200 kcal/day floor.
Men, minimum intake	—	1,500 kcal/day floor.
Pregnancy or breastfeeding	—	Do not attempt without clinical supervision.
History of eating disorder	—	Do not attempt without clinical supervision.

What people get wrong

The most common mistake is treating the 500 kcal deficit as a fixed rule. It is not. The 500 kcal figure comes from dividing 3,500 kcal (the approximate energy in one pound of adipose tissue) by 7 days. That arithmetic assumes the energy content of adipose tissue equals the energy cost of weight loss, which is false. Early weight loss includes water and glycogen; later, maintenance falls as mass falls. A 500 kcal deficit at the start may become a 300 kcal deficit after 10 kg of loss, because TDEE drops.

A second mistake is ignoring the floor. A 30% deficit for a woman with a TDEE of 1,600 kcal produces an intake below 1,200 kcal. The floor exists because it is difficult to meet micronutrient needs below it. It is not a target.

A third mistake is assuming the equation is precise. Mifflin-St Jeor has a 10% standard error. Two people with identical inputs can have different maintenance. The deficit that works for one may be too large or too small for another.

These mistakes are natural because the arithmetic is simple and the numbers are published without their uncertainty. The 500 kcal figure is memorable; the 10% error is not.

Adjusting over time

As mass falls, maintenance falls. A deficit that was 20% at the start may become 25% after several kilograms of loss. To keep the deficit at the chosen percentage, intake must be recalculated periodically. The Mifflin-St Jeor equation can be rerun with the new weight, and the new TDEE multiplied by the same factor (0.80 for a 20% deficit).

This is not a recommendation to diet indefinitely. It is a description of the arithmetic. Any plan that continues for more than a few weeks should be reviewed with a clinician or registered dietitian, especially if intake approaches the floor.

Summary of figures

Mifflin-St Jeor RMR: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5 (men) or −161 (women). Standard error ~10%.
TDEE: RMR × activity factor (1.2–1.9).
Rate of loss for resistance-trained athletes: about 0.5–1% of body weight per week, deficit kept modest (Helms et al., 2014).
Metabolic adaptation shrinks the deficit over time (Trexler et al., 2014).
500 kcal deficit: derived from 3,500 kcal per pound of adipose tissue; not a constant.
Floors: 1,200 kcal/day for women, 1,500 kcal/day for men.
7,700 kcal per kg: describes adipose tissue energy, not body weight change; wrong early (water) and late (maintenance falls).

This page is for information only. It does not diagnose, treat, or replace clinical advice. If you are considering a calorie deficit, consult a qualified professional. If you are struggling with disordered eating, call or text 988 in the United States, or contact Beat on 0808 801 0677 in the United Kingdom.

How Big a Calorie Deficit? A Reference Guide