Any calorie target, whether for losing weight, keeping it steady or building muscle, starts from an estimate of how much energy your body uses in a day. That estimate has two parts: basal metabolic rate, the energy you burn at complete rest, and everything you add by moving, working and exercising. This guide shows the standard formulas, a worked example and why the result should be treated as a starting point to adjust, not a precise figure.

BMR and TDEE: two terms to know

BMR, basal metabolic rate, is the energy your body needs to stay alive at rest: breathing, circulation, temperature control and cell repair. For most people it is the largest part of daily energy use, often around 60 to 70 percent.

TDEE, total daily energy expenditure, is BMR plus the energy used for movement, exercise and digesting food. It is the number of calories you would need to eat to keep your weight stable.

The Mifflin-St Jeor equation

This equation, published in 1990, is widely used and has been found to be more accurate for most people than older formulas. It uses weight in kilograms, height in centimetres and age in years.

For men: BMR = 10 × weight + 6.25 × height − 5 × age + 5.

For women: BMR = 10 × weight + 6.25 × height − 5 × age − 161.

The result is in kilocalories per day. The sex-based constant reflects average differences in body composition, so the formula is an estimate for a typical person of that size.

Worked example

A 30-year-old woman who weighs 65 kg and is 165 cm tall.

BMR = 10 × 65 + 6.25 × 165 − 5 × 30 − 161 = 650 + 1,031.25 − 150 − 161 = 1,370 kcal per day (rounded).

A 40-year-old man who weighs 80 kg and is 180 cm tall.

BMR = 10 × 80 + 6.25 × 180 − 5 × 40 + 5 = 800 + 1,125 − 200 + 5 = 1,730 kcal per day.

From BMR to TDEE: the activity multiplier

Multiply BMR by a factor that matches your usual week. Common values are: 1.2 for little or no exercise and a desk job; 1.375 for light exercise one to three days a week; 1.55 for moderate exercise three to five days a week; 1.725 for hard exercise six to seven days a week; and 1.9 for very hard exercise combined with a physical job.

For the woman above with light exercise: 1,370 × 1.375 = about 1,884 kcal per day. For the man with moderate exercise: 1,730 × 1.55 = about 2,682 kcal per day.

People tend to overestimate their activity, so if you are unsure, choose the lower multiplier.

Turning TDEE into a goal

To lose weight, eat below your TDEE; to gain, eat above it. A common rule of thumb says that a deficit of around 500 kcal a day leads to a loss of roughly half a kilogram a week, but this is only an approximation, because the body adapts as you lose weight and your TDEE falls.

A moderate deficit of 10 to 20 percent below TDEE is easier to sustain than a large one. Very low intakes are not suitable for most people without medical supervision. Eating much below your BMR for long periods can cause fatigue, loss of muscle and nutritional gaps.

Why the result is only an estimate

The equations were built from averages. Two people with the same age, height and weight can differ by a couple of hundred calories a day because of muscle mass, genetics, hormones, medication and sleep. Muscle uses more energy than fat at rest, so a muscular person burns more than the formula predicts.

The activity multiplier is also a rough guess. And food labels and portion sizes are not exact, so calorie counting has its own error.

The practical way to use the number is to follow it for two to three weeks while tracking your weight, and then adjust. If your weight stays the same on the planned intake, that intake is your real TDEE.

Frequently asked questions

Is Mifflin-St Jeor better than Harris-Benedict? In studies of healthy adults it has generally predicted resting energy more closely, which is why many dietitians prefer it.

Does the formula work for children, pregnant women or athletes? No. It is designed for healthy adults. Children, pregnancy and breastfeeding, and very lean athletes need different guidance.

Should I eat the same number every day? Not necessarily. A weekly average matters more than a single day.

Can I trust a fitness watch's calorie figure? Treat it as a rough guide. Studies find that wearables can be off by a large margin, especially for energy burned during exercise.

Calculate yours

Use our calorie and BMR calculators to enter your age, height, weight and activity level. This article is general information, not medical or dietary advice. If you have a medical condition or want a weight-loss plan, talk to a doctor or a registered dietitian.