How your calorie needs are estimated
Your body burns energy in two broad ways: keeping you alive at rest (your basal or resting metabolic rate, which is usually 60–70% of the total) and everything else — walking, working, exercising and digesting food. The calculator estimates the first with the Mifflin-St Jeor equation, then multiplies by an activity factor to get your total daily energy expenditure (TDEE). Eating that many calories should hold your weight steady; eating less or more moves it down or up.
The formulas
Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161 TDEE = BMR × activity factor (1.2 / 1.375 / 1.55 / 1.725 / 1.9) Target = TDEE − (lb per week × 3,500 / 7)
Worked example
A 35-year-old woman, 5 ft 5 in (165.1 cm), 150 lb (68.04 kg), moderately active:
BMR = 10 × 68.04 + 6.25 × 165.1 − 5 × 35 − 161 = 680.4 + 1,031.9 − 175 − 161 ≈ 1,376 kcal.
TDEE = 1,376 × 1.55 ≈ 2,133 kcal/day to maintain. To lose about 1 lb a week: 2,133 − 500 ≈ 1,633 kcal/day. A 2 lb/week target (1,133 kcal) would fall below the 1,200-calorie floor, so the calculator flags it.
Limitations
- Equations predict the average person of your size and age. Individuals commonly differ by 10% or more — watch your weight trend for two to three weeks and adjust.
- Activity factors are broad categories. Fitness trackers can help, but their calorie counts also carry large errors.
- Not designed for pregnancy, breastfeeding, children, competitive athletes or medical conditions affecting metabolism.
Sources: Mifflin, M. D., St Jeor, S. T., et al. (1990), “A new predictive equation for resting energy expenditure in healthy individuals,” American Journal of Clinical Nutrition 51(2):241–247; Frankenfield, D. et al. (2005), Journal of the American Dietetic Association 105(5):775–789; Dietary Guidelines for Americans.
Frequently asked questions
How many calories should I eat to lose weight?
A common starting point is about 500 calories a day below your maintenance level, which works out to roughly one pound a week. Faster loss is possible but harder to sustain, and most guidelines advise not going below about 1,200 calories a day for women or 1,500 for men without medical supervision.
Why is the 3,500-calorie rule only an approximation?
A pound of body fat stores roughly 3,500 calories, but as you lose weight your body burns a little less, and early loss includes water. Real-world loss usually slows over time compared with the straight-line estimate. Recalculate every 10 pounds or so.
Which activity level should I choose?
Pick the level that describes a typical week, not your best one. Most people with desk jobs who exercise a few times a week are "light" or "moderate". Overestimating activity is the most common reason calorie targets don't work as expected.
Why Mifflin-St Jeor?
In a 2005 review by the American Dietetic Association, the Mifflin-St Jeor equation predicted resting metabolic rate within 10% of measured values for more people than other common equations, including Harris-Benedict. It is the default used by many dietitians.
Does it matter where the calories come from?
For weight change, total calories matter most, but protein, fiber and overall food quality affect hunger, muscle retention and health. The Dietary Guidelines for Americans are a good reference for building a balanced plan.
This calculator is for general information and screening only and is not medical advice. Talk to a doctor or registered dietitian before making changes based on the result, especially if you are pregnant, under 18, or managing a health condition.