BMI and TDEE Calculator
Enter your height, weight, age, sex, and activity level to get your BMI, basal metabolic rate, and total daily energy expenditure, plus calorie targets for cutting, maintaining, or gaining.
Last updated: October 2026
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What BMI tells you (and what it does not)
Body mass index (BMI) is a simple ratio of your weight to your height. Doctors and insurers use it as a quick screening tool because, across large populations, higher BMIs correlate with higher health risks. It is not a diagnosis and it cannot see body composition: a muscular athlete and a sedentary person can share the same BMI while carrying very different amounts of body fat.
The standard adult categories are: under 18.5 is underweight, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 or above is obesity. Treat the number as a starting point for a conversation with your doctor, not a verdict on your health.
How your BMR is calculated: the Mifflin-St Jeor equation
Your basal metabolic rate (BMR) is the energy your body burns at complete rest just to keep you alive: breathing, circulation, brain function, and cell repair. The Mifflin-St Jeor equation, published in 1990 and still the most validated formula for healthy adults, estimates it from weight, height, age, and sex:
Women: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age - 161
Weight is the biggest driver because maintaining more tissue costs more energy. Age subtracts a little each year, reflecting the slow metabolic decline most adults experience. The sex term accounts for average differences in muscle mass and body composition.
From BMR to TDEE: the activity multiplier
Total daily energy expenditure (TDEE) is your BMR plus everything you do: walking, working, exercising, even fidgeting. The standard method multiplies BMR by an activity factor. Be honest here, since most people overestimate how active they are, and the multiplier you pick moves the result by hundreds of calories.
| Activity level | Multiplier | What it means |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no exercise |
| Lightly active | 1.375 | Light exercise 1 to 3 days a week |
| Moderately active | 1.55 | Moderate exercise 3 to 5 days a week |
| Very active | 1.725 | Hard exercise 6 to 7 days a week |
| Extremely active | 1.9 | Physical job plus daily training |
Worked example
Take a 30-year-old man who is 180 cm tall, weighs 80 kg, and exercises moderately (3 to 5 days a week):
- BMI: 80 / (1.80 x 1.80) = 80 / 3.24 = 24.7, in the healthy range
- BMR: 10 x 80 + 6.25 x 180 - 5 x 30 + 5 = 800 + 1125 - 150 + 5 = 1,780 calories
- TDEE: 1,780 x 1.55 = 2,759 calories per day to maintain weight
- Mild cut: 2,759 - 500 = 2,259 calories (roughly 1 lb of fat loss per week)
- Lean bulk: 2,759 + 300 = 3,059 calories (slow muscle gain with minimal fat)
Setting cut, maintain, and bulk targets
To maintain your weight, eat at your TDEE. Small day-to-day swings are normal; watch the two-week trend, not the daily scale reading.
To lose fat, a deficit of 300 to 500 calories per day produces steady, sustainable loss of roughly half a pound to one pound per week. Larger deficits work faster but cost more muscle and are harder to stick with. Keep protein high (around 1.6 to 2.2 grams per kilogram of body weight) and keep lifting so most of the weight you lose is fat.
To gain muscle, a surplus of 200 to 300 calories per day is the sweet spot for most people. Bigger surpluses mostly add fat. Expect roughly half a pound to one pound of total weight gain per week when bulking, and adjust if the scale moves much faster.
Limitations worth knowing
BMI misclassifies muscular people, since muscle is dense: many athletes land in the overweight range with low body fat. It also becomes less useful at the extremes of height and during pregnancy. The calorie formulas are population averages, so real-world results vary by plus or minus 10 to 15 percent. If your weight does not move as predicted after two to three consistent weeks, adjust your intake by 100 to 200 calories rather than assuming the math is broken. And if you have a medical condition, take medication that affects weight, or have a history of disordered eating, work with a professional instead of a calculator.
BMI and TDEE FAQs
What is a healthy BMI?
For adults, a BMI between 18.5 and 24.9 is considered the healthy range. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is classified as obesity. These are screening cutoffs, not diagnoses.
Is BMI accurate for muscular people?
Not really. Muscle is denser than fat, so muscular athletes often land in the overweight or even obese BMI range despite low body fat. For muscular individuals, waist measurement, body-fat percentage, or progress photos are more useful than BMI.
What is the difference between BMR and TDEE?
BMR (basal metabolic rate) is the calories your body burns at complete rest to stay alive. TDEE (total daily energy expenditure) is BMR plus all activity: exercise, walking, work, and daily movement. You eat at TDEE to maintain weight.
How accurate is the Mifflin-St Jeor equation?
It is the most validated BMR formula for healthy adults, typically within 10 percent of lab-measured values. Individual metabolism still varies with muscle mass, genetics, and medical conditions, so treat the result as a starting point and adjust based on real results.
How many calories should I eat to lose weight?
Eat 300 to 500 calories below your TDEE for steady fat loss of roughly half a pound to one pound per week. The calculator shows a mild cut target of TDEE minus 500. Keep protein high and keep training so the weight you lose is mostly fat.
What does moderately active actually mean?
In this calculator it means moderate exercise 3 to 5 days per week, such as brisk workouts, sports, or active training sessions, on top of normal daily movement. A desk job with evening workouts usually fits here, not very active.
Should I eat back the calories I burn exercising?
Usually not fully. Fitness trackers and machines tend to overestimate calorie burn, and the activity multiplier in your TDEE already accounts for your training. If you add exercise calories on top of an active multiplier, you can easily erase your deficit.