BMR and TDEE Calculator Using Mifflin-St Jeor

You want to lose weight, gain muscle, or just understand how many calories your body actually needs. The starting point is always the same: figure out your BMR (basal metabolic rate — what you'd burn lying in bed all day) and your TDEE (total daily energy expenditure — what you actually burn given your activity level).

How the math works

The BMR/TDEE calculator uses the Mifflin-St Jeor equation, which is widely considered the most accurate formula for estimating metabolic rate. For men: 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 5. For women: the same formula minus 161 instead of minus 5.

Enter your sex, age, height, weight, and activity level. The tool gives you your BMR in kilocalories per day and then multiplies by an activity factor to get your TDEE. The activity levels range from sedentary (multiply by 1.2) to very active (multiply by 1.9).

What the numbers tell you

Your TDEE is roughly the number of calories you need to eat to maintain your current weight. Eat less than your TDEE and you'll lose weight over time. Eat more and you'll gain. The calculator shows three targets: weight loss (TDEE minus 500 kcal), maintenance, and weight gain (TDEE plus 500 kcal). A 500 kcal daily deficit works out to roughly half a kilogram per week.

There's also a basic macronutrient split showing suggested grams of protein, carbohydrates, and fat for each calorie target. These are general guidelines, not personalized nutrition advice, but they give you a reasonable starting framework.

Be honest about activity

The most common mistake is overestimating your activity level. "I go to the gym three times a week" sounds like moderate activity, but if you spend the other 160 hours of the week sitting at a desk, sedentary or light is probably more accurate. The TDEE calculator will give you a number regardless — it's only as accurate as the inputs you give it.

The tool supports both metric and imperial units, so you don't have to convert before entering your data. Everything runs locally in your browser.