What is the difference between BMR and TDEE?

BMR is what your body would burn lying completely still for 24 hours, keeping your organs running. TDEE is that figure multiplied by an activity factor to account for everything else you do — moving, digesting, exercising. TDEE is the number that matters for planning what to eat; BMR is the floor it is built on.

Why Mifflin-St Jeor rather than Harris-Benedict?

Mifflin-St Jeor was published in 1990 using a more representative sample and has been shown in validation studies to be more accurate for modern populations, typically within about 10 percent for most people. The older Harris-Benedict equation, from 1919, tends to overestimate.

How accurate is this for me personally?

Expect roughly plus or minus 10 percent, and occasionally more. The equation cannot see your body composition, thyroid function, genetics or how much you fidget. Treat the number as a starting point, then adjust based on what actually happens to your weight over two or three weeks.

Which activity level should I honestly pick?

Most people overestimate. If you have a desk job and train three times a week, that is moderate, not active. Very active is for manual labour or two training sessions a day, not an enthusiastic gym habit. Choosing one level too high is the single most common reason a calorie plan fails to work.

Does BMR fall as you lose weight?

Yes, for two reasons. A smaller body costs less to maintain, and prolonged dieting produces some additional adaptive reduction beyond what the size change alone predicts. This is why weight loss slows over time and why recalculating every few kilograms is worthwhile.

Can I raise my metabolic rate?

Modestly and slowly. Building muscle helps, since lean tissue costs more to maintain than fat, though the effect is smaller than commonly claimed — roughly 13 kcal per kilogram of muscle per day. Staying more active generally, and eating enough protein, both help more than any supplement.

BMR Calculator: Metabolic Rate and TDEE

How to use this calculator

Choose metric or imperial units.
Enter your sex at birth, age, weight and height. In imperial, height is total inches.
Pick the activity level that honestly matches your week.

The headline number is your total daily energy expenditure — what you actually burn. The BMR underneath is the portion your body uses just staying alive.

How the calculation works

This uses the Mifflin-St Jeor equation, published in 1990:

male:   BMR = 10 × kg + 6.25 × cm − 5 × age + 5
female: BMR = 10 × kg + 6.25 × cm − 5 × age − 161

BMR is then multiplied by an activity factor to give TDEE:

Activity level	Factor
Sedentary — desk job, no exercise	1.20
Light — 1 to 3 days a week	1.375
Moderate — 3 to 5 days a week	1.55
Active — 6 to 7 days a week	1.725
Very active — physical job or twice daily	1.90

Mifflin-St Jeor is used rather than the older Harris-Benedict equation from 1919, which was derived from a small and unrepresentative sample and tends to overestimate. Validation studies have consistently found Mifflin-St Jeor closer for modern populations.

A worked example

A 34-year-old male, 78 kg, 175 cm, exercising 3 to 5 days a week.

BMR = 10 × 78 + 6.25 × 175 − 5 × 34 + 5
BMR = 780 + 1093.75 − 170 + 5
BMR = 1708.75

TDEE = 1708.75 × 1.55 = 2648.56


BMR (at rest)	1,709 kcal
Daily calories burned	2,649 kcal
To lose weight	2,149 kcal
To gain weight	3,149 kcal

Note the size of the BMR: 65 percent of everything burned happens at rest, keeping organs running, tissue repaired and body temperature stable. Exercise is a smaller share of the total than most people assume.

BMR versus TDEE

BMR is what you would burn lying completely still for 24 hours in a thermally neutral room. It is the floor, not a target — eating at BMR means eating far less than you burn.

TDEE is BMR plus everything else: moving around, digesting food, fidgeting, and formal exercise. This is the number that matters for planning what to eat. Eat at TDEE and weight holds; below it and weight falls; above it and it rises.

Choosing an activity level honestly

This is where most people go wrong, and it is the single largest source of error in the whole calculation.

If you have a desk job and train three times a week, that is moderate, not active. “Very active” means manual labour or two training sessions a day, not an enthusiastic gym habit.

The difference is not trivial. For the example above, choosing “active” instead of “moderate” raises TDEE from 2,649 to 2,948 — nearly 300 calories a day. Plan a deficit from the inflated figure and you eliminate most of it before you start, then conclude the calculator was wrong.

If in doubt, pick one level lower than feels right.

How accurate is this?

Expect roughly plus or minus 10 percent, and occasionally more. The equation cannot see your body composition, thyroid function, genetics, or how much you unconsciously move.

The practical approach is to treat it as a starting hypothesis. Eat at the predicted maintenance level for two to three weeks, track weight as a weekly average, and adjust. If weight is stable, that is your true maintenance regardless of what the formula said.

Common mistakes to avoid

Eating at BMR rather than TDEE. BMR is the resting floor. Eating there creates an enormous deficit that is difficult to sustain and costs muscle.

Not recalculating as weight changes. A smaller body burns less. Lose 10 kilograms and TDEE falls by roughly 100 to 150 calories a day, which is why loss slows and eventually stops at what used to be a deficit.

Expecting exercise to dominate. A 45-minute run burns roughly 400 to 500 calories, around 15 percent of the daily total for our example. Exercise is excellent for health and a modest lever on energy balance.

Trusting fitness tracker calorie burns. Tracker estimates for exercise are frequently overstated, sometimes by 50 percent or more, and many then add that back as food you have “earned”. This is one of the most reliable ways to eat more than intended.

When this calculator is not the right tool

For a calorie target aimed at a specific rate of weight change, with a macronutrient split, the calorie calculator does that directly. For body composition, the body fat calculator is more informative than any weight-based number. And during pregnancy or breastfeeding these equations are not calibrated — the right figure should come from a midwife or doctor.

BMR Calculator