TDEE & BMR Calculator
Estimates basal metabolic rate from the Mifflin-St Jeor equation and multiplies by an activity factor to give total daily energy expenditure. It is a starting estimate rather than a measurement, and the honest error band is wide enough that you should treat the output as a hypothesis to test.
How to use it
- Enter weight, height, age, and sex.
- Set an activity multiplier between 1.2 and 1.9.
- Read BMR and TDEE in kilocalories per day.
Why Mifflin-St Jeor
The equation, published in 1990, is: for men, 10 times weight in kg plus 6.25 times height in cm minus 5 times age plus 5; for women, the same with minus 161 instead of plus 5.
It replaced the older Harris-Benedict equations from 1919, which overestimate BMR by roughly 5 percent in modern populations. Mifflin-St Jeor is accurate to within 10 percent for around 80 percent of people, which is the best available from height, weight, age, and sex alone.
The Katch-McArdle equation performs better where lean body mass is known, since muscle is the main determinant of resting metabolism, but it requires a body composition measurement that most people do not have. If you have a reliable DEXA figure, it is worth using instead.
A worked case: an 80 kg, 180 cm, 30-year-old man. BMR is 800 plus 1,125 minus 150 plus 5, which is 1,780 kcal. At a 1.55 multiplier the TDEE is about 2,759.
The four components of expenditure
Total expenditure is not one thing, and understanding the split explains why deficits stop working.
- BMR, typically 60 to 70 percent of the total. This is the cost of staying alive at rest, dominated by organ and lean tissue metabolism rather than muscle used for movement.
- The thermic effect of food, roughly 10 percent. Digestion costs energy, and it varies by macronutrient: protein costs 20 to 30 percent of its own calories to process, carbohydrate 5 to 10 percent, and fat 0 to 3 percent. This is part of why higher-protein diets show a modest metabolic advantage.
- Exercise activity, which is highly variable and usually smaller than people assume. An hour of moderate exercise is commonly 300 to 500 kcal, which is a fraction of the daily total.
- Non-exercise activity thermogenesis, or NEAT — fidgeting, walking, standing, posture. This varies by up to 2,000 kcal a day between individuals and is the single largest source of variation between two people with the same BMR.
Activity multipliers are the weakest input
The standard bands are 1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for very active, and 1.9 for extremely active. The spread from 1.2 to 1.9 on a 1,780 kcal BMR is 2,136 to 3,382 — a range of over 1,200 kcal, which is larger than most people intended deficit.
Self-assessment is unreliable in a consistent direction. Most people overestimate their activity level, partly because a few gym sessions feel more significant than eight hours of sitting, and partly because a desk job with three workouts a week is genuinely closer to 1.375 than 1.55 once the sedentary hours are counted.
A more defensible approach is to pick the lower plausible multiplier, eat at that level for two to three weeks while tracking weight, and adjust from the observed trend. Body weight change over time is a direct measurement of energy balance, which no equation can be. Two weeks minimum, because water weight fluctuations of a kilogram or more can mask the underlying trend over shorter periods.
The estimate moves as you use it
TDEE is not a constant, and it declines as you lose weight for reasons beyond the obvious.
Part is straightforward: a lighter body costs less to maintain and to move, so BMR falls with weight. But there is also adaptive thermogenesis, where expenditure falls further than the weight change alone predicts — commonly 10 to 15 percent below the value expected for the new body weight. Reduced NEAT accounts for much of it, largely unconsciously: less spontaneous movement, less fidgeting, lower body temperature.
The practical consequence is that a deficit calculated once will gradually stop producing weight loss. This is the plateau, and it is not a failure of adherence. Recalculating TDEE every 5 to 10 kg of change, and accepting that the number keeps moving, is the correct response.
Aggressive deficits accelerate all of this and cost lean mass as well, which lowers BMR further. A deficit of 20 to 25 percent below TDEE, with adequate protein and resistance training, preserves lean mass considerably better than a larger one and is more sustainable over the months these changes actually take.
At a glance
| Equation | Mifflin-St Jeor, 1990 |
|---|---|
| Accuracy | Within about 10 percent for 80 percent of people |
| Multiplier range | 1.2 sedentary to 1.9 extremely active |
| Transmitted | Nothing |
Frequently asked questions
Which activity multiplier should I use?
The lower plausible one. Most people overestimate. A desk job with three workouts a week is closer to 1.375 than 1.55.
How accurate is the estimate?
Within about 10 percent for roughly 80 percent of people. Track weight for two to three weeks and adjust from the observed trend rather than trusting the number.
Why did my weight loss stall?
TDEE falls as you lose weight, and adaptive thermogenesis takes it 10 to 15 percent lower still through reduced spontaneous movement. Recalculate every 5 to 10 kg.
Why does protein matter for expenditure?
Protein costs 20 to 30 percent of its own calories to digest, against 5 to 10 percent for carbohydrate and near zero for fat.
Read more
Health and fitness math — Every formula here is an estimate with a known error band. Knowing the band is what makes them useful.