How this instrument works
Total daily energy expenditure is a full day's calorie cost — not just staying alive, but digesting food, walking across a room, fidgeting at a desk, and any deliberate exercise layered on top. This instrument builds it in two stages: it computes basal metabolic rate first, using Mifflin-St Jeor, then multiplies that resting figure by an activity factor drawn from five tiers, from desk-bound to twice-daily training.
Its resting half dates to a 1990 study by Mifflin, St Jeor and colleagues, who measured 498 adults by indirect calorimetry and fitted a simple line to their results. Its multiplying half is older in spirit: nutrition scientists have scaled resting metabolism by an activity ratio since FAO/WHO/UNU energy-requirement reports spanning 1985 to 2001, and this five-tier version — sedentary through athlete — is a convention that dietitians and fitness software converged on for everyday use.
Neither half measures you directly. Its equation predicts population-level averages for someone your size, sex and age; its multiplier then layers on a further average for how active people at that tier tend to be. Confirming true daily energy expenditure means doubly labelled water tracked over two weeks — laboratory work, not four numbers on paper. Treat this output as a starting point to weigh against how your own body weight moves over several weeks, not a target to hit exactly.
- Set Sex — it changes a closing constant in this resting-metabolism line, +5 for male or −161 for female.
- Enter Weight and Height, switching either unit menu to pounds, stone, feet or inches if you don't use metric.
- Enter Age (years); each year older removes five kcal from this resting estimate.
- Pick your Activity level from five tiers — sedentary, light, moderate, very active or athlete.
- Read Basal metabolic rate (kcal/day) first, then Daily energy expenditure (kcal/day) below it — that second figure is your resting figure times your chosen tier.
Worked example — 70 kg, 175 cm, age 30, moderate activity
Enter 70 kilograms, 175 centimetres, age 30 and Sex set to male. Its resting line runs 10 times 70 plus 6.25 times 175 minus 5 times 30 plus 5, which is 700 + 1093.75 − 150 + 5 — a basal metabolic rate of 1,648.75 kcal per day, a cost of this body doing nothing at all.
Set Activity level to Moderate, a middle tier for someone exercising three to five days a week, which carries a multiplier of 1.55. Multiplying 1,648.75 by 1.55 gives 2,555.5625, which this readout rounds to 2,556 kcal per day — an estimated total for a day that includes typical training, not just rest.
Questions
How accurate is a TDEE estimate?
It's a population-average prediction, not a measurement of you specifically — expect it to sit within roughly 10 to 20 percent of lab measurements like doubly labelled water. Use it as a starting point, then adjust any intake you build from it based on how your weight actually trends over two to three weeks, a more reliable signal than this formula alone.
Why does another calculator give me a different TDEE?
Calculators disagree because they mix different pieces: some use an older Harris-Benedict equation from 1919 instead of Mifflin-St Jeor, others use Katch-McArdle, and activity-tier multipliers vary slightly between sites. A gap of one or two hundred kcal between two reasonable tools is normal and doesn't mean either one is wrong.
Should I eat exactly at my TDEE figure?
That figure describes an estimate of maintenance, not an instruction. Whether to eat above, at, or below it depends on a goal this instrument has no way to know, and its underlying estimate carries meaningful uncertainty in both directions — treat it as a starting number to adjust from, not a target to hit precisely.
Why does changing my activity level move this number so much?
Because that multiplier scales entire resting metabolism, not just calories spent exercising. This instrument's own example reads 1,978.5 kcal sedentary and 2,555.6 kcal at moderate activity for one 70 kg, 175 cm, 30-year-old man — a 577-kcal gap from a single dropdown menu, which is why an honest activity estimate matters more than a precise scale reading.
Does a fitness tracker's exercise calories add on top of TDEE?
Usually not without double-counting. Activity tiers already fold a typical week of training into their multiplier, so someone who selects Very active and then adds tracker exercise calories on top is often counting the same movement twice. Pick a tier matching your typical week, or start from Sedentary and add tracked exercise calories separately — not both at once.
What's the difference between BMR and TDEE?
Basal metabolic rate is a resting half of this sum — an energy cost of staying alive with no activity at all. Daily energy expenditure takes that resting figure and multiplies it by your activity tier, so it typically comes out larger than BMR unless your activity factor equals 1.
References
- Mifflin et al. 1990, Am J Clin Nutr — original BMR equation (PubMed)
- NIH NIDDK — Body Weight Planner
- CDC — Physical Activity Basics
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.