Every diet that works is, underneath the branding, an argument about one number: how much energy you actually use in a day. Basal metabolic rate estimates the floor of that number, and total daily energy expenditure extends it to your real life. This guide explains the equation behind both, why dietitians trust Mifflin-St Jeor over its rivals, and — most importantly — the calibration routine that turns any equation’s guess into your measured truth.
Worth stating plainly at the outset: no equation on this page knows about your thyroid, your muscle mass, your medications or your history of dieting. It knows four numbers. The calibration routine at the end of this article exists because those unmeasured things are real — the loop is how the equation apologises for them.
BMR: the cost of being alive
Basal metabolic rate is the energy your body spends in a day at complete rest: powering the brain, liver, heart, kidneys, and the constant repair work of staying alive. It is startlingly large — for most adults, 60–70% of everything they burn — and startlingly stable, set mainly by body mass, height, age and sex. Exercise, for all its virtues, is a minority shareholder in your daily burn, which is why ‘I trained, so I can eat anything’ arithmetic fails so reliably.
Key takeaways
- BMR is 60–70% of a typical day’s burn
- It depends mostly on mass, height, age and sex
- Exercise is a smaller share than most people assume
Why Mifflin-St Jeor wins
Several equations compete. Harris-Benedict dates to 1919 and was fitted on a narrow, lean population. Katch-McArdle needs an accurate body-fat figure most people do not have. Mifflin-St Jeor, published in 1990 and validated repeatedly since — including a major review by the Academy of Nutrition and Dietetics — lands within about 10% of measured resting burn for the largest share of adults, lean and heavier alike. That is why it powers this site’s calculator: 10 × weight + 6.25 × height − 5 × age, then +5 for men or −161 for women.
| Equation | Year | Needs | Typical verdict |
|---|---|---|---|
| Harris-Benedict | 1919 | Weight height age | Often overestimates today |
| Mifflin-St Jeor | 1990 | Weight height age | Closest for most adults |
| Katch-McArdle | — | Accurate body fat % | Excellent if the input is honest |
The activity multiplier: honest guesswork
TDEE multiplies BMR by a factor from 1.2 (desk-bound) to 1.9 (athlete or heavy manual work). This multiplier is the weakest joint in the whole structure: self-assessment of activity is notoriously optimistic, and two ‘moderately active’ lives can differ by 400 kcal a day. The defences are to describe your day in behaviours, not adjectives — our calculator’s options are written that way — and to avoid double-counting: if your watch adds exercise calories on top, do not also select a training-level multiplier.
What BMR is not: the ‘metabolism’ of folklore
Popular talk treats metabolism as a personality — fast people burn, slow people store — and imagines differences of thousands of calories between similar bodies. Measured data is ruder: once body size and composition are accounted for, resting metabolic rates cluster tightly, with most of the remaining spread explained by fat-free mass. The friend who ‘eats anything’ usually moves more, fidgets more, and misremembers lunches at the same rate as everyone else. This is oddly hopeful news: the number is not a curse or a gift, it is mostly a description of the machine’s size, and machines can be nudged at the margins that matter — mass, muscle, movement.
Where real outliers exist — thyroid disorders, certain medications, recovery from illness — they belong to medicine, and an equation’s job is only to make the outlier visible by comparison with your trend.
NEAT: the multiplier hiding inside your day
Between formal exercise and rest sits non-exercise activity thermogenesis — NEAT: the fidgeting, standing, stair-taking, pacing-on-phone-calls burn of ordinary life. Studies that overfeed volunteers find NEAT is where the responders separate: some people unconsciously move hundreds of calories more per day when overfed, and store correspondingly less. For TDEE purposes the lesson is that two desk workers with identical workouts can honestly differ by 300–500 kcal through NEAT alone — which the activity factor cannot see and calibration can. If your measured maintenance keeps surprising the calculator, NEAT is probably the explanation, and a step count is its rough gauge.
It is also the most painless lever available: raising NEAT — standing calls, walking errands, the long way round — costs no recovery and no willpower budget, and our steps calculators exist largely to make it visible.
Protein, and the part of TDEE nobody multiplies
The calculators divide your day into burn and intake, but the composition of intake changes the arithmetic at the margins. Protein costs more to process than carbohydrate or fat — its thermic effect runs near 20–30% of its calories against roughly 5–10% for carbohydrate and 0–3% for fat — so two diets of identical labelled calories can differ by 50–100 kcal of actual net energy. More importantly, protein defends the tissue that sets BMR: while losing weight, intakes around 1.6 g per kilogram preserve measurably more lean mass, and lean mass is tomorrow’s metabolic rate. Our calculator prints a protein range with the calorie targets for exactly this reason: the deficit decides whether you shrink; the protein helps decide what you shrink from.
None of this rescues a fantasy number — a 100 kcal thermic nuance cannot out-argue a 600 kcal weekend — but it explains why two equally ‘compliant’ dieters diverge, and why the scale is not the only progress report worth reading.
Calibration: the fortnight that tells the truth
Eat at your estimated maintenance for two to three weeks, weigh most mornings after the bathroom and before food, and average each week. Flat trend: the estimate was your TDEE. Drifting down half a kilo a week: your real maintenance is about 500 kcal above the estimate. Adjust by the observed amount and the argument is over — you now own a measured number no equation can outrank. This loop is unglamorous and is the entire secret. Every successful long-term plan is this loop with better marketing.
Common mistakes
- Picking the activity factor that matches your identity instead of your Tuesday
- Adding watch exercise calories on top of a high multiplier
- Judging by single weigh-ins instead of weekly averages
Questions this raises
Usually a mix of an optimistic activity factor, under-counted intake (oils, drinks, weekends), and genuine individual variation in resting burn. The calibration loop absorbs all three without needing to assign blame.
Very lean athletes with a trustworthy body-fat measurement may do better with Katch-McArdle. Everyone else is better served by Mifflin plus calibration.
Put a number on it.
The Mifflin-St Jeor equation is the best one-line estimate of resting burn — and still only an estimate. How to use it as a starting bid, then let your own data correct it.
Open the BMR & TDEE Calculator