How to Calculate Your TDEE by Hand (And Why the Number Is a Hypothesis)
Total daily energy expenditure is the number every calorie target is built on. Here are the four equations used to estimate it, what each one needs, where each is most accurate, and the error all of them carry — plus how to replace the estimate with a measurement in about three weeks.
The two-step structure
Every method works the same way. Calculate basal metabolic rate — what you would burn at complete rest — then multiply by an activity factor.
TDEE = BMR × activity factorWorth knowing before you start: the activity multiplier usually introduces more error than the equation does, because it is self-reported and self-reported activity is consistently overestimated.
Mifflin-St Jeor
The formula
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161What it needs
Weight, height, age, sex. Nothing you have to measure specially.
Where it is most accurate
General adult populations in the normal-to-overweight range. This is the default in most clinical guidance and the one to use if you are picking one.
Its known error
Roughly ±10% for an individual, more at the extremes of body composition, because it treats a kilogram of muscle and a kilogram of fat as the same input.
Harris-Benedict (revised)
The formula
Men: BMR = 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
Women: BMR = 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)What it needs
The same four inputs.
Where it is most accurate
Largely historical. It is the oldest of these and still widely embedded in calculators, which is the main reason to recognise it.
Its known error
Tends to overestimate, commonly by 5% or so against Mifflin-St Jeor, and more in people with higher body fat. If a calculator gives you a suspiciously generous number, this is often why.
Katch-McArdle
The formula
BMR = 370 + (21.6 × lean body mass in kg)
where lean body mass = weight × (1 − body fat %)What it needs
Body fat percentage, from a method you actually trust — DEXA, or calipers used consistently. Bioimpedance scales are too variable to build on.
Where it is most accurate
People at the extremes of body composition, where weight-based equations fail most: very lean athletes and people with higher body fat. It is the only one here that distinguishes muscle from fat.
Its known error
Inherits whatever error is in your body fat measurement, and that is usually the larger term. A good equation fed a bad input is not an improvement.
Cunningham
The formula
BMR = 500 + (22 × lean body mass in kg)What it needs
Lean body mass, as above.
Where it is most accurate
Trained athletic populations, where it typically returns a higher figure than Katch-McArdle and is closer to measured values.
Its known error
Overestimates for untrained people. It is a specialist tool, not a general one.
The activity factor
| Multiplier | Description | What it usually means in practice |
|---|---|---|
| 1.2 | Sedentary | Desk job, little deliberate exercise |
| 1.375 | Lightly active | Light exercise 1–3 days a week |
| 1.55 | Moderately active | Moderate exercise 3–5 days a week |
| 1.725 | Very active | Hard exercise 6–7 days a week |
| 1.9 | Extremely active | Physical job plus hard daily training |
Choose one lower than you think. This is the single most common error in the whole calculation. People select "moderately active" for three gym sessions a week while spending the other 160 hours seated, and the gap between 1.375 and 1.55 on a 1,600 BMR is nearly 300 calories a day.
A worked example
A 35-year-old woman, 68 kg, 168 cm, three gym sessions a week and a desk job.
BMR = (10 × 68) + (6.25 × 168) − (5 × 35) − 161
= 680 + 1050 − 175 − 161
= 1394 kcal
TDEE = 1394 × 1.375 = 1917 kcalSo the estimate is about 1,900. And the honest reading of that number is that her true maintenance is somewhere around 1,600 to 2,200.
Why it is a hypothesis, not a target
Individual error on these equations runs to roughly ±10–15%. On a 2,500 estimate that is a band of about 375 calories in either direction — larger than most deficits people try to run.
Two people identical on age, sex, height and weight can genuinely differ by several hundred calories a day, through lean mass, thyroid function, non-exercise activity thermogenesis and adaptive responses to previous dieting. The equations describe a population; you are one person.
How to replace the estimate with a measurement
Three weeks, and you no longer need any of the equations above.
- Log intake accurately, every day. Accurately is doing the work in that sentence — see below.
- Weigh daily, same conditions. Morning, after the bathroom, before eating.
- Use the weekly average, never a single day. Water, glycogen and digestive contents move daily weight by more than a week of deficit does.
- Read the trend at three weeks. Stable average weight means your average intake is your maintenance. If it moved, roughly 7,700 kcal corresponds to a kilogram, so the weekly change gives you the size and direction of the gap.
This is what adaptive-target apps automate, and it is why their numbers beat any calculator: they are measuring you rather than estimating a population.
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If your real TDEE comes out below the estimate
Three explanations, in order of how often they are the answer:
1. Under-reported intake. The most common by a distance. Cooking oil, drinks, bites while cooking, portions estimated rather than weighed, and days that never got logged. The literature puts the average gap around 20%.
2. An overestimated activity multiplier. Drop one level and the discrepancy often disappears entirely.
3. Adaptive reduction. Genuine after prolonged dieting, and smaller than the first two. It is real, and it is rarely the whole explanation people want it to be.
The summary
Use Mifflin-St Jeor unless you have trustworthy body fat data, in which case use Katch-McArdle. Pick an activity multiplier one level below your instinct. Treat the result as a starting point, run it for three weeks with accurate logging and a weekly weight average, and then throw the equation away — you will have a measurement, which is strictly better than anyone's formula.
Frequently Asked Questions
How do you calculate TDEE?
Calculate BMR with Mifflin-St Jeor, then multiply by an activity factor from 1.2 (sedentary) to 1.9 (extremely active). Treat the result as a hypothesis: individual error runs to roughly 10-15% either way.
Which TDEE equation is most accurate?
Mifflin-St Jeor for general use, and Katch-McArdle if you have reliable body fat data, because it works from lean mass. But no equation is accurate for an individual — the equation you pick matters less than correcting the result with your own data.
How accurate are TDEE calculators?
Roughly ±10-15%, which on a 2,500 estimate is a band of about 375 calories. The activity multiplier contributes more of that error than the equation does, because self-reported activity is consistently overestimated.
Why is my TDEE lower than the calculator says?
Usually under-reported intake — the average gap between logged and actual is around 20% — or an activity multiplier set one level too high. Adaptive reduction after prolonged dieting is real but smaller than either.