Whether you want to lose fat, maintain weight or build muscle, the first number you should know is your daily calorie maintenance. Not the generic one printed on packaging ("the average diet is 2000 kcal"), but your number — the one that depends on your weight, height, age, sex and activity level.
Without that number you're guessing. You eat "a bit less" without knowing how much less; you train "hard" without knowing if it actually offsets dinner. The good news: working it out takes a minute and one formula that holds up for most people.
BMR: what you burn at rest
BMR stands for Basal Metabolic Rate — the energy your body spends at complete rest, just to stay alive: breathing, heartbeat, body temperature, brain activity, organ function. It's the floor of your daily expenditure: even if you stayed in bed for 24 hours you'd still burn at least your BMR.
The most accurate formula in clinical use today is Mifflin-St Jeor (1990), recommended over the older Harris-Benedict by the Academy of Nutrition and Dietetics.
BMR (men) = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
BMR (women) = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
Worked example
Mark, 35, 78 kg (172 lb), 178 cm (5'10"):
BMR = (10 × 78) + (6.25 × 178) − (5 × 35) + 5
= 780 + 1112.5 − 175 + 5
= 1722 kcal/day
Mark would burn around 1,722 kcal even lying still all day. That's the starting point, not the destination.
TDEE: what you actually burn
TDEE stands for Total Daily Energy Expenditure — your real calorie need, because it covers everything beyond breathing: walking, working, training, climbing stairs, even chewing and digesting (yes, that's a real cost — it's called TEF, Thermic Effect of Food).
You get TDEE by multiplying BMR by an activity factor:
| Level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, little movement, no sport | × 1.2 |
| Light | Semi-active job or 1-3 light workouts/wk | × 1.375 |
| Moderate | 3-5 medium-intensity workouts/wk | × 1.55 |
| Active | 6-7 hard workouts/wk or physical job | × 1.725 |
| Very active | Athlete, double sessions or heavy labor | × 1.9 |
Full worked example
Julia, 30, 65 kg (143 lb), 165 cm (5'5"), goes to the gym 3 times a week and walks 30 minutes a day → moderate.
BMR = (10 × 65) + (6.25 × 165) − (5 × 30) − 161
= 650 + 1031.25 − 150 − 161
= 1370 kcal
TDEE = 1370 × 1.55 = 2124 kcal/day
Julia's real maintenance is roughly 2,124 kcal. If she ate 1,700 kcal/day (a 20% deficit) she'd lose about 0.5 kg of fat per week.
The mistakes that wreck the math
Even with the right starting number, most people fail for two well-documented reasons.
1. Underestimating food intake (by 20–40%)
Classic studies like Lichtman et al. (NEJM, 1992) showed that people convinced they were "barely eating" were under-reporting actual intake by up to 47%. It's not lying — it's the splash of olive oil, the handfuls of nuts, the slice of cheese "just to taste", the glass of wine forgotten. They add up.
The fix isn't weighing food forever, but doing a reality check for 1-2 weeks: weigh today, log everything, and you'll see exactly where eyeballing breaks down.
2. Overestimating activity
A "hard" hour at the gym rarely burns 700 kcal like the heart-rate monitor claims. More realistically it's 300-400 for an average person, and some of those would have been burned anyway. Same with "I walk a lot" — usually that's 5,000 steps, not 12,000.
What to do with the number
Once you know your TDEE, you have three options:
- Maintain weight — eat at TDEE, don't worry about ±5% swings.
- Lose fat — subtract 10–20% (caloric deficit). For Julia: 2124 − 20% ≈ 1700 kcal. Expect 0.5–1 kg/week.
- Gain muscle — add 10–15% (surplus). For Mark: 2900 × 1.10 ≈ 3200 kcal. Expect 0.2–0.4 kg/week, mostly muscle if you lift.
Important: this is a predictive estimate, not a measurement. The formulas have a 10–15% error margin compared to indirect calorimetry (the lab measurement). Adjust over the next 2-3 weeks based on what the scale and the mirror actually show.
Ready to calculate yours?
If you want to skip the manual math, we have a free maintenance calorie calculator that does it in 30 seconds and even suggests a macro split. No account, no email — just the number.
And if you want to actually understand where your calories go every day without weighing food for the rest of your life, dyllo does it for you: write or speak what you ate, the AI parses and logs it. That simple.
