Metabolism, BMR and Daily Calorie Needs
Basal metabolic rate (BMR) is the energy your body uses at complete rest — breathing, circulating blood, repairing cells, running your brain. It is the largest single component of what you burn each day, usually 60–70% of the total. Everything else — digestion, walking, fidgeting, training — sits on top of it.
What makes up your daily burn
| Component | Share of daily total | What it is |
|---|---|---|
| Basal metabolic rate | 60–70% | Keeping you alive at rest |
| Thermic effect of food | ~10% | Digesting and processing what you eat |
| Non-exercise activity | 15–30% | Walking, standing, fidgeting, everyday movement |
| Exercise | 0–15% | Deliberate training |
The proportions are worth sitting with. For most people deliberate exercise is the smallest component, and incidental daily movement is several times larger. That is why activity level dominates calorie calculations far more than the choice of BMR equation.
Calculators in this section
- BMR Calculator: Calories Burned at Rest
- TDEE Calculator: Daily Calorie Needs
- BMR vs TDEE: Which Number Should You Use? Coming soon
- Macro Calculator: Protein, Carbs and Fat Targets
- Calorie Deficit Calculator for Weight Loss
- How Many Calories to Lose 1 Pound a Week? Coming soon
How BMR is estimated
Measuring BMR properly requires indirect calorimetry in a lab — analysing your exhaled air after an overnight fast. Everything you will find online instead uses a regression equation fitted to population data. Two dominate:
Mifflin-St Jeor (1990)
Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161
Harris-Benedict (revised 1984)
Men: 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
Women: 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)
Mifflin-St Jeor is the current default in clinical and sports-nutrition practice because it fits contemporary populations better. Neither equation knows anything about your body composition, so both overestimate for people carrying high body fat and underestimate for very muscular people. If that describes you, an equation based on lean mass — such as Katch-McArdle — will be closer, but it needs a body-fat measurement first.
Calculate your BMR using both equations side by side.
From BMR to daily calories
Total daily energy expenditure is BMR multiplied by an activity factor. These multipliers are broad brush, and the honest reading is that they define a range rather than a number.
| Level | Typical pattern | Multiplier |
|---|---|---|
| Sedentary | Desk job, little or no exercise | × 1.2 |
| Lightly active | Light exercise 1–3 days a week | × 1.375 |
| Moderately active | Moderate exercise 3–5 days a week | × 1.55 |
| Very active | Hard exercise 6–7 days a week | × 1.725 |
| Extremely active | Physical job or twice-daily training | × 1.9 |
Note the spread: moving from sedentary to extremely active multiplies the same BMR by 1.58. For a BMR of 1,600 that is a swing of roughly 1,100 calories a day. Since activity level is self-reported and people consistently overestimate it, this single input introduces more error than every other part of the calculation combined. When in doubt, pick the lower option and adjust based on what actually happens over a few weeks.
Calculate your TDEE and see calorie targets for losing, maintaining or gaining.
Why the numbers move
A calorie target is not a fixed property of your body. As you lose weight there is less of you to maintain, so BMR falls — and beyond that purely mechanical effect, energy expenditure tends to fall somewhat further during sustained restriction, partly through reduced spontaneous movement. The practical consequence is that a deficit which worked at the start becomes smaller in effect over time.
This is why recalculating every few kilograms beats holding one figure for months, and why a stall is usually a moved target rather than a failure of effort.
Common questions
- What does metabolism actually mean?
- Metabolism is the sum of every chemical process that keeps you alive — converting food into energy, building and repairing tissue, and clearing waste. In everyday use, "metabolism" usually means metabolic rate: how many calories those processes consume.
- Can you have a slow metabolism?
- Metabolic rate varies between people of the same size, but the range is narrower than commonly believed — typically within about 200 to 300 calories a day once body size and composition are accounted for. Most large differences in daily calorie burn come from body size and activity, not from an unusually slow or fast metabolism.
- Does metabolism slow down with age?
- Yes, though less dramatically and later than the popular account suggests. Research published in 2021 found that energy expenditure adjusted for body composition stays broadly stable from about age 20 to 60, then declines gradually. Much of the weight gain attributed to a slowing metabolism reflects falling activity and lost muscle mass instead.
- Why do calorie calculators give me different numbers?
- Two reasons. They use different BMR equations — Mifflin-St Jeor and Harris-Benedict can differ by around 100 calories — and they apply different activity multipliers. The activity factor causes far more variation than the equation does, and self-reported activity is the least reliable input in the whole calculation.