BMR — Basal Metabolic Rate — is one of those acronyms that sounds technical but is actually quite simple. Understanding yours is the foundation of any sensible weight-loss or muscle-gain plan.
What is BMR, in plain English?
Your BMR is the number of calories your body burns just to stay alive — keeping your heart beating, your lungs breathing, your brain thinking, your kidneys filtering and your cells repairing. If you lay perfectly still in bed for 24 hours and didn't move a muscle, you'd still burn this many calories.
For most UK adults, BMR sits between 1,200 and 1,800 calories per day, though it varies considerably by sex, age, height and weight.
Typical BMR for UK adults
| Profile | Approximate BMR |
|---|---|
| Woman, 30, 5'4", 11 stone | ~1,400 kcal |
| Woman, 50, 5'6", 13 stone | ~1,450 kcal |
| Man, 30, 5'10", 14 stone | ~1,800 kcal |
| Man, 50, 6'0", 15 stone | ~1,750 kcal |
How is BMR calculated?
The most accurate widely-used formula is Mifflin–St Jeor (1990), which the British Dietetic Association endorses:
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
Worked example
A 35-year-old woman, 165cm tall, 65kg:
BMR = (10 × 65) + (6.25 × 165) − (5 × 35) − 161
= 650 + 1,031 − 175 − 161
= 1,345 kcal/day
That's the calories her body burns at complete rest. Anything she does on top — walking, cooking, working — burns more.
BMR vs TDEE — what's the difference?
This is where most people get confused.
- BMR = calories burned at rest (just staying alive)
- TDEE = Total Daily Energy Expenditure (BMR + everything you do all day)
To get TDEE from BMR, multiply by an activity factor:
| Lifestyle | Multiplier |
|---|---|
| Sedentary (desk + Netflix) | BMR × 1.2 |
| Lightly active (1–3 workouts/week) | BMR × 1.375 |
| Moderately active (3–5 workouts/week) | BMR × 1.55 |
| Very active (6–7 workouts/week) | BMR × 1.725 |
So our 35-year-old woman with a BMR of 1,345 kcal who exercises 3 times a week has a TDEE of roughly 1,345 × 1.375 = 1,850 kcal/day. That's how many calories she actually burns on a typical day.
Why does BMR matter for weight loss?
Three reasons:
- It sets your floor. You should never eat below your BMR for any extended period — your body will dial down all the systems it can to compensate (energy levels, mood, hormones, muscle).
- It tells you how big a deficit you can run. Subtract 500–600 kcal from your TDEE for steady weight loss. If your TDEE is 1,800, your target is 1,200–1,300. If your TDEE is 2,500, your target is 1,900–2,000.
- It changes as you lose weight. Lighter body = lower BMR. This is why weight loss slows over time and why you need to recalculate every stone or so.
What affects your BMR?
- Muscle mass — every kg of muscle burns ~13 kcal/day even at rest. More muscle = higher BMR.
- Age — BMR drops about 1–2% per decade after 30, mainly due to muscle loss.
- Sex — men typically have higher BMRs because they have more muscle and less body fat.
- Body size — bigger bodies burn more, even at rest.
- Genetics — yes, some people genuinely have faster metabolisms. The variation is about ±10% though, smaller than people often assume.
- Thyroid function — an underactive thyroid lowers BMR significantly. If yours feels unusually low, your GP can do a blood test.
Can you increase your BMR?
The biggest lever is building muscle. A 5kg increase in muscle adds about 65 kcal/day to your BMR — modest but compounding. Resistance training 2–3 times a week is the most efficient way.
Things that don't meaningfully increase BMR despite the marketing:
- Green tea (effect is real but tiny — ~80 kcal/day at most)
- "Fat-burning" supplements
- Eating "small frequent meals" (myth — total daily intake is what matters)
- Spicy food (effect is negligible)
The bottom line
Your BMR is the calories you burn just being alive — typically 1,200–1,800 kcal for UK adults. Multiply by an activity factor to get TDEE (your real daily burn), then subtract 500–600 for weight loss. Use the calculator below to find yours.
Sources: Mifflin MD, St Jeor ST, et al. (1990) "A new predictive equation for resting energy expenditure in healthy individuals." American Journal of Clinical Nutrition. British Dietetic Association. NHS Live Well.