BMR Calculator
Work out your Basal Metabolic Rate — the calories your body burns just staying alive. It is the floor you should never eat below, and the number most diets get catastrophically wrong. Free, and we do not hold your result hostage for an email.
What is BMR?
Your Basal Metabolic Rate is the number of calories your body burns doing nothing at all — keeping your heart beating, your lungs working, your brain running, your cells repairing. If you lay motionless in bed for 24 hours, this is what you would still burn.
It is roughly 70% of everything you burn in a day. Most people assume exercise is the big lever. It is not. Simply being alive is.
Why your BMR is the floor, not the target
This is the number that matters most, and almost nobody uses it correctly.
Your BMR is a floor. You do not eat below it. Ever.
Eat under your BMR and your body does not politely burn fat. It starts breaking down muscle, downshifts your thyroid output, tanks your energy, wrecks your training, and quietly reduces how much you move without you noticing. You lose weight for a few weeks, then stall — and the moment you eat normally again it comes back, often with interest.
That is not a willpower failure. That is what happens when you eat below the line your body needs to run itself.
BMR vs RMR vs TDEE
Three numbers, constantly confused:
- BMR — complete rest. Nothing but survival.
- RMR (Resting Metabolic Rate) — measured at rest but includes digestion and small movements. Typically within 10% of BMR, and slightly higher. In practice the two are used interchangeably.
- TDEE — everything: BMR plus digestion, plus daily movement, plus training. This is the number you actually eat against.
BMR tells you the floor. TDEE tells you the target. You need both.
Your BMR falls as you lose weight
A smaller body costs less to run. Lose 20 pounds and your BMR drops — which means the calorie target that was working now is not.
This is the single most common reason fat loss stalls at month three, and why people conclude their metabolism is broken. It is not broken. It is just smaller. Recalculate every 10 pounds or so.
Related free tools
- TDEE Calculator — the calories you burn in a full day, including activity.
- Macro Calculator — split those calories into protein, carbs and fat.
- The Edit — our nutrition and training library.
BMR, answered
What happens if I eat below my BMR?
You lose muscle alongside fat, your energy and training quality drop, and your body compensates by reducing how much you move throughout the day. Progress stalls, and the weight returns as soon as you eat normally. This calculator will not return a target below your BMR — if the deficit you select would go under it, we raise the number and tell you why.
How accurate is a BMR calculator?
The Mifflin-St Jeor equation used here is the most accurate of the common formulas for the general population, with a realistic margin of roughly plus or minus 10%. It cannot account for your muscle mass, genetics or medical history. Use it as a floor and a starting point, not a verdict.
Is BMR the same as metabolism?
Not quite. BMR is the biggest single component of your metabolism, but your total metabolism — your TDEE — also includes digestion, daily movement and exercise. When people say their metabolism is slow, they usually mean their TDEE is lower than they assumed, often because they move far less than they think.
Can I increase my BMR?
Somewhat, and slowly. Muscle is more metabolically active than fat, so building and holding muscle raises your BMR over time — which is exactly why we protect it with adequate protein and a sensible deficit rather than crash dieting. There is no supplement, food or trick that meaningfully changes this.
Most diets fail because they ignore this number. A 1,200-calorie target handed to a woman whose BMR is 1,450 is not a diet. It is a slow-motion muscle loss programme with a rebound built in.
We will not do that to you. Book a free 15-minute consult and we will tell you honestly what your body actually needs.