Every calorie target you have ever seen — every "eat 1,800 calories a day" — is built on one number most people have never calculated: your basal metabolic rate, or BMR. It is the energy your body burns just staying alive. Before a single step, your body has already spent a measurable number of calories keeping your heart beating and your brain running. Here is that number, worked out by hand.
What BMR Actually Is
Basal metabolic rate is the calories you burn at complete rest — lying still, warm, fasting. For most adults it is the biggest slice of the day's burn, roughly 60 to 70 percent of total daily energy expenditure; the rest is walking, working, exercising, digesting.
Take a 35-year-old woman who weighs 70 kg and is 165 cm tall. Her BMR is about 1,395 calories a day — roughly 58 calories every hour, including the hours she spends asleep. Her total daily burn, lightly active, is 1,918, so about 73 percent of everything she burns in a day happens before activity is even counted. Get BMR wrong and everything built on it is wrong too.
The Formula: Mifflin-St Jeor
The equation dietitians use most is Mifflin-St Jeor, published in 1990 and still the recommended default because it beats the older formulas in head-to-head validation:
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
The only difference between the two lines is the final constant: −161 for women, +5 for men.
Worked Example: A 35-Year-Old Woman
Our example woman: 35 years old, 70 kg, 165 cm.
- Weight term: 10 × 70 = 700
- Height term: 6.25 × 165 = 1,031.25
- Age term: 5 × 35 + 161 = 336 to subtract
- Put it together: 700 + 1,031.25 − 336 = 1,395.25
Her BMR is about 1,395 calories a day. Skip the arithmetic anytime with the free bmr calculator — it runs the same Mifflin-St Jeor math and compares all three formulas for you.
Three Formulas, One Woman — and a 71-Calorie Spread
The same woman gets three different answers:
- Mifflin-St Jeor: 1,395 calories a day (uses weight, height, age, sex)
- Katch-McArdle: 1,459 — it uses lean body mass instead of total weight. Her body fat is 28%, so her lean mass is 70 × (1 − 0.28) = 50.4 kg, and BMR = 370 + 21.6 × 50.4 = 1,459
- Harris-Benedict (1919): 1,466 — the oldest formula, using its precise original coefficients
The spread is 71 calories, all within about 5 percent. That is normal: every BMR equation is a population estimate with roughly ±10 percent error for an individual. If two calculators disagree by 50 to 100 calories, neither is broken. Use Mifflin-St Jeor unless you know your body fat percentage, in which case Katch-McArdle is usually the better pick. A basal metabolic rate calculator that shows all three side by side makes the comparison visible instead of mysterious.
BMR Is Not Your Calorie Target
This is the mistake that matters most. BMR is what you burn lying still all day — the floor, not the target. Total daily energy expenditure (TDEE) multiplies BMR by an activity factor: sedentary 1.2 gives her 1,674, lightly active 1.375 gives 1,918, moderately active 1.55 gives 2,163, and extra active 1.9 gives 2,651. Diet and weight-loss targets are built from TDEE, never from BMR. Eating at your BMR while exercising daily is an accidental crash diet — and the reason so many "1,400-calorie diets" feel impossible is that 1,400 is someone's BMR, not their TDEE.
Average BMR by Age: The Slow Slide
BMR falls with age, and Mifflin-St Jeor shows exactly why: the −5 × age term removes 5 calories a day for every year, or 50 per decade. For a woman of 60 kg and 165 cm: age 20, BMR 1,370; age 30, 1,320; age 40, 1,270; age 50, 1,220; age 60, 1,170. For a man of 75 kg and 178 cm it runs 1,768 down to 1,568. The decline is mostly muscle loss, not age itself. Muscle burns roughly 6 calories per pound per day at rest, against about 2 for fat — so the person who keeps their muscle keeps much more of their BMR.
Can You Increase Your BMR?
Yes, but honestly and slowly. Build muscle: five kilograms (11 lb) of extra muscle adds roughly 66 calories a day to BMR (11 × 6) — strength training 2–3 times a week is how you get it. Eat enough protein: its thermic effect is 20–30%, so your body burns a fifth to nearly a third of protein calories just digesting them, against 5–10% for carbs and 0–3% for fat. At 1.6–2.2 g per kg, a 70 kg person needs about 112–154 g a day. Don't crash-diet: severe restriction triggers metabolic adaptation and costs muscle along with fat, which lowers BMR — the opposite of the goal. Forget "metabolism-boosting" drinks: the lever is muscle, built over months.
5 Mistakes That Break the Calculation
- Using pounds and inches. Convert first: the formula needs kilograms and centimetres (154 lb is 70 kg; 5 ft 5 in is 165 cm).
- Eating at your BMR. BMR is the floor. Build every target from TDEE instead.
- Using the wrong formula for your body. Very muscular or very lean? If you know your body fat percentage, Katch-McArdle beats Mifflin-St Jeor for you.
- Forgetting BMR moves. Lose 5 kg and your BMR drops — our example woman at 65 kg has a BMR of 1,345, not 1,395. Recalculate every few weeks.
- Trusting a single number to the calorie. Every formula carries roughly ±10% error for an individual. Treat 1,395 as "about 1,400", not as gospel.
The Bottom Line
Your BMR is the calories you burn doing nothing — about 1,395 a day for our example woman, 58 an hour, roughly 73 percent of her lightly-active day. One formula, four numbers in, and you can check any app's number by hand. From there, multiply by your activity factor to get TDEE, and build every eating target on that. For the instant version, use the free bmr calculator.
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