Your basal metabolic rate is the energy your body burns doing nothing at all, fully at rest, not even digesting. It typically covers 60-70% of your daily calories, which means the biggest burner in your life is you. One equation, Mifflin-St Jeor, estimates it from four numbers you already know, and it's accurate to within about 10% for most healthy adults. Here's the math, worked examples, what a normal BMR looks like, and what to do with the number once you have it.
BMR is the cost of being alive. Your brain, liver, heart, and kidneys alone account for most of it, with breathing, blood circulation, and body temperature regulation making up the rest. It's measured in a lab under strict conditions: fasted, lying still, in a room that's neither warm nor cold. No digestion, no movement, no thinking harder than usual.
Everything you burn in a day stacks on top of it. Digesting food adds roughly 10% of your intake. Physical activity, from workouts down to walking to the mailbox, adds the rest. That total is your TDEE (total daily energy expenditure), and BMR is its floor, which is why two couch potatoes of different sizes can burn wildly different amounts doing absolutely nothing.
The recommended general-population formula is Mifflin-St Jeor, published in 1990:
Worked examples. A 30-year-old man, 75 kg, 175 cm: 750 + 1,094 − 150 + 5 = 1,699 kcal/day. A 30-year-old woman, 63 kg, 165 cm: 630 + 1,031 − 150 − 161 = 1,350 kcal/day. Notice what drives the two apart: not metabolism, just the inputs. The sex constant alone is a 166 kcal/day gap, and each kilogram of body weight carries about 10 kcal/day with it.
You can run your own numbers in ten seconds with the BMR calculator, which also applies the activity multipliers described below.
Several equations are still in circulation, and they don't agree, which is why different calculators give you different answers:
| Formula | Year | Inputs | Notes |
|---|---|---|---|
| Mifflin-St Jeor | 1990 | Sex, weight, height, age | Current general-population default; within ~10% of lab values for most adults |
| Harris-Benedict (rev. 1984) | 1919/1984 | Sex, weight, height, age | Still powers many websites; runs roughly 5% high for most people |
| Katch-McArdle | — | Lean body mass | Better for muscular or lean people; requires knowing body fat % |
| Cunningham | 1980 | Lean body mass | Similar logic to Katch-McArdle; favored in some sports-science settings |
The Katch-McArdle formula is 370 + 21.6 × lean mass in kg. Run our example man through it assuming he's 17% body fat: lean mass of 62.25 kg gives 370 + 1,345 = 1,715 kcal/day, a whisker from Mifflin's 1,699. When the two disagree badly, trust the lean-mass version, because it's the only one that can see your composition.
There's no single normal, size sets the range. But computed reference points help you sanity-check a result:
| Profile (Mifflin-St Jeor) | Age 25 | Age 45 | Age 65 |
|---|---|---|---|
| Woman, 163 cm (5'4"), 63 kg (139 lb) | 1,363 | 1,263 | 1,163 |
| Man, 175 cm (5'9"), 77 kg (170 lb) | 1,744 | 1,644 | 1,544 |
Most adult BMRs land somewhere between roughly 1,100 and 1,900 kcal/day, with big people and muscular people above that band. If your calculator returns 1,400 and a friend's returns 1,800, that's body size, not one of you having a broken metabolism.
Enter age, sex, height, and weight to get your BMR plus maintenance estimates at three activity levels.
BMR Calculator →BMR on its own is a lab number; you don't eat lying perfectly still. Multiply it by an activity factor to get maintenance calories:
| Activity level | Multiplier | Maintenance for BMR 1,699 |
|---|---|---|
| Sedentary (desk job, little exercise) | 1.2 | 2,039 |
| Light (exercise 1-3 days/week) | 1.375 | 2,336 |
| Moderate (exercise 3-5 days/week) | 1.55 | 2,633 |
| Active (exercise 6-7 days/week) | 1.725 | 2,931 |
| Very active (hard training or physical job) | 1.9 | 3,228 |
The honest warning: nearly everyone picks a multiplier one notch too high. If you sit for work and train three evenings, "light" is your truthful answer. From maintenance, subtract 250-500 kcal to lose or add them to gain, then let two to three weeks of actual weight data correct the estimate. The calorie calculator does the full chain in one step, and the macro calculator splits the result into protein, carbs, and fat.
By the formula, age subtracts a flat 5 kcal/day per year, about 50 per decade, which alone wouldn't matter much. The real driver is what the formula can't see: adults who don't train lose muscle steadily from their 30s on, and each lost kilogram of lean mass takes roughly 10-20 kcal/day of resting burn with it. A 60-year-old who lifts and keeps protein high can hold a BMR most 40-year-olds would be happy with.
Generally not on purpose. Deficits are meant to come off TDEE, not BMR, and eating below your resting burn while living a normal life creates an aggressive deficit that's hard to sustain and easy to rebound from. Commonly cited floors without medical supervision are about 1,200 kcal/day for women and 1,500 for men. There are supervised exceptions, but for everyone self-directing, moderate deficit, adequate protein, resistance training is the version that actually sticks. BMR equations are also off-limits as guidance during pregnancy and unreliable during illness.
BMR is your resting energy bill, typically 60-70% of everything you burn, and Mifflin-St Jeor estimates it from sex, weight, height, and age within about 10%. Multiply by an honest activity factor to get maintenance, adjust in 250-500 kcal steps for your goal, and recalibrate against the scale rather than the formula. The number is a starting point, not a diagnosis.
Almost, and the two get used interchangeably. BMR is measured under strict lab conditions, fasted and lying still in a temperature-neutral room. RMR (resting metabolic rate) is measured under looser conditions and comes out slightly higher. For practical purposes, treat formulas labeled either way as the same estimate.
No. BMR covers only resting functions: brain, heart, kidneys, breathing, temperature regulation. Digestion adds roughly 10% of intake on top (the thermic effect of food), and activity adds the rest. Stack all three and you get TDEE, total daily energy expenditure.
Usually because they use different equations on the same body. The 1919 Harris-Benedict formula and its 1984 revision still power many sites, and they run roughly 5% higher than Mifflin-St Jeor for most people, a gap of 100 kcal/day or more. Fitness trackers add their own estimates on top of either formula.
Genuinely low BMR from a medical cause, like untreated hypothyroidism, exists but is uncommon. For most people the difference between a 'fast' and 'slow' metabolism is lean mass and daily movement, both of which respond to training. If unexplained weight change comes with fatigue or other symptoms, that's a doctor conversation, not a formula problem.