BMR Calculator

What Is BMR?

Basal Metabolic Rate (BMR) is the minimum number of calories your body needs to perform basic life-sustaining functions — breathing, blood circulation, cell production, nutrient processing, and temperature regulation. Think of it as the energy cost of simply being alive.

Your brain alone consumes roughly 20% of your BMR despite making up only about 2% of your body weight. Your liver, kidneys, and heart account for another large chunk.

The Mifflin-St Jeor Equation

This calculator uses the Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate BMR formula for the general population. It replaced the older Harris-Benedict equation (1919).

The formulas:

• Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
• Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

Worked Example: BMR for a Man and a Woman

Adam is 35, stands 180 cm and weighs 82 kg. Maria is also 35, stands 165 cm and weighs 65 kg. Here is every term of the equation filled in, nothing hidden.

Adam burns 430 kcal more at rest, and the gap splits three ways. His extra 17 kg is worth 170 kcal, his extra 15 cm is worth 94 kcal, and the male constant sits 166 kcal above the female one. Age cancels out, because both are 35.

Neither figure is a calorie target. BMR only covers what the body spends lying still, so you multiply it by an activity factor to reach TDEE, the number a diet actually runs on. The difference between BMR and TDEE goes deeper on where those factors come from.

Notice the overlap. Maria training six days a week needs 2,321 kcal, more than Adam sitting at a desk on 2,130 kcal, even though his BMR is 430 kcal higher than hers. What you do with your day moves the number further than how big you are.

BMR Reference Table by Age and Weight

Use these to sanity-check the result above. The men's grid assumes 175 cm, the women's 163 cm. Differ in height? Add or subtract 63 kcal for every 10 cm.

Two patterns fall straight out of the grid: every 10 kg of body weight is worth 100 kcal, and every 20 years of age costs 100 kcal. If your calculated BMR sits more than about 150 kcal away from the nearest cell, check the units you typed before you trust it. Women's numbers cluster low enough that a standard deficit gets tight fast; women's calorie needs covers what that means in practice.

Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle

Three equations dominate the field. They ask for different inputs, and they disagree by amounts worth knowing about.

• Mifflin-St Jeor (1990): needs sex, weight, height and age. Frankenfield's 2005 review ranked it first among the predictive equations tested, in both lean and obese adults. It's the default here and in most clinical software. How Mifflin and St Jeor derived it.
• Harris-Benedict (1919): the same four inputs. Its coefficients came from 239 subjects measured before 1918, when average body composition looked different from today's. Roza and Shizgal published revised coefficients in 1984 that most modern implementations use.
• Katch-McArdle: lean body mass only, via 370 + (21.6 × lean mass in kg). It ignores sex, height and age because lean mass already encodes them. That only helps if the body-fat figure behind it is real.

Harris-Benedict comes out 83 kcal high for Adam and 73 kcal high for Maria, roughly 5% in both cases. The Katch-McArdle row assumes 18% body fat for Adam and 28% for Maria. Move Adam to 25% body fat at the same 82 kg and Katch-McArdle drops 124 kcal, while Mifflin-St Jeor doesn't shift at all. It never asked.

Factors That Affect Your BMR

Your BMR isn't fixed — several factors influence how many calories you burn at rest:

• Muscle mass: Muscle burns ~6 kcal/lb at rest compared to fat at ~2 kcal/lb. More muscle = higher BMR.
• Age: You lose ~3–8% of muscle mass per decade after 30 (a process called sarcopenia), which gradually lowers BMR.
• Sex: Men typically have a 5–10% higher BMR than women of similar size due to greater lean mass.
• Genetics: Variation of up to 200–300 kcal/day between individuals of similar size and composition (Bouchard, 1990).
• Thyroid function: Hypothyroidism can decrease BMR by 5–15%; hyperthyroidism can increase it.
• Temperature: Cold exposure activates brown fat and increases BMR — your body burns extra calories to stay warm.

How Accurate Is a BMR Calculator?

Frankenfield's headline number cuts both ways. The equation lands within 10% of measured values for 82% of people, which leaves about one person in five outside that band. On Adam's 1,775 kcal, 10% is 178 kcal, a plate of pasta either way.

The equation reads four numbers off you. It can't see what sits underneath them:

• Body composition: two men at 82 kg and 180 cm get an identical answer whether they carry 12% or 30% body fat, though their lean mass differs by about 15 kg.
• Thyroid output: hypothyroidism can pull resting burn down 5–15%. No predictive equation asks about it.
• Adaptation to dieting: Leibel, Rosenbaum and Hirsch (NEJM, 1995) found total energy expenditure running below what body size alone predicted after a 10% weight loss. The formula just reads the new weight and returns the textbook figure.
• Measurement conditions: true BMR demands an overnight fast, complete rest and a thermoneutral room. Labs usually report RMR instead, which tends to run about 10% higher.

Treat the result as a starting estimate carrying a couple of hundred kcal of uncertainty. Replacing it with something real takes two to three weeks: hold intake at a fixed number, weigh yourself on the same mornings, and read the trend. Flat weight means that intake is your true maintenance, measured rather than predicted. The logging errors that ruin this test are worth reading first, because a 20% under-report makes the whole exercise meaningless.

What to Do With Your BMR Number

BMR is an input, not an answer. Four steps turn it into something you can eat by.

• 1. Pick the activity factor honestly. Most people who call themselves moderately active train closer to lightly active. For Adam that single choice is worth 310 kcal a day.
• 2. Take the deficit from TDEE, never from BMR. Cutting 500 kcal off Adam's moderately active 2,751 leaves 2,251 kcal, still 476 kcal clear of his resting burn. How a calorie deficit actually works explains why that headroom matters.
• 3. Check the floor. If the target lands under 1,200 kcal, either the arithmetic slipped or the deficit is too steep. Run it past a doctor or dietitian before eating there.
• 4. Set protein before anything else. Protein carries a 20–30% thermic effect and defends lean mass while you're in a deficit, which defends the BMR you just calculated. See how much protein you need per day.

The TDEE calculator handles step one, and the calorie deficit calculator handles step two with a target date attached. If you're over 40 and the numbers feel lower than they used to be, tracking calories after 40 covers what changes.

How to Increase Your BMR

While you can't control your age or genetics, there are evidence-based ways to raise your resting metabolism:

• Resistance training: Building muscle is the most effective long-term strategy. Muscle tissue is metabolically active even at rest.
• Higher protein intake: Protein has a thermic effect of 20–30% (your body burns 20–30% of protein calories just digesting them), compared to 5–10% for carbs and 0–3% for fat.
• Avoid crash diets: Extreme calorie restriction (below 1,200 kcal) can lower BMR by 15–20% through metabolic adaptation — your body fights back by burning less.

BMR Questions People Actually Ask

• Is my BMR the same as my daily calorie goal?
  No. BMR covers rest only. Multiply it by an activity factor between 1.2 and 1.9 to reach TDEE, then add or subtract from there. Adam's BMR is 1,775 kcal and his moderately active maintenance is 2,751 kcal, a difference of 976 kcal.
• Why does another site give me a different BMR?
  It's running a different equation. Harris-Benedict returns 1,858 kcal for the same man Mifflin-St Jeor puts at 1,775, an 83 kcal gap of roughly 5%. Neither site is measuring you; both are predicting.
• How accurate is a BMR calculator?
  Frankenfield et al. (2005) found Mifflin-St Jeor within 10% of measured values for 82% of the adults tested. About one in five sits outside that, which on a 1,775 kcal result means more than 178 kcal in either direction.
• Can I eat at my BMR to lose weight?
  BMR isn't built to be a target. It's the rest-only floor of your expenditure, and you burn above it every day you get out of bed, so deficits are taken from TDEE instead. If a calculated target falls below 1,200 kcal, check it with a doctor or dietitian first.
• What's the difference between BMR and RMR?
  BMR is measured after an overnight fast, at complete rest, in a thermoneutral room. RMR uses looser conditions and typically comes out about 10% higher. Mifflin's 1990 study actually measured resting energy expenditure, so the two labels get used interchangeably in practice.
• Does BMR really drop with age?
  The equation subtracts 5 kcal per year, so a 65-year-old and a 25-year-old of identical size differ by 200 kcal. Muscle loss does the real work underneath: adults lose 3–8% of muscle mass per decade after 30 unless they train against it.
• Do I need my body fat percentage?
  Not for this calculator. Mifflin-St Jeor uses sex, weight, height and age only. Body fat matters for Katch-McArdle, which derives BMR from lean mass alone and shifts 124 kcal when body fat moves from 18% to 25% at a fixed weight.
• Does BMR change when I lose weight?
  Yes, twice over. Ten kilograms off lowers the equation's answer by 100 kcal. On top of that, Leibel and colleagues (NEJM, 1995) measured expenditure sitting below prediction after a 10% weight loss, so the real drop tends to exceed the arithmetic one.