Basal metabolic rate (BMR)

What your body burns doing nothing at all, what it includes, what actually moves it, and how big a deficit you need to lose weight at a given rate.

What basal metabolic rate means

Basal metabolic rate is the energy your body spends staying alive while doing nothing at all: lying still, awake, at a comfortable temperature, having not eaten for twelve hours. It is not the energy of a lazy day. It is the floor beneath every day.

For most people BMR accounts for 60 to 75 percent of total daily energy expenditure, which makes it by far the largest single component, and the one you have the least direct control over.

What BMR actually pays for

The energy goes to organ tissue that never stops working. Approximate shares in a typical adult:

The distribution explains a common frustration. Organ mass is roughly fixed, and resting muscle burns only about 13 kcal per kg per day — so gaining 5 kg of muscle, which takes years of serious training, raises BMR by only about 65 kcal a day. Fat tissue burns about 4.5 kcal per kg per day, so it is not metabolically inert either, just quieter.

BMR, RMR and TDEE

• BMR is measured under strict fasted, rested, thermoneutral conditions.
• RMR (resting metabolic rate) is measured under looser conditions and comes out about 5 to 10 percent higher. Most calculators, including this one, blur the two; the difference is smaller than the error in any formula.
• TDEE is BMR plus everything else: the thermic effect of food (roughly 10 percent of intake), non-exercise activity, and deliberate exercise. TDEE is the number that decides whether you gain or lose.

The formulas

Every BMR calculator is a regression equation fitted to measured data. They give an estimate with a real error bar, typically plus or minus 10 percent for about 80 percent of people, and worse at the extremes of body composition.

Mifflin-St Jeor (1990) — the default

Derived from a more modern and more representative sample than its predecessor. The Academy of Nutrition and Dietetics recommends it for non-obese and obese adults alike, and it is the formula most clinical software defaults to.

Harris-Benedict (1919, revised 1984)

The original, still widely quoted. It tends to overestimate by roughly 5 percent in modern populations, largely because the 1919 cohort was leaner than today's.

Katch-McArdle

The only one of the three that makes no distinction by sex, because lean mass already carries that information. It is the best choice for very lean or very muscular people, but only if your body fat figure is genuinely accurate — and consumer bioimpedance scales frequently are not.

What actually changes your BMR

Things you cannot change much

• Body size. The dominant factor. A bigger body has more tissue to maintain, which is why weight appears in every formula.
• Age. A decline of roughly 1 to 2 percent per decade after age 20, driven mostly by lean-mass loss. Recent large-scale metabolic research suggests the drop is smaller and later than long assumed, with expenditure fairly stable from about 20 to 60 once body composition is accounted for.
• Genetics. Two people matched on age, sex, weight, and body composition can still differ by 100 to 200 kcal a day.
• Thyroid function. Hypothyroidism can lower BMR by 15 to 30 percent; hyperthyroidism raises it. This is a medical matter, not a lifestyle one.

Things you can influence

• Build muscle. Real but modest: roughly 13 kcal per kg of muscle per day. The larger benefit is indirect — more muscle means harder training, which raises total expenditure well beyond the BMR change.
• Do not crash diet. Aggressive restriction reduces BMR beyond what the weight loss alone predicts, an effect called adaptive thermogenesis, typically 10 to 15 percent. Moderate deficits with adequate protein blunt it.
• Eat enough protein. Protein has a thermic effect of 20 to 30 percent of its own energy, against 5 to 10 percent for carbohydrate and 0 to 3 percent for fat. This raises TDEE rather than BMR, but it lands in the same budget. It also preserves lean mass during a deficit, which protects BMR.
• Sleep properly. Restriction to 4 to 5 hours measurably lowers resting expenditure and disrupts the hormones governing appetite.
• Move more outside the gym. Non-exercise activity thermogenesis — walking, standing, fidgeting, housework — varies by up to 2,000 kcal a day between individuals. It is not BMR, but it is the largest lever most people have.

How many calories to cut to lose weight

Weight loss requires taking in less energy than you spend. The arithmetic starts from the energy stored in body fat:

So a sustained deficit of 500 kcal a day is 3,500 kcal a week, which is about 1 lb or 0.5 kg. That is where the standard advice comes from.

Three caveats matter more than the table does:

1. The 7,700 kcal rule overpredicts long-run loss. It assumes a constant deficit against a constant expenditure. In reality, as you get lighter your TDEE falls, so the same intake becomes a smaller deficit and weight loss decelerates and eventually stops. Expect the first weeks to track the prediction and later weeks to fall behind it.
2. Early loss is mostly water. Each gram of stored glycogen holds about 3 g of water. The dramatic first-week drop on a low-carbohydrate diet is largely that, and it returns the moment carbohydrate does.
3. Percentages beat fixed numbers. A 500 kcal deficit is 20 percent for someone maintaining at 2,500 and 33 percent for someone at 1,500. Aiming for a 15 to 25 percent reduction from TDEE scales sensibly to body size.

To convert any of these targets into kilojoules, the converter on the home page does it in one step. To see how a given deficit maps onto real food, the food pages list calories and kilojoules per portion.

Common questions