Your basal metabolic rate, or BMR, is the number of calories your body burns at rest just to stay alive — to breathe, circulate blood, keep your organs running, and maintain body temperature. For a typical 35-year-old woman weighing 70 kg, that resting burn is about 1,395 calories a day, before a single step or workout is counted.
That number is the foundation of every calorie target: BMR usually accounts for 60 to 70 percent of all the calories you burn in a day. This guide explains what BMR is, what it covers, what raises or lowers it, and how to use it.
What Is BMR?
BMR stands for basal metabolic rate. It is the minimum energy your body needs to carry out its basic, life-sustaining functions while you are completely at rest — lying still, awake, in a comfortable temperature, having not eaten for several hours. Nothing else is included: no walking, no talking, no digesting food, no exercise.
Think of it as your body’s idling speed: your heart, lungs, liver, kidneys, and brain never stop working, and all of that costs energy.
A free bmr calculator estimates this number from your age, sex, height, and weight in seconds. But the number is more useful when you understand what sits behind it, so this guide works through the ideas — and the arithmetic — step by step.
What Does Your BMR Actually Cover?
Your resting burn is not one job; it is the sum of many small ones. The biggest shares go to the organs that never switch off. Your brain, for example, uses roughly 20 percent of your resting energy despite being only about 2 percent of your body weight. Your liver, heart, kidneys, and digestive organs take another large share, and your resting muscles account for much of the rest.
That split explains why two people of the same weight can have very different BMRs. Muscle tissue is metabolically active even at rest, while fat tissue burns far less energy per pound. A muscular 80 kg person and a sedentary 80 kg person weigh the same on the scales, but the muscular one has a higher BMR, because more of those 80 kg is lean tissue doing expensive work around the clock.
It also explains why BMR is measured under strict conditions. A true BMR reading is taken in a laboratory after an overnight fast, lying motionless in a warm, dark room. Almost nobody is ever measured that way outside research, which is why every BMR figure you see — including the ones in this guide — is an estimate from a formula, not a laboratory measurement.
How Is BMR Different from RMR?
You will often see a second term, resting metabolic rate (RMR), used as if it meant the same thing. The two are close cousins, not twins. BMR is measured under the strict laboratory conditions described above. RMR is measured under looser, real-world conditions: you may have eaten a few hours earlier, sat upright rather than lying flat, and moved around a little beforehand.
Because of those extras, RMR usually comes out slightly higher than BMR, typically by around 10 percent. In practice, most calculators use the two terms interchangeably. If an app tells you your “resting calories” are a little higher than the number you calculate here, the BMR-versus-RMR gap is the most likely reason, not an error.
Worked Example: A 35-Year-Old Woman’s BMR
The standard formula dietitians use is Mifflin-St Jeor, published in 1990 and validated against measured resting energy in hundreds of adults. A 2005 review by the Academy of Nutrition and Dietetics found it was the equation most often within 10 percent of the measured value. For women it is:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Take a 35-year-old woman who weighs 70 kg and is 165 cm tall:
- Weight term: 10 × 70 = 700
- Height term: 6.25 × 165 = 1,031.25
- Age term: 5 × 35 = 175
- BMR = 700 + 1,031.25 − 175 − 161 = 1,395 calories a day
So her body burns about 1,395 calories a day before she does anything at all. Two other respected formulas give nearby answers for the same woman, which is a useful cross-check. The Katch-McArdle formula, which uses her 28 percent body fat to work from lean mass (70 × 0.72 = 50.4 kg of lean mass), gives 370 + 21.6 × 50.4 = 1,459 calories a day. The older Harris-Benedict formula gives 1,466 calories a day. The three answers sit within 71 calories of each other — a reminder that the choice of formula matters far less than getting your activity level right, which moves the final number by hundreds. Our guide to how to calculate BMR works through all three formulas step by step.
What Affects Your BMR?
Your BMR is not a fixed number. It moves with your body, and five factors do most of the moving:
- Age. BMR falls by roughly 5 calories a year, mostly because muscle mass declines with age. A 55-year-old with the same weight, height, and activity as a 35-year-old burns about 100 fewer calories a day at rest.
- Sex. Men typically have a higher BMR than women of the same weight and age, because they usually carry more muscle and less body fat. In the Mifflin-St Jeor formula this shows up as a 166-calorie gap: the same 70 kg, 165 cm, 35-year-old as a man would have a BMR of 1,561, not 1,395.
- Weight and height. Bigger bodies cost more to run. Every extra kilogram of body weight adds about 10 calories a day to BMR in the Mifflin-St Jeor formula, and every extra centimetre of height adds 6.25.
- Body composition. This is the factor you can actually change. Muscle burns far more energy at rest than fat — roughly 6 calories per pound of muscle per day, against about 2 per pound of fat. Two people at the same weight can differ by a hundred calories or more purely because one is more muscular.
- Hormones and health. Thyroid hormones set the pace of your metabolism, so an overactive thyroid raises BMR and an underactive one lowers it. Fever, pregnancy, and recovering from illness or injury also raise resting burn temporarily.
Genetics plays a role too, but age, sex, size, and muscle explain most of the difference you will ever see between two real people.
From BMR to the Calories You Actually Burn
BMR is only the starting point. You do not lie in bed all day, so your real daily burn — your total daily energy expenditure, or TDEE — is your BMR multiplied by an activity factor:
| Activity level | What it looks like | Multiplier |
|---|---|---|
| Sedentary | Desk job, little or no exercise | 1.2 |
| Lightly active | Light exercise 1–3 days a week | 1.375 |
| Moderately active | Moderate exercise 3–5 days a week | 1.55 |
| Very active | Hard exercise 6–7 days a week | 1.725 |
| Extremely active | Physical job plus daily training | 1.9 |
For our 35-year-old woman, lightly active means 1,395 × 1.375 = 1,918 calories a day. That is her maintenance level: eat about 1,918 calories and her weight stays roughly stable. Most people pick one activity level too high, which is the single most common reason calorie targets come out wrong.
BMR Is a Floor, Not a Target
The most important practical rule about BMR is this: do not eat below it for long. Your BMR is what your body needs to keep its basic systems running. Eating below it day after day means your body has to cut corners — and the corners it cuts are muscle, energy, and hormone function.
Weight loss works by eating below your TDEE, not below your BMR. For our example woman, a sensible weight-loss target is 500 calories below her 1,918-calorie maintenance, or about 1,418 calories a day — still above her 1,395 BMR. See also our guide to BMR vs TDEE and our free BMI calculator. A free basal metabolic rate calculator gives you your BMR and your TDEE together, so you can see both numbers before you choose a target, and our calorie calculator turns the pair into a daily eating plan. If you also know your body fat percentage, our body fat calculator result on the same page uses the Katch-McArdle formula to give you the lean-mass version of your BMR.
FAQ: What Is BMR?
What is BMR in simple terms?
BMR is the number of calories your body burns at rest just to stay alive — breathing, circulating blood, and keeping your organs working. For a typical 35-year-old woman weighing 70 kg, it is about 1,395 calories a day.
Is a higher BMR better?
A higher BMR means you burn more calories at rest, which makes maintaining or losing weight easier. But BMR mostly reflects your size and muscle mass, so a higher number is not automatically healthier — a very tall person has a high BMR simply because they are large.
What is a normal BMR for a woman?
Most adult women have a BMR between about 1,300 and 1,500 calories a day, and most adult men between about 1,600 and 1,900. Your own number depends on your weight, height, age, and muscle mass.
Does BMR change when you lose weight?
Yes. Losing weight lowers your BMR, because there is less body to maintain — roughly 10 fewer calories a day for every kilogram lost. That is why weight loss often slows after a few months, and why recalculating with your new weight matters.
Can you increase your BMR?
Yes, mainly by building muscle through strength training. Each pound of muscle burns about 6 calories a day at rest, so adding 5 kg (11 lb) of muscle raises BMR by roughly 66 calories a day. Our guide to how to increase your BMR covers seven science-backed ways.
Should I eat my BMR calories or my TDEE calories?
Neither exactly. Eat below your TDEE to lose weight, above it to gain, and around it to maintain — but stay above your BMR, and above the 1,200-calorie (women) or 1,500-calorie (men) floor unless a doctor advises otherwise.