BMR (Basal Metabolic Rate)

What is BMR? The definition in 20 seconds

Your BMR is the energy your body needs per day at complete rest: breathing, circulation, brain, organs, temperature. It is measured exactly in a lab, fasted and in the morning, with breath gas analysis. In real life you use a formula and check the result on the scale. Together with NEAT, EAT and TEF it makes up your daily expenditure.

• Baseline, not daily total: BMR is the largest single item in your expenditure, but only one of them. Eating at your BMR means eating like you spent the day in bed.
• Hard to move: It depends mostly on your fat-free mass. You shift it slowly by building muscle, not with tricks.
• For hardgainers this means: A low BMR rarely explains why you are not gaining. Far more often the difference sits one floor up: in your daily movement and in what you actually eat.

The anchors in the system: the Metabolism System, metabolism and rate of gain.

The formula: a starting value, not a verdict

For healthy adults the Mifflin-St Jeor equation is the most widely used estimate. You need your weight in kg, your height in cm and your age in years.

• Men: 10 times weight plus 6.25 times height minus 5 times age plus 5.
• Women: the same calculation, but minus 161 instead of plus 5.
• Example: 70 kg, 180 cm, 25 years gives a BMR of about 1,705 kcal. Times an activity factor of 1.6 that is about 2,730 kcal per day. On top come 150 to 250 kcal per day, our deliberately cautious starting value for the surplus. Your target lands at about 2,880 to 2,980 kcal.

How accurate is that? In a comparison of the common equations, Mifflin-St Jeor landed within ten percent of the measured value more often than any other. For an individual it can still be clearly off. Ten percent of 1,700 kcal is 170 kcal. Times 1.6 that becomes about 270 kcal in your daily target, more than your entire surplus.

That is why the formula stays a starting value. Your weight trend gives the answer.

What you control and what you don't

You can barely move BMR itself. What you control is what you do with the number.

• Fat-free mass: It explains most of the difference between people. More muscle raises your BMR. It grows with your build, slowly.
• What you don't control: Age, height and a remainder no measurement explains. The thyroid hormone thyroxine plays a part too. With unexplained weight swings, feeling cold all the time or persistent exhaustion, get your thyroid hormones checked by a doctor. That does not belong in your training plan.
• The activity factor: This is where the big mistakes happen. If you call yourself “lightly active” but spend all day on your feet, you undercount yourself. Pick the factor honestly, not the one with the nicest number.
• Recalculate: After a few kilos of gain the old number no longer holds. Run it again and check it on the scale.

Practice: the 14-day check

• Day 0, set your starting value: Calculate BMR with Mifflin-St Jeor, multiply by your activity factor, write down your daily target.
• Document daily: Eat that amount consistently. Bodyweight every morning, steps every evening as your measure for NEAT. How much your steps burn is shown by the NEAT Calculator.
• Day 7, checkpoint: Build the first weekly average. Change nothing.
• Day 14, check the trend: Second weekly average. If your weight holds, you have found maintenance. If it falls, the formula ran low for you. Then adjust in steps of 100 to 200 kcal instead of throwing the formula out.

You do not measure BMR, you measure its result. And that result is called maintenance calories.

Common misconceptions

• “My metabolism is broken, that's why I can't gain.” BMR does vary between people, and most of that variation comes from fat-free mass. Far more often the problem sits between “feels like a lot” and “actually enough”.
• “I'm an ectomorph, my BMR is just high.” Your BMR follows your fat-free mass above all. If you are lean and light, you have rather little of it. For many, the difference lies in daily movement, not in resting expenditure.
• “I eat my BMR, that's enough.” The most expensive mistake on this page. In the example above there are almost 1,200 kcal between BMR and the gaining target.
• “Any online calculator will do.” Some still run Harris-Benedict from 1919. In the Mifflin study that equation overestimated measured resting expenditure by 5 percent on average. Check which formula sits behind it.

Common questions