BMR vs TDEE: what's the difference?

BMR is the calories your body burns just to stay alive at complete rest: breathing, pumping blood, keeping warm. TDEE is everything you burn in a full day, so it is your BMR plus the energy used moving around, exercising and digesting food. You work out BMR first with a formula, then multiply it by an activity number to get TDEE. TDEE is the figure that matters, because it tells you roughly how many calories keep your weight steady, and that is the number you set every target from. Eat below it to lose, above it to gain.

Here is how the two connect.

What BMR is

BMR stands for basal metabolic rate. It is the energy your body would use if you lay still in bed all day and did nothing. Even at rest your organs are working constantly, and that baseline usually accounts for the biggest slice of what you burn.

The most widely used way to estimate it is the Mifflin-St Jeor equation. It takes your weight in kilograms, height in centimetres and age in years:

• Men: BMR = (10 x weight) + (6.25 x height) − (5 x age) + 5
• Women: BMR = (10 x weight) + (6.25 x height) − (5 x age) − 161

Take a 30 year old woman, 165cm tall, 68kg. That is (10 x 68) + (6.25 x 165) − (5 x 30) − 161. So 680 plus 1,031 minus 150 minus 161, which comes to about 1,400 kcal. That is her resting burn before she has got out of bed.

BMR rises with more weight and height, and falls with age. It is an estimate, not a lab reading, but it is close enough to plan from.

The activity multiplier

BMR on its own is not useful for planning, because nobody lies still all day. To get your real daily burn, you multiply BMR by an activity factor that reflects how much you move. The standard range runs from about 1.2 to 1.9:

The multiplier covers everything BMR leaves out: walking, fidgeting, workouts, and the energy spent digesting food. Most people who work at a desk and train a few times a week sit somewhere around 1.375 to 1.55. When in doubt, pick the lower one, since people tend to overrate how active they are.

Getting to TDEE

TDEE stands for total daily energy expenditure. It is simply BMR multiplied by your activity number.

Back to our example: a BMR of 1,400 kcal at a moderately active 1.55 gives 1,400 x 1.55, so about 2,170 kcal. That is her TDEE, the rough number of calories that keeps her weight where it is.

Why you set your target from TDEE

TDEE is your maintenance line, and every goal is set relative to it.

• To lose weight, eat below your TDEE. A deficit of roughly 500 kcal a day is a common, steady pace. Since about 7,700 kcal roughly equals 1kg of body fat, that works out to around half a kilogram a week.
• To maintain, eat at your TDEE.
• To gain, eat above it, a modest surplus of a few hundred calories if you are trying to build muscle without piling on fat.

For our example, a 500 kcal deficit means a target near 1,670 kcal a day for gradual weight loss.

Setting a target from BMR instead of TDEE is a classic mistake. BMR ignores all the calories you burn moving, so a target built on it is far too low, leaves you hungry and worn out, and rarely lasts. Always plan from TDEE.

One thing to hold loosely: your TDEE is an estimate. If the scale is not moving the way the maths predicts after a couple of weeks, adjust your intake by 100 to 200 kcal and watch again. Real bodies vary, and the formula gets you a sensible starting point, not a guarantee.

Let the app do the maths

Working out BMR by hand, then a multiplier, then a deficit, gets fiddly. Fettle (stayfettle.com) takes your details, runs the Mifflin-St Jeor maths and your activity level, and sets your daily calorie and macro targets from your TDEE. It has UK foods and barcode scanning, it is free, and there is no account to set up.

One honest note

These are general estimates for healthy adults, not medical advice. If you have a health condition, are pregnant, or are managing your weight for medical reasons, check with your GP or a registered dietitian before setting a calorie target.