BMR calculator

Your basal metabolic rate (BMR) is the number of calories your body burns just to keep you alive, even if you stayed in bed all day. It powers the basics, such as:

• Breathing
• Pumping blood
• Keeping your body temperature steady

For most people who don't exercise much, BMR is the biggest part of the calories they burn in a day, around 60 to 70%.

• Muscle and fat: muscle uses more energy than fat, so more muscle means a higher BMR.
• Body size: a taller or heavier body needs more energy to run.
• Age: BMR slowly drops as you get older, mostly because muscle declines.
• Sex: men tend to have a higher BMR, mainly because they usually carry more muscle.
• Hormones: thyroid hormones in particular speed up or slow down how fast you burn energy.
• Genes: some of the difference between people is simply inherited.

This calculator uses the Mifflin-St Jeor equation, the one most dietitians and researchers use today:

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

For example, a 35-year-old woman who weighs 160 lbs (72.6 kg) and is 5 ft 6 in (167.6 cm) tall has a BMR of about 1,437 calories a day.

The equation was built from measurements of men and women, and it adds or takes away a fixed amount for each. That reflects average differences in muscle and body fat, not anything about who you are.

There isn't one number that is normal for everyone, because BMR depends on your size, age, sex and muscle. For example, a 30-year-old woman who is 5 ft 4 in and 140 lbs has an estimated BMR of about 1,340 calories a day, while a 30-year-old man who is 5 ft 10 in and 180 lbs is closer to 1,780.

A higher or lower BMR isn't good or bad on its own. It is a starting point for understanding how much energy your body needs.

• How many calories you burn in a whole day: moving, exercising and digesting food add to it. Pick an activity level in your result to see your whole day.
• Your body composition: two people with the same BMR can have very different amounts of muscle and fat.
• What to eat: it says nothing about protein, carbs, fat, vitamins or fiber.
• Whether you'll gain or lose weight: that depends on what you eat compared with everything you burn.
• Your overall health: it doesn't measure blood sugar, hormones or other conditions.

Your BMR is the floor of your daily calorie burn. Multiply it by your activity level and you get the calories you burn in a typical day. Your result shows this for the activity level you pick.

To lose weight, you eat a bit less than that daily number. A gap of about 500 calories a day works out to roughly 1 lb a week. Going much lower is hard to keep up and can cost you muscle, so talk to a clinician or dietitian before cutting back sharply.

As you lose weight your BMR drops a little too, so it's worth working it out again every few months. Strength training helps here: building muscle nudges your BMR up.

People often use them to mean the same thing, but they're measured differently.

• BMR is measured under strict lab conditions: just after waking, after a 12-hour fast and with no recent activity.
• RMR (resting metabolic rate) is measured under looser conditions and includes a little everyday activity, so it usually comes out slightly higher.