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Your BMR: what it is, how to estimate it, and why the number is only a starting point

By raphael.fit team · Oct 23, 2026 · 4 min read

Your basal metabolic rate (BMR) is the energy your body burns at complete rest just to keep you alive: breathing, circulation, keeping your organs running. For most people it's the biggest slice of the calories they burn in a day. It's a useful number to know, as long as you treat it as an estimate and not a rule.

BMR, RMR and TDEE

• BMR is measured under strict lab conditions: lying down, after an overnight fast, at a comfortable temperature. A study from the Rowett Research Institute in Scotland calls it the largest component of daily energy demand in Western societies.
• RMR (resting metabolic rate) is measured under slightly looser conditions. It's usually a bit higher than BMR, and most online calculators actually estimate RMR. In everyday use, people treat the two as the same.
• TDEE (total daily energy expenditure) is everything you burn in a day: BMR plus the energy to digest food, plus all your movement, from training to walking to fidgeting.

If you want to set calories for a goal, TDEE is the number that matters. BMR is the base it's built on.

How BMR is estimated

The gold standard is measuring it with indirect calorimetry, which tracks the oxygen you use and the carbon dioxide you breathe out. Almost nobody does that, so we use equations.

The most widely recommended one is the Mifflin-St Jeor equation, published in 1990:

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

Example: a 35-year-old man, 80 kg (176 lb) and 178 cm (5'10"), comes out at about 1,750 kcal a day.

A systematic review for the American Dietetic Association (now the Academy of Nutrition and Dietetics) compared the four most common equations. Mifflin-St Jeor was the most reliable, landing within 10% of measured values in more people than the others, with the narrowest error range.

From BMR to TDEE

To get TDEE, calculators multiply your BMR by an activity factor, usually somewhere between about 1.2 for mostly sitting and 1.9 for very hard daily training. Those multipliers are rough. Two people who both call themselves "moderately active" can move very differently. If you want something better than a single multiplier, the NIH Body Weight Planner from the National Institute of Diabetes and Digestive and Kidney Diseases uses a more detailed model of how your body changes over time.

What actually changes your BMR

• Fat-free mass. In the Rowett study of 150 adults, fat-free mass explained 63% of the differences in BMR between people, fat mass 6% and age 2%. Sex wasn't significant once body composition was accounted for. About a quarter of the variation was left unexplained.
• Body size. Bigger bodies burn more, at rest and while moving. Mayo Clinic lists body size and composition, sex and age as the main drivers.
• Age, but less than you think. A 2021 analysis in Science of more than 6,000 people, led by Herman Pontzer at Duke University, found that energy expenditure adjusted for body size stays fairly stable from about 20 to 60 and declines after that. Much of the slowdown people blame on their 30s and 40s probably comes from moving less and losing muscle, not from age itself.
• Dieting. When you lose weight, your BMR drops, partly because there's less of you. It can also drop more than your new size predicts. In an NIH follow-up of "The Biggest Loser" contestants, resting metabolic rate was still about 500 kcal a day lower than predicted six years later. That's an extreme case: 14 people who lost huge amounts of weight very fast. Don't assume it applies to a normal, gradual diet.

The limits of calculators

Here's the counterpoint. "Within 10%" sounds precise until you do the math. For someone with a true BMR of 1,750 kcal, 10% is 175 kcal a day either way, and some people fall outside that range. The same systematic review noted that older adults and many ethnic groups were underrepresented in the studies these equations come from, so be more skeptical if that's you. Equations also don't know how much muscle you carry. A lean, muscular person will often burn more than the formula says.

The other problem is that the body isn't a fixed calculator. Researchers at the NIH showed that weight changes slow down over time as your energy needs shift with your weight. The old rule of "cut 500 calories a day, lose a pound a week, forever" overestimates long-term weight loss.

How to use it

1. Get an estimate. Use Mifflin-St Jeor and an honest activity factor to get a starting TDEE.
2. Test it for 2 to 3 weeks. Eat around that number and track your food and weight. Snap your meals in the free food tracker to keep it low effort.
3. Adjust from real data. If your weekly average weight isn't moving the way you want, adjust by 100 to 200 kcal a day and test again. Your trend beats any formula.
4. Protect muscle. Lifting and enough protein help keep fat-free mass, which is the biggest thing you control in your BMR.

The bottom line

BMR is real and worth understanding, but every calculator gives you a guess. Use the estimate to start, then let your own numbers over a few weeks tell you what your body actually needs.

This article is for general information and isn't medical advice. Talk to a qualified health professional about your own situation.

Sources

1. Mifflin et al. 1990, Am J Clin Nutr: a new predictive equation for resting energy expenditure
2. Frankenfield et al. 2005, J Am Diet Assoc: comparison of predictive equations for resting metabolic rate (systematic review)
3. Johnstone et al. 2005, Am J Clin Nutr: factors influencing variation in basal metabolic rate
4. Pontzer et al. 2021, Science: daily energy expenditure through the human life course
5. Fothergill et al. 2016, Obesity: persistent metabolic adaptation 6 years after "The Biggest Loser"
6. Hall et al. 2011, Lancet: quantification of the effect of energy imbalance on bodyweight
7. Mayo Clinic: Metabolism and weight loss
8. NIDDK: Body Weight Planner