What you need to know about BMR

To explore the variety of factors that influence BMR and feed into this calculator, we’ve published an in-depth series of articles digging into the research and explaining the ins-and-outs of BMR. But, these are the highlights:

Background of BMR formulas

The earliest attempts to measure BMR stretch back into the 1800s, and the earliest formulas developed to estimate BMR from factors like height, weight, age, and sex date back to the 1910s. Today’s best BMR formulas are based on meta-analytic research, pooling the results of multiple studies with hundreds, thousands, or even tens of thousands of subjects. The BMR calculator above builds on the two best “off-the-shelf” BMR formulas developed from this type of meta-analytic research: the Cunningham equation (sometimes erroneously referred to as the Katch-McArdle equation), and the Oxford/Henry equations.

You can learn more about the background of BMR formulas here.

Determinants of BMR, and the accuracy of BMR calculators

While formulas to estimate BMR have improved over the years, they still at best only provide a rough approximation of your BMR. That’s because your BMR is primarily determined by the mass of your high-metabolic-rate organs: your brain, liver, kidneys, and heart. They account for less than 5% of your total body mass, while accounting for more than 50% of your total BMR. Easily observable factors like height, weight, age, sex, and body composition are associated with the mass of your high-metabolic-rate organs, but those associations are typically fairly weak. So, the output of BMR calculators shouldn’t be treated as gospel – they’re only intended to provide you with a decent ballpark estimate. That’s why the BMR calculator above provides you with both a point estimate of your BMR (the most likely value), and the range in which your actual BMR is likely to fall (which accounts for the typical error of BMR calculators, and the typical variability of human BMRs).

You can learn more about the determinants of BMR here.

The impact of sex

Women tend to have lower BMRs than men. When adjusted for factors like height, weight, and age, men generally have BMRs that are about 150-200 Calories higher. However, that difference is primarily due to differences in body composition: at a given height and weight, men tend to have a bit less body fat (which has a very small impact on BMR), and a bit more lean body mass (which has a larger impact on BMR). So, BMR calculators and formulas that account for body composition apply equally to both men and women – in other words, the relationship between total fat-free mass and BMR is the same for both sexes. An interesting upshot of that is that women actually have higher BMRs per unit of fat-free mass, on average, because a greater proportion of their total fat-free mass is typically comprised of high-metabolic-rate organs. The same relationship applies within each sex as well (smaller women typically have a higher BMR per unit of fat-free mass than larger women, and smaller men typically have a higher BMR per unit of fat-free mass than larger men).

You can learn more about the impact of sex on BMR here.

The impact of age

BMR tends to decrease as we age for a couple of reasons. First, most people lose muscle mass as they age. Second, tissue-specific metabolic rates decrease with age. In both cases, these effects occur very gradually at first, such that aging has very little impact on BMR from the start of adulthood until about 60 years old. Past age 60, age-related declines in BMR begin to accelerate.

You can learn more about the impact of age on BMR here.

The impact of athletic status

Athletes tend to have higher BMRs than non-athletes, but not for the reasons most people expect. It’s commonly believed that athletes have higher BMRs because they tend to be leaner and have more muscle mass than non-athletes. While those body composition differences do have an effect, research finds that athletes still have higher BMRs than non-athletes even when adjusting for those differences in body composition. Instead, athletes primarily have higher BMRs than non-athletes because the mass of their high-metabolic-rate organs (particularly the heart, kidneys, and liver) scales more strongly with overall body size than the relationship observed in non-athletes. So, at a given level of total fat-free mass, athletes tend to have BMRs that are about 10-12% higher than non-athletes. However, this difference is larger for larger athletes, and smaller for smaller athletes.

This BMR difference is observed at all levels of athletic performance – both recreational athletes and elite athletes have higher BMRs than non-athletes, but there aren’t major BMR differences between recreational athletes and elite athletes. Furthermore, the impact appears to be similar for all different types of athletic performance – strength/power athletes, endurance athletes, and athletes in sports with mixed demands all appear to have BMRs that are elevated to a similar degree.

You can learn more about the impact of athletic status on BMR here.

The impact of weight loss

While you’re losing weight, your BMR decreases. Part of this decrease is due to the loss of body mass – it takes less energy to fuel a smaller body, after all – and part of this decrease is due to “metabolic adaptation.” Metabolic adaptation refers to decreases in metabolic rate that are disproportionate to the amount of tissue lost. Fears of metabolic adaptation are often blown way out of proportion (i.e., fears of entering “starvation mode”), but we do tend to find that BMRs decrease during weight loss to a level that’s about 5-10% lower than we’d expect to observe in people with similar characteristics at energetic maintenance.

However, about half of the metabolic adaptation that occurs during weight loss goes away once you complete your diet and enter a state of weight maintenance. Research on people who’ve lost significant amounts of weight, and maintained their weight loss over time, find that their BMRs are only 3-5% lower than people who aren’t in a weight-reduced state, on average. In real terms, that’s generally a difference of just 30-100 Calories per day.

You can learn more about the impact of weight loss on BMR here.

The impact of weight gain

Unlike weight loss, we don’t typically observe “reverse” metabolic adaptation when people gain weight. For unknown reasons, BMR does appear to immediately increase by about 5-10% when people begin intentionally overfeeding, but this isn’t a metabolic “advantage” that’s maintained over time. During periods of sustained weight gain, BMR tends to increase in a manner that’s proportional to the tissue that’s gained. In other words, BMR during periods of weight gain is very similar to BMR during periods of weight maintenance.

You can learn more about the impact of weight gain on BMR here.

What is the range of normal human BMRs?

At the extremes, researchers have observed BMRs as low as 700 Calories per day in patients with anorexia, and as high as 3700 Calories per day in exceptionally muscular athletes. But, on average, most women tend to have BMRs between about 1300 and 1600 Calories per day, and men tend to have BMRs between 1700 and 2100 Calories per day.

However, plenty of people have BMRs that fall above or below these relatively narrow ranges. In representative research on people in the general population, we sometimes see women with BMRs below 1000 Calories per day or above 2400 Calories per day, and men with BMRs below 1100 Calories per day or above 2800 Calories per day. So, don’t be too surprised if your BMR is above or below the relatively narrow “norms” you may have encountered previously.

You can learn more about the range of human BMRs here.

Why is this BMR calculator better?

This BMR calculator uses a novel set of formulas that extend and build upon the (already very good) Cunningham and Oxford/Henry equations. Namely, there were four obvious areas where these formulas could be improved.

First, they’re both linear equations. However, we know that metabolic rate actually scales non-linearly (allometrically) with body size. Linear equations can provide a decent approximation of allometric relationships for most people, but they inherently produce less accurate estimates for people who are considerably smaller or larger than average.

Second, the impact of age on BMR is complex. Age has very little impact on BMR up to about 60 years old, and a much larger impact on BMR past age 60. The Oxford/Henry equations tackle this problem in a manner that’s relatively effective, but not particularly elegant (they just use entirely different equations for adults aged 18-30, 30-60, and 60+, with differing coefficients that don’t directly reflect the underlying physiological realities of aging), and the Cunningham equation doesn’t directly account for age at all.

Third, they don’t account for athletic status, which has a very notable impact on BMR. Furthermore, there weren’t pre-existing equations for athletes developed using meta-analytic methods. So, we did the legwork of filling that gap, and developed an equation and BMR calculator that is based on data from 30 studies and nearly 2000 athletes.

Fourth, none of these equations account for the known metabolic impact of being in an energy deficit (metabolic adaptation) or a weight-reduced state. 

So, we built upon a strong foundation, and made key improvements that will help our equations be at least a bit more accurate for everyone (on average), with particularly large increases in accuracy for people aiming to lose weight, athletes, people who are considerably smaller than average, and people who are considerably larger than average. And, instead of only presenting your estimated BMR as a single number (which gives an unwarranted impression of extreme precision), this BMR calculator shows you the research-based range of plausible values, so that you can feel more confident with self-experimentation if your actual BMR (and therefore, your actual energy requirements) turns out to be a bit higher or a bit lower than our single best guess.

You can read more about our methodology for developing these equations here.