RMR vs BMR: The Difference and Which Number You Should Use

2 days ago

5 min read

Search for a calorie calculator and you will be asked for your BMR. Book a metabolic test and you will be given your RMR. The two terms get used as if they are the same thing. They are not, and the difference explains why a measured result and a calculator result can disagree even before you get to the question of accuracy.

Comparison cards showing how BMR, RMR and calculator estimates are obtained and the conditions each requires

The definitions

Basal Metabolic Rate is the minimum energy required to sustain life under strictly controlled conditions. In the research literature, a true BMR measurement requires the subject to have slept at the testing facility, to be measured immediately on waking, to have fasted for at least 12 hours, to be in a thermoneutral environment, and to have done no physical activity beforehand.

Resting Metabolic Rate is the same physiological quantity measured under practical conditions. You arrive at the clinic having fasted, you rest quietly for a period before the measurement, and you are measured awake and supine.

Because the conditions are less strict, an RMR measurement is slightly higher than a true BMR would be for the same person. You have travelled to the clinic, you are awake, and your digestive system has not been idle for twelve hours. Those small costs are included.

The practical consequence is that RMR and BMR are close, and the gap is small relative to the error in any equation that estimates either.

Why almost nobody measures BMR

The conditions that define BMR make it impractical outside a research setting.

Requiring a subject to sleep overnight at the facility, be woken under controlled conditions, and be measured before moving means a metabolic ward, an overnight stay, and staff at dawn. The cost of that is enormous relative to the additional precision it buys.

So the field measures RMR instead, and the published best practice protocols are written for RMR rather than BMR. The accepted procedure requires a 10 to 20 minute rest before measurement begins, a total measurement window of around 10 minutes, discarding the first 5 minutes of data, and a coefficient of variation under 10% across the remaining period (Compher et al., Journal of the American Dietetic Association).

The same review confirmed that a short rest of 20 minutes or less is sufficient for the metabolic effect of ordinary daily activity to dissipate, which is what makes a practical clinic protocol defensible in the first place.

The confusing part: the equations are labelled wrongly

Here is where the terminology genuinely causes problems.

The Harris Benedict equation is almost always presented as a BMR formula. The Mifflin St Jeor equation is usually presented as a BMR formula too. Both were developed and validated against measurements taken under resting rather than strict basal conditions.

So when a calculator asks for your details and returns "your BMR", it is returning an estimate of something closer to your RMR, generated by an equation validated against RMR measurements. The label is wrong, the underlying quantity is roughly right, and the accuracy is the real issue.

That accuracy is worth knowing. In a validation study of 337 adults measured by indirect calorimetry, Mifflin St Jeor was the only unbiased equation tested, with an accuracy rate of 82% within 10% of the measured value. Every other equation tested, including Harris Benedict, tended to overestimate (Frankenfield, Clinical Nutrition).

Term	What it is	How it is obtained	Practical use
BMR	Minimum energy to sustain life, strict conditions	Metabolic ward, overnight stay	Research only
RMR	Same quantity, practical conditions	Indirect calorimetry, 15 minute clinic test	Setting calorie targets, detecting suppression
Estimated BMR from a calculator	An equation's guess at RMR	Weight, height, age, sex	A starting point when nothing is measured

So which number should you be using

Use a measured RMR.

Not because BMR is wrong, but because BMR is unobtainable for you and the distinction between the two is far smaller than the distinction between measured and estimated. The meaningful choice is not BMR versus RMR. It is measured versus predicted.

To make the point with numbers. The gap between a true BMR and a properly measured RMR is a modest percentage. The gap between a measured RMR and an equation estimate exceeded 10% in roughly one in five people in the Frankenfield validation, and was worse in obese participants, where Mifflin St Jeor accuracy fell to 75%.

You are arguing about a small systematic difference while ignoring a large individual one.

What to do if you already have a calculator number

Do not throw it out. Use it as a starting hypothesis and then check it.

Note what your calculator says your BMR is.
Get a measured RMR.
Compare. If they agree within a few percentage points, your equation happened to fit you and you can use calculator-based planning with reasonable confidence in future.
If they disagree meaningfully, you now know the direction and magnitude of your personal error, which is information you cannot get any other way.

That last point is the underrated one. A single measured test does not just give you today's number. It tells you whether equations work for you at all.

Common questions

Do I need to fast for 12 hours for an RMR test? No. The canonical fast at Precision Body Lab is 4 hours. Twelve hours is a BMR requirement. Food, alcohol, caffeine and nicotine all affect resting metabolic rate for a variable period after consumption, which is why they are controlled rather than eliminated.

Does my body composition scale give me a BMR? It gives you an equation output based on an impedance estimate of your lean mass. It is a prediction layered on top of another prediction.

Is a sleeping metabolic rate the same as BMR? No. Sleeping metabolic rate is typically lower than BMR, since some metabolic processes slow during sleep. It is a third distinct quantity.

Which number goes into a TDEE calculation? RMR, multiplied by an activity factor. The activity factor is now the weakest link in the chain, which is a separate problem worth understanding on its own.

The short version

BMR is measured under strict laboratory conditions and is not available to you. RMR is the same quantity measured under practical clinic conditions and is slightly higher. Calculators labelled BMR are really estimating RMR. The difference between the two definitions is small. The difference between measuring and estimating is not.

An RMR test at Precision Body Lab is $149, or $279 with a DEXA scan as a Metabolic Baseline. The test takes about 15 minutes and requires a 4 hour fast. Book online or call 1300 910 163.

RMR vs BMR: The Difference and Which Number You Should Use

The definitions

Why almost nobody measures BMR

The confusing part: the equations are labelled wrongly

So which number should you be using

What to do if you already have a calculator number

Common questions

The short version

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