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Every weight-loss plan, every muscle-gain diet, and every calorie target starts from the same foundation: the number of calories the body burns at rest. That number has a name, resting energy expenditure, or REE, and a REE calculator estimates it from a few simple inputs. But the estimate is only as useful as the understanding behind it. Different formulas give different answers, and knowing which one a calculator uses, and why, is the difference between a useful baseline and a misleading number.
This guide explains what a REE calculator does, how the main equations work, how to use the tool step by step, how to read the result, and where the estimate’s limits lie. Note: this article is for general information only and is not medical advice; anyone with a health condition should consult a qualified professional before changing their diet.
What Is Resting Energy Expenditure?
Resting energy expenditure is the amount of energy, measured in calories (kilocalories) per day, that the body uses while at rest to keep its essential functions running. Breathing, circulation, cell repair, brain activity, and maintaining body temperature all cost energy, and REE is the total of those costs.
REE is typically the largest single component of daily energy use, accounting for roughly 60 to 75 percent of total daily energy expenditure in a sedentary person. The rest comes from digesting food and from physical activity. That dominance is exactly why REE matters: any calorie target that ignores it is built on sand.
REE vs BMR vs RMR vs TDEE
These four terms get mixed up constantly, so here is the clean distinction:
- BMR (Basal Metabolic Rate): The energy used under strict laboratory conditions: complete rest, fasting, thermoneutral environment, just after waking. It is a theoretical minimum.
- REE (Resting Energy Expenditure): The energy used at rest under less strict conditions. In practice, REE and BMR differ by only a few percent, and the terms are often used interchangeably in calculators.
- RMR (Resting Metabolic Rate): Essentially the same concept as REE under a different name. Some tools label their output RMR; the math is identical.
- TDEE (Total Daily Energy Expenditure): REE plus the energy from food digestion plus all physical activity. TDEE is always larger than REE, and it is the number that actually determines whether someone gains, loses, or maintains weight.
A REE calculator answers “what do I burn doing nothing.” A TDEE calculator answers “what do I burn on a normal day.” The REE number is the input the TDEE calculation starts from. For the full picture of daily energy, see the TDEE calculator guide.
The Inputs a REE Calculator Needs
Every standard REE calculator asks for the same four inputs:
- Age (years). Metabolic rate declines with age, so older inputs produce lower outputs.
- Sex. The equations use different constants for men and women, reflecting average differences in body composition.
- Weight. Heavier bodies cost more energy to maintain. Enter it in kilograms; if the tool accepts pounds, it converts internally.
- Height. Taller bodies have more tissue to maintain. Enter it in centimeters or inches, matching the tool’s units.
Some calculators also ask for body fat percentage. That input unlocks the Katch-McArdle equation, which estimates REE from lean body mass rather than total weight and is more accurate for very muscular or very lean people.
The Mifflin-St Jeor Equation
The Mifflin-St Jeor equation, published in 1990, is the formula most modern REE calculators use by default. Research has repeatedly found it the most accurate of the simple equations for the general population, typically landing within about 10 percent of measured values.
For men: REE = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) + 5
For women: REE = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) – 161
Worked example: a 30-year-old man weighing 70 kg at 175 cm.
REE = (10 x 70) + (6.25 x 175) – (5 x 30) + 5 = 700 + 1093.75 – 150 + 5 = 1648.75 calories per day
And a 30-year-old woman with the same weight and height:
REE = 700 + 1093.75 – 150 – 161 = 1482.75 calories per day
The same body at different sexes differs by 166 calories, which is why the sex-specific constants matter.
The Harris-Benedict Equation
The Harris-Benedict equation is the older classic, dating to 1919, with a revised version by Roza and Shizgal published in 1984. Many calculators still offer it as an option, and it is worth knowing because older nutrition literature references it constantly. It tends to overestimate REE by roughly 6 to 15 percent compared to measured values, which is why Mifflin-St Jeor is generally preferred today.
Original Harris-Benedict, men: REE = 66.5 + (13.75 x weight in kg) + (5.003 x height in cm) – (6.755 x age)
Original Harris-Benedict, women: REE = 655.1 + (9.563 x weight in kg) + (1.850 x height in cm) – (4.676 x age)
Revised (Roza-Shizgal, 1984), men: REE = 88.362 + (13.397 x weight in kg) + (4.799 x height in cm) – (5.677 x age)
Revised (Roza-Shizgal, 1984), women: REE = 447.593 + (9.247 x weight in kg) + (3.098 x height in cm) – (4.330 x age)
Using the revised equation on the same 30-year-old man (70 kg, 175 cm):
REE = 88.362 + (13.397 x 70) + (4.799 x 175) – (5.677 x 30) = 88.362 + 937.79 + 839.825 – 170.31 = 1695.67 calories per day
Compare with Mifflin-St Jeor’s 1648.75: the Harris-Benedict result runs about 47 calories higher, a typical gap. For a step-by-step walkthrough of this equation in calculator form, see the full set of free calculators on the homepage.
The Katch-McArdle Equation
The Katch-McArdle equation is the specialist of the three. Instead of using total weight, it uses lean body mass, which makes it more accurate for athletes and anyone whose body composition differs a lot from average.
REE = 370 + (21.6 x lean body mass in kg)
Lean body mass is total weight minus fat mass. A 90 kg person at 15 percent body fat has 76.5 kg of lean mass:
REE = 370 + (21.6 x 76.5) = 370 + 1652.4 = 2022.4 calories per day
The catch: it requires an honest body fat measurement, which most people do not have. Without one, the equation cannot be used, and Mifflin-St Jeor is the better choice.
How to Use a REE Calculator Step by Step
- Measure weight and height accurately. Morning weight, after using the bathroom and before eating, is the most consistent. Height should be measured, not guessed.
- Select the equation. Choose Mifflin-St Jeor for the general case, Harris-Benedict if comparing with older references, Katch-McArdle if body fat percentage is known.
- Enter age, sex, weight, and height. Double-check the units: kilograms vs pounds, centimeters vs inches. A unit mix-up is the most common cause of absurd results.
- Read the result in calories per day. This is the resting estimate, not a daily calorie target.
- Sanity-check the number. Typical adult REE values fall roughly between 1200 and 2200 calories per day. A result far outside that range usually means an input error.
From REE to Daily Calories: The Activity Multiplier
REE alone does not say how much to eat, because nobody spends the whole day at rest. The standard bridge to daily needs is multiplying REE by an activity factor:
| Activity level | Multiplier |
|---|---|
| Sedentary (little or no exercise) | 1.2 |
| Lightly active (light exercise 1-3 days/week) | 1.375 |
| Moderately active (moderate exercise 3-5 days/week) | 1.55 |
| Very active (hard exercise 6-7 days/week) | 1.725 |
| Extremely active (very hard exercise, physical job) | 1.9 |
Example: the man above with REE of 1648.75, moderately active: 1648.75 x 1.55 = 2555.6 calories per day estimated TDEE. That number, minus a moderate deficit for weight loss or plus a surplus for gain, becomes the practical calorie target — and students juggling fitness with studies can also track academics with the grade and GPA calculator.
What Affects REE Beyond the Formula Inputs
The equations capture the big four inputs, but several other factors move REE in real life:
- Muscle mass. Muscle tissue burns more energy at rest than fat tissue. Two people with identical weight and height can have meaningfully different REEs.
- Age. REE declines with age, partly from muscle loss and partly from other metabolic changes.
- Hormones. Thyroid function is a major lever; thyroid disorders can shift REE substantially in either direction.
- Temperature and climate. The body spends extra energy maintaining core temperature in very hot or very cold environments.
- Caffeine and stimulants. These temporarily raise energy expenditure.
- Pregnancy and illness. Both raise energy demands above the formula’s prediction.
This is why the calculator gives an estimate, not a measurement. For most healthy adults the estimate is close enough to plan from; for clinical situations it is not.
How REE Is Measured in a Lab
The gold standard is indirect calorimetry: the person rests under a hood or connected to a metabolic cart while the equipment measures oxygen consumed and carbon dioxide produced. From those gas exchange numbers, the abbreviated Weir equation computes 24-hour energy expenditure. It is accurate but requires specialized equipment and trained staff, which is why predictive equations exist for everyday use.
Healthline’s overview of metabolism and calorie needs is a solid next read for anyone who wants the physiology behind these numbers without the textbook density: Healthline on metabolism.
Common Mistakes When Using a REE Calculator
- Treating REE as a calorie target. Eating exactly at REE while living a normal active life creates an unintended deficit.
- Using pounds as kilograms. A 154 “kg” entry that was really pounds nearly doubles the result.
- Picking the highest activity level out of optimism. The multiplier should reflect the average week, not the best week.
- Comparing equations without realizing it. Two calculators can disagree simply because one uses Mifflin-St Jeor and the other Harris-Benedict.
- Expecting precision to the single calorie. The output is an estimate with a real error band; rounding to the nearest 50 is honest.
Frequently Asked Questions
What does a REE calculator tell me?
It estimates the calories your body burns per day at complete rest, based on your age, sex, weight, and height. It does not include activity or digestion.
What is a good REE for my age?
There is no single “good” number; REE depends on sex, weight, and height too. Typical adult values range roughly from 1200 to 2200 calories per day. What matters is whether the estimate is accurate for your body, not whether it hits some ideal.
Which REE formula is most accurate?
The Mifflin-St Jeor equation is generally considered the most accurate for the general population, usually within about 10 percent of measured values. Katch-McArdle can be better for muscular individuals if body fat is known.
Is REE the same as BMR?
Nearly. BMR is measured under stricter laboratory conditions, while REE is measured under more relaxed resting conditions. In practice the numbers differ by only a few percent, and most calculators use the terms interchangeably.
How do I convert REE to TDEE?
Multiply REE by an activity factor from 1.2 (sedentary) to 1.9 (extremely active). The result is an estimate of total daily energy expenditure.
Why do two REE calculators give different results?
They are probably using different equations. Mifflin-St Jeor and Harris-Benedict routinely differ by 50 to 150 calories for the same person. Check which formula each tool uses before comparing.
Can REE change over time?
Yes. Gaining muscle raises it, losing weight lowers it, and aging gradually reduces it. Recalculating every few months, or after significant weight change, keeps the estimate current.
