RMR / BMR Calculator (Mifflin-St Jeor)
Resting Metabolic Rate (RMR/BMR) calculator. Mifflin-St Jeor, Harris-Benedict (1918) and two lean-mass equations side by side, plus TDEE at five activity levels.
RMR Calculator
Four equations compared
TDEE at activity levels
| Sedentary (×1.2) | — |
| Lightly active (×1.375) | — |
| Moderately active (×1.55) | — |
| Very active (×1.725) | — |
| Extra active (×1.9) | — |
The five multipliers are a calculator convention, not part of any of these papers. FAO/WHO/UNU (2004) class a sedentary or light lifestyle as 1.40–1.69 times basal metabolism, so the ×1.2 row sits below even that.
How to use the RMR calculator
Enter sex, age, weight, height
Weight in kg (pounds × 0.45359237, exactly). Height in cm (inches × 2.54, exactly). Age 18 to 100: these are adult equations. Every field is required; a blank box is an error, not a default.
Enter body fat % (optional)
Used only by the two lean-mass equations, which work from fat-free mass (weight × (1 − body fat %)) instead of total weight. Leave it blank if you do not have a measured figure: a guessed body-fat percentage moves these results more than it seems to.
Read RMR (kcal/day at rest)
Mifflin-St Jeor (1990) is the headline figure: a 2005 systematic review found it the most reliable of the four common equations it compared. Harris-Benedict (1918) is the original. Cunningham (1980) and the textbook Katch-McArdle form use lean mass only. Where they disagree, that spread is a fair picture of how uncertain any single estimate is.
Apply activity multiplier for TDEE
Total Daily Energy Expenditure (TDEE) = RMR × activity multiplier. Sedentary (1.2): desk job, no formal exercise. Lightly active (1.375): 1-3 days/week light exercise. Moderately active (1.55): 3-5 days. Very active (1.725): 6-7 days hard. Extra active (1.9): athlete + physical job. The ladder is a convention; if unsure, pick the lower rung and adjust from results.
Use TDEE for weight management
Maintenance: eat about your TDEE. Lose weight: TDEE − 500 kcal/day is about 0.45 kg a week at first by the old 3,500-kcal-per-pound rule, and slower as your weight falls (Hall et al., Lancet 2011). Gain weight: a modest surplus. Re-calculate as your weight changes, since every equation here scales with body mass.
RMR — what your body burns just to exist
Resting Metabolic Rate (RMR) — sometimes used interchangeably with Basal Metabolic Rate (BMR) though technically slightly different — is the energy your body burns at complete rest, performing only essential functions: breathing, circulation, cellular maintenance, brain activity, organ function. Basal metabolism makes up about 45 to 70% of total daily energy expenditure in adults (FAO/WHO/UNU, 2004). The rest is physical activity plus the thermic effect of food, which for a mixed diet is about 10% of the energy eaten (Westerterp, 2004). Knowing your RMR is the foundation of any structured nutrition plan — for weight loss, weight gain, or athletic performance. Estimating it accurately matters more than people realise.
Why Mifflin-St Jeor replaced Harris-Benedict
Harris and Benedict published their equations in 1918 from indirect-calorimetry measurements, and tabulated them for ages 21–70, heights of 151–200 cm and weights of 25–124.9 kg. Roza and Shizgal re-fitted them in 1984 with Benedict's later subjects added and found they estimate resting expenditure in a normal subject "with a precision of 14%". Mifflin and colleagues fitted a new equation in 1990 to 498 healthy adults aged 19–78, 234 of them obese. A 2005 systematic review for the American Dietetic Association (Frankenfield et al.) compared four common equations and found Mifflin-St Jeor "the most reliable, predicting RMR within 10% of measured in more nonobese and obese individuals than any other equation", with the narrowest error range — while warning that older adults and ethnic minorities were under-represented in the studies behind all of them. At this page's default inputs (a man of 35, 175 cm, 75 kg) the two give 1,674 and 1,737 kcal a day.
An equation predicts the average person of your sex, size and age. Only a measurement can see you.
The Katch-McArdle advantage for lean individuals
Mifflin-St Jeor and Harris-Benedict use total body weight, but tissues differ: at rest skeletal muscle uses about 13 kcal per kg a day and fat tissue about 4.5 (Elia's values, checked against MRI and calorimetry by Wang et al., 2010). Cunningham re-analysed Harris and Benedict's own subjects in 1980 and found lean body mass "the single predictor of BMR", proposing BMR = 500 + 22 × lean mass. The widely quoted Katch-McArdle form, 370 + 21.6 × lean mass, comes from a textbook we could not trace to an original paper, so it is shown beside Cunningham and labelled as such. For a 30-year-old man of 180 cm and 90 kg at 10% body fat (81 kg lean), Mifflin-St Jeor gives 1,880 kcal, Harris-Benedict 2,002, Katch-McArdle 2,120 and Cunningham 2,282. Both lean-mass equations are only as good as the body-fat figure behind them; without a measured one, Mifflin-St Jeor is the safer estimate.
Activity multiplier reality check
The activity ladder (×1.2 to ×1.9) is a calculator convention, not something any of these papers fitted. For scale, FAO/WHO/UNU (2004) class lifestyles by physical activity level, as multiples of basal metabolism: 1.40–1.69 for a sedentary or light lifestyle, 1.70–1.99 for an active one and 2.00–2.40 for a vigorous one, adding that values above 2.40 are difficult to maintain for long. Pick the rung that matches your week rather than your best week, then let a few weeks of your own weight trend correct it.
Why two people of the same weight burn different amounts at rest
Mifflin-St Jeor (1990) was fitted to 498 healthy adults aged 19–78; a 2005 review found it the most reliable of four common equations.
Basal metabolism is about 45–70% of total daily energy expenditure in adults (FAO/WHO/UNU, 2004).
At rest, skeletal muscle uses about 13 kcal per kg a day; fat tissue about 4.5 (Wang et al., 2010).
Adjusted for fat-free mass, daily energy expenditure stays stable from 20 to 60 and declines only in older adults (Pontzer et al., 2021).
Cunningham (1980) found lean body mass the single predictor of BMR in Harris and Benedict's own data: BMR = 500 + 22 × lean mass.
Roza and Shizgal (1984) put Harris-Benedict's precision for a normal individual at ±14%; indirect calorimetry measures instead of predicting.
Thermic effect of food: protein 20–30%, carbohydrate 5–10%, fat 0–3% of their energy (Westerterp, 2004).
Harris and Benedict tabulated their 1918 equations only for ages 21–70, 151–200 cm and 25–124.9 kg; the page notes when you are outside that.
NEAT, the energy of everything that is not sleep, eating or exercise, is high in manual and agricultural workers and falls with industrialisation (Levine, 2002).
At the same weight, height and age, Mifflin-St Jeor puts a man 166 kcal a day above a woman: its sex terms are +5 and −161.
Frequently asked questions
BMR is measured under strict standard conditions: just after waking, fasted, at rest, in a thermoneutral room. RMR (or REE) is measured at rest under looser conditions, so it runs a little higher. In practice the terms are used interchangeably. Mifflin-St Jeor was fitted to resting measurements; Harris and Benedict, and Cunningham, to basal ones.
Two reasons: (1) a smaller body burns less, and every equation here scales with body mass; (2) "adaptive thermogenesis": measured expenditure can fall further than the lost mass alone predicts. How much varies a great deal between people and studies. Recalculate as your weight changes.
Modestly. Build muscle: skeletal muscle uses about 13 kcal per kg a day at rest (Wang et al., 2010), so a few kilograms of muscle is worth tens of kilocalories, not hundreds. Eat enough protein: the ISSN gives 1.4–2.0 g/kg a day for most people who exercise, and protein has the highest thermic effect of the three macronutrients. Avoid crash diets, which cost lean mass along with fat.
Different equations and different ways of estimating activity, most of them proprietary and unpublished. None is calibrated to your individual physiology. Use any one source consistently and judge it by your weight trend; don't mix numbers from several.
Thyroid hormone sets much of resting metabolism, so an under- or over-active thyroid can move your real RMR well away from any equation, which assumes a healthy adult. If you have symptoms (fatigue, weight change despite a steady diet, feeling unusually cold or hot), see a doctor; thyroid function is a blood test, and an equation is no substitute for it.
The phrase is dramatic; the underlying phenomenon, adaptive thermogenesis, is real, but its size varies widely between people and studies, and it is not permanent damage in the everyday sense. Weight loss also slows simply because a smaller body needs less energy, which is why a fixed deficit loses less each month (Hall et al., Lancet 2011).
Track results over 2-3 weeks. If you're losing weight on stated TDEE − 500, the multiplier is right. If maintaining weight on stated TDEE − 500, the multiplier is overestimating. Adjust down a tier (×1.55 → ×1.375) and retest. Calorie tracking accuracy + label inaccuracy can also contribute.
No. All inputs stay in your browser.
Yes. At the same weight, women typically carry more fat and less muscle than men, and the equations encode that: Mifflin-St Jeor's sex terms differ by exactly 166 kcal a day; Harris-Benedict uses different coefficients altogether, so its gap depends on your size. The lean-mass equations have no sex term at all. For transgender people on hormone therapy, body composition shifts over months; a lean-mass equation with a measured body-fat figure sidesteps the choice.
Mifflin MD et al. Am J Clin Nutr 1990;51:241-247. Harris JA, Benedict FG. Proc Natl Acad Sci 1918;4:370-373. Cunningham JJ. Am J Clin Nutr 1980;33:2372-2374. Frankenfield D et al. J Am Diet Assoc 2005;105:775-789 (systematic review). FAO/WHO/UNU, Human Energy Requirements (2004), chapter 5.
Method & sources
How it computes
Mifflin-St Jeor 1990 (10 × kg + 6.25 × cm − 5 × age, +5 men / −161 women) is the headline. Compared: Harris-Benedict 1918 as printed in PNAS, Cunningham 1980 (500 + 22 × lean mass) and the textbook Katch-McArdle form (370 + 21.6 × lean mass), lean mass = weight × (1 − body fat %), skipped when body fat is blank. TDEE = Mifflin × 1.2/1.375/1.55/1.725/1.9, a calculator convention (EnergyNeedsKernel).
What this tool implements
- Mifflin-St Jeor resting energy expenditure equation (1990), sex-separated simplified form as published; fitted to 498 healthy adults aged 19-78, so ages outside that are computed and noted, and under-18s refused.
- Harris-Benedict 1918, exactly as printed: men 66.4730 + 13.7516 w + 5.0033 s − 6.7550 a; women 655.0955 + 9.5634 w + 1.8496 s − 4.6756 a (w kg, s cm, a years). The authors tabulated it for 25.0-124.9 kg, 151-200 cm and 21-70 years; inputs outside that are noted on the result.
- Lean-mass equations: Cunningham 1980, BMR = 500 + 22 × lean body mass, from Harris and Benedict's own subjects; the 370 + 21.6 × lean mass "Katch-McArdle" form is a textbook equation not traced to an original paper and is labelled so on the page.
- Frankenfield et al. 2005 (systematic review for the American Dietetic Association): Mifflin-St Jeor was the most reliable of four common equations, within 10% of measured RMR in more people than any other, with the narrowest error range.
- Activity factors 1.2-1.9 are a calculator convention; FAO/WHO/UNU 2004 Table 5.3 puts a sedentary or light lifestyle at PAL 1.40-1.69, and the page says the ×1.2 row sits below it. BMR is about 45-70% of total expenditure in adults (FAO/WHO/UNU 2004, §5.2).
- Corrected 2026-10-16: Harris-Benedict switched from the Roza & Shizgal 1984 coefficients (not readable at source) to the 1918 equations; empty fields no longer fall back to 35 y / 75 kg / 175 cm / 20% body fat; ages 15-17 refused; body fat optional; "RMR ≈ 60-75% of TDEE" → FAO 45-70%; Mifflin "~10% under-estimate" vs Harris-Benedict "5-10% over-estimate", "50-150 kcal/day" and "ADA adopted" → Frankenfield's own conclusion; Katch-McArdle example with no stated height or age (Mifflin "1,950") → a fully specified case (1,880 / 2,002 / 2,120 / 2,282); "muscle ~3× fat" and "~15 kcal per kg of muscle" → 13 vs 4.5 kcal/kg/day (Wang 2010); "RMR declines 1-3% per decade after 30" → Pontzer 2021; unsourced percentages for indirect calorimetry, starvation mode, NEAT, thyroid, adaptive thermogenesis, BMR vs RMR and BMI-to-body-fat removed; device claims about Apple Health, Garmin and Whoop removed; an unsourced pull-quote replaced.
Sources
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51(2):241-7. PMID 2305711
- Harris JA, Benedict FG. A biometric study of human basal metabolism. Proc Natl Acad Sci U S A 1918;4(12):370-3. PMID 16576330. Text read from the JSTOR Early Journal Content scan: https://archive.org/details/jstor-83837
- Cunningham JJ. A reanalysis of the factors influencing basal metabolic rate in normal adults. Am J Clin Nutr 1980;33(11):2372-4. PMID 7435418
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr 1984;40(1):168-82. PMID 6741850 (abstract: "precision of 14%")
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc 2005;105(5):775-89. PMID 15883556
- FAO/WHO/UNU. Human Energy Requirements, Food and Nutrition Technical Report Series 1, 2004: §5.2 (BMR 45-70% of TEE) https://www.fao.org/4/y5686e/y5686e06.htm and Table 5.3 (PAL categories) https://www.fao.org/4/y5686e/y5686e07.htm
- Westerterp KR. Diet induced thermogenesis. Nutr Metab (Lond) 2004;1(1):5. PMID 15507147; full text https://europepmc.org/article/PMC/PMC524030
- Wang Z, Ying Z, Bosy-Westphal A, et al. Specific metabolic rates of major organs and tissues across adulthood. Am J Clin Nutr 2010;92(6):1369-77. PMID 20962155
Abridged — the full review record for this tool runs longer than the list above.
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