Predictive equations are estimates: a 2005 systematic review found Mifflin-St Jeor within 10 % of measured resting energy for most adults, but not all.
At rest your body uses about 1,649 kcal a day by Mifflin-St Jeor; at the “moderately active” level that comes to about 2,556 kcal a day to hold your weight steady. The 3 formulas span 1,580–1,696 kcal at rest.
BMR by formula
Daily calories by activity level (5 rows)
Activity level
Factor
Mifflin-St Jeor
Harris-Benedict (revised)
Katch-McArdle
Sedentary
1.2
1,979
2,035
1,896
Lightly active
1.375
2,267
2,332
2,172
Moderately active
1.55
2,556
2,628
2,448
Very active
1.725
2,844
2,925
2,725
Extra active
1.9
3,133
3,222
3,001
How it's calculated S
Inputs in kilograms, centimetres and years
w=70kg,h=175cm,a=30
Lean body mass from body fat
LBM=70×(1−10020)=56.00kg
BMR by Mifflin-St Jeor
BMR=10×70+6.25×175−5×30+5=1,648.75kcal/day
Multiply by the activity factor
TDEE=1,648.75×1.55=2,555.56kcal/day
Calories are displayed as whole numbers, rounded half-up.
About the BMR and TDEE calculator
Basal metabolic rate (BMR) is the energy the body uses at complete rest. Mifflin-St Jeor computes it as 10 × weight in kg + 6.25 × height in cm − 5 × age in years, plus 5 for men or minus 161 for women; the revised Harris-Benedict equation and the Katch-McArdle equation, which works from lean body mass, are shown beside it. Total daily energy expenditure (TDEE) multiplies BMR by an activity factor from 1.2 (sedentary) to 1.9 (extra active).
TDEE is the usual starting point for a calorie target to lose, hold or gain weight. A 30-year-old man weighing 70 kg at 175 cm has a Mifflin-St Jeor BMR of 1,649 kcal a day, and at the moderately active factor of 1.55 his TDEE is about 2,556 kcal.
Both numbers are population estimates. Mifflin-St Jeor was fitted to 498 adults aged 19 to 78, and the activity factors are conventional multipliers rather than measurements, so an individual's measured needs can differ by 10 % or more.
Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle
Mifflin-St Jeor and Harris-Benedict were fitted to resting energy measured by indirect calorimetry, which works out heat output from the oxygen a person breathes in and the carbon dioxide they breathe out. The three equations differ in what they ask for and in how well they have held up when tested on new people.
Equation
Inputs
Built from
How it performed in later tests
Mifflin-St Jeor (1990)
Weight, height, age, sex
498 adults aged 19 to 78, 234 of them obese
More than 10 % off in 22 % of 130 adults; under 50 % accurate in recreational athletes
Harris-Benedict, Roza-Shizgal revision (1984)
Weight, height, age, sex
The 1919 Harris-Benedict subjects plus 98 more from Benedict's later work
More than 10 % off in 33 % of the same 130 adults, and in 36 % of the obese ones
Katch-McArdle
Lean body mass only
Cunningham's 1991 review of lean-mass studies
A lean-mass equation of the same form was the accurate one in athletes
The 130-adult comparison is Frankenfield and colleagues (2003), whose volunteers had BMIs from 18.8 to 96.8. The Owen equation, tested alongside, missed by more than 10 % in 35 %. Their abstract does not say which version of Harris-Benedict they used; the athlete study below tested the 1984 revision the calculator uses.
The athlete result comes from ten Haaf and Weijs (2014), who measured 90 recreational athletes training about nine hours a week. Mifflin-St Jeor, Harris-Benedict, WHO, Schofield and Owen each predicted fewer than half of them within 10 %, while Cunningham's lean-mass equation (500 + 22 × lean mass in kg) did so for 85 % of the men and 78 % of the women. Katch-McArdle, 370 + 21.6 × lean mass, has the same structure: its constants are the ones Cunningham proposed in a 1991 review, where lean mass explained 65 to 90 % of the variation in resting energy. Cunningham (1980) had found, reanalysing 223 of Harris and Benedict's original subjects, that lean body mass alone predicted basal metabolic rate and that sex and age added little once it was known.
How to calculate BMR and TDEE by hand
Take a 52-year-old woman who weighs 68 kg, is 163 cm tall, has 32 % body fat and is lightly active.
Harris-Benedict (revised, women): 447.593 + 9.247 × 68 + 3.098 × 163 − 4.330 × 52 = 447.593 + 628.796 + 504.974 − 225.160 = 1,356.203 kcal. For men the constants are 88.362, 13.397, 4.799 and 5.677.
Katch-McArdle: Lean mass is 68 × (1 − 0.32) = 46.24 kg, so BMR is 370 + 21.6 × 46.24 = 1,368.784 kcal. Sex and age do not enter the equation.
TDEE: Multiply the chosen BMR by the activity factor. By Mifflin-St Jeor at the lightly active factor, 1,277.75 × 1.375 = 1,756.9, which the calculator shows as 1,757 kcal a day. The same factor gives 1,865 kcal from Harris-Benedict and 1,882 kcal from Katch-McArdle.
The three BMRs span 91 kcal, 7.1 % of the Mifflin figure. Keep every decimal until the last step; the calculator rounds only the displayed result, half-up to whole calories.
Activity multipliers table
The factors are the ratio of total daily energy use to BMR. The FAO/WHO/UNU expert consultation calls the same ratio the physical activity level (PAL), and its 2004 report on human energy requirements gives measured ranges for it. The last column places each factor in those ranges; the TDEE figures are for the woman above.
Activity level
Factor
Description
TDEE (Mifflin)
FAO/WHO/UNU lifestyle band
Sedentary
1.2
Desk job, little or no exercise
1,533 kcal
Below the lowest band (1.40)
Lightly active
1.375
Light exercise 1–3 days a week
1,757 kcal
Below the lowest band
Moderately active
1.55
Moderate exercise 3–5 days a week
1,981 kcal
Sedentary or light, 1.40 to 1.69
Very active
1.725
Hard exercise 6–7 days a week
2,204 kcal
Active or moderately active, 1.70 to 1.99
Extra active
1.9
Physical job or training twice a day
2,428 kcal
Active or moderately active, 1.70 to 1.99
The two lowest factors are conservative. A meta-analysis the report cites found a typical PAL of 1.60 in 411 adults with a mostly sedentary Western lifestyle, and an obesity task force suggested 1.50 to 1.55 for sedentary people. The report treats a PAL of 1.21 as a short-term survival figure for totally inactive, dependent people, and sets the vigorous band at 2.00 to 2.40, noting that higher values are hard to keep up. An hour a day of jogging, cycling or similar exercise raises PAL from about 1.55 to 1.75.
For the woman above, the gap between the sedentary factor and a PAL of 1.60 is 511 kcal a day. The report's sedentary examples are office workers who drive and rarely exercise, and even they start at 1.40, above the calculator's lightly active factor.
What BMR measures and what TDEE adds
Basal metabolic rate is measured under strict conditions: awake, lying down, after 10 to 12 hours without food and 8 hours of physical rest, relaxed, at a room temperature that neither chills nor warms you. Mifflin-St Jeor was fitted to resting energy expenditure (REE, also called RMR); the calculator, like most sources, reports its result as BMR.
Mifflin-St Jeor is the default and did best in the 130-adult comparison above. Switch to Katch-McArdle when you are muscular or train hard and have a body fat figure from a reliable method; the body fat calculator and lean body mass calculator give estimates you can enter.
Mistakes that move the answer most:
A guessed body fat in Katch-McArdle. Each 5 percentage points shifts this woman's lean mass by 3.4 kg and her BMR by 73 kcal.
Choosing the level from gym sessions alone. The FAO bands describe a whole lifestyle, including work and getting around.
Treating the number as exact. Even for Mifflin-St Jeor, one person in five was more than 10 % off in Frankenfield's test. At 1,757 kcal, 10 % is 176 kcal a day.
Using it outside the fitted range. Mifflin-St Jeor was built on adults up to 78, and the calculator warns above that age.
The most reliable check is your own weight trend. If your weight holds steady for two or three weeks on a known intake, that intake is your real TDEE. To turn the estimate into a calorie target for losing or gaining weight, with macros, use the calorie and macro calculator.
BMR is the energy used at complete rest to keep the body running; TDEE adds the energy spent moving and digesting food. TDEE is BMR times an activity factor: 1.2 for sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active and 1.9 extra active. A BMR of 1,649 kcal becomes 1,979 kcal a day at the sedentary factor and 2,556 kcal at 1.55.
Which BMR formula is most accurate?
For most adults, Mifflin-St Jeor. A 2005 systematic review for the American Dietetic Association (Frankenfield and colleagues) found it predicted resting metabolic rate within 10 % of the measured value in more non-obese and obese people than the Harris-Benedict, Owen or WHO/FAO/UNU equations, with the narrowest error range. Katch-McArdle works from lean mass instead, so its accuracy depends on the body fat figure entered.
Does BMR go down with age?
Yes. In the Mifflin-St Jeor equation BMR falls by 5 kcal a day for each year of age at the same weight and height, so a 70 kg, 175 cm man drops from 1,649 kcal at 30 to 1,549 kcal at 50. Much of the fall reflects loss of lean mass: Mifflin and colleagues found fat-free mass the best single predictor of resting energy expenditure (R² = 0.64).
Why is Harris-Benedict higher than Mifflin-St Jeor?
Harris-Benedict was fitted in 1919 to 239 people, and Mifflin and colleagues found in 1990 that it overestimated measured resting energy by 5 %. Roza and Shizgal's 1984 revision, used here, refitted the data with 98 more subjects and reports a precision of ±14 %. For the default 30-year-old man it gives 1,696 kcal, 2.8 % above Mifflin-St Jeor's 1,649 kcal.
How accurate is the BMR and TDEE calculator?
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “Man, 30, 70 kg, 175 cm, moderately active” is checked against Python 3.8 decimal: 10·70 + 6.25·175 − 5·30 + 5 = 1648.75; × 1.55 = 2555.5625; Roza–Shizgal 88.362 + 13.397·70 + 4.799·175 − 5.677·30 = 1695.667; 370 + 21.6·56 = 1579.6.
Where does the method come from?
Mifflin MD, St Jeor ST et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51:241–247; Roza AM, Shizgal HM. The Harris Benedict equation reevaluated. Am J Clin Nutr 1984;40:168–182; Cunningham JJ. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation. Am J Clin Nutr 1991;54:963–969 (source of 370 + 21.6 × lean mass); McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy, and Human Performance (the textbook that named the Katch-McArdle equation); Frankenfield D et al. Comparison of predictive equations for resting metabolic rate. J Am Diet Assoc 2005;105:775–789.