BMR Formula: Mifflin-St Jeor, Harris-Benedict, and More
All the major BMR formulas compared — with worked examples, activity multipliers for TDEE, and guidance on which formula to use for your situation.
What Is BMR?
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — lying still, fasted, in a warm environment — just to keep essential functions running: breathing, circulation, cell repair, and temperature regulation. It represents the minimum energy your body needs to survive.
BMR is distinct from Total Daily Energy Expenditure (TDEE), which is the total calories burned in a day after accounting for physical activity. To find TDEE, you multiply BMR by an activity factor (see below). Understanding your BMR is the foundation for any evidence-based approach to calorie targets.
The Mifflin-St Jeor Formula
Published in 1990 by researchers M.D. Mifflin and S.T. St Jeor, this is the formula recommended by the Academy of Nutrition and Dietetics and is considered the most accurate for the general population without known body composition data.
− (5 × age) + 5
− (5 × age) − 161
Weight term: 10 × 75 = 750
Height term: 6.25 × 178 = 1,112.5
Age term: 5 × 30 = 150
BMR = 750 + 1,112.5 − 150 + 5 = 1,717.5 kcal/day
This person needs roughly 1,718 kcal/day at complete rest.
Activity Multipliers: From BMR to TDEE
To estimate how many calories you burn in a normal day, multiply your BMR by the activity factor that best matches your lifestyle. These multipliers were popularised by Harris and Benedict and are sometimes called the Harris-Benedict activity factors.
| Activity level | Multiplier | Example (1,718 kcal BMR) |
|---|---|---|
| Sedentary (desk job, little exercise) | × 1.2 | 2,062 kcal/day |
| Lightly active (1–3 days/week exercise) | × 1.375 | 2,362 kcal/day |
| Moderately active (3–5 days/week) | × 1.55 | 2,663 kcal/day |
| Very active (hard training 6–7 days/week) | × 1.725 | 2,963 kcal/day |
| Extremely active (physical job + 2× daily training) | × 1.9 | 3,264 kcal/day |
The Harris-Benedict Formula
Developed in 1919 by James Arthur Harris and Francis Gano Benedict, this was the dominant BMR formula for over 70 years. A revised version by Roza and Shizgal (1984) corrected errors in the original using a larger dataset.
+ (5.003 × cm) − (6.755 × age)
+ (4.799 × cm) − (5.677 × age)
BMR = 66.5 + (13.75 × 75) + (5.003 × 178) − (6.755 × 30)
= 66.5 + 1,031.25 + 890.5 − 202.65
= 1,785.6 kcal/day — about 68 kcal higher than Mifflin-St Jeor
The original Harris-Benedict formula tends to overestimate BMR by 5–15% compared to modern indirect calorimetry measurements. The revised Roza and Shizgal version is more accurate but still slightly exceeds Mifflin-St Jeor.
Katch-McArdle Formula
The Katch-McArdle formula skips weight and height entirely, instead using lean body mass (LBM) — the mass of everything except body fat. This makes it the most accurate formula for people who know their body fat percentage, particularly athletes and muscular individuals whose BMR is underestimated by the other formulas.
To use this formula, first calculate your lean body mass: LBM = total weight × (1 − body fat fraction). For example, a 75 kg person at 20% body fat has an LBM of 75 × 0.80 = 60 kg. Their BMR = 370 + (21.6 × 60) = 1,666 kcal/day. Use the Lean Body Mass calculator to find your LBM.
Which Formula Should You Use?
| Formula | Best for | Limitation |
|---|---|---|
| Mifflin-St Jeor | Most adults — general population | Less accurate at extremes of weight |
| Harris-Benedict (revised) | When Mifflin is not available | Tends to overestimate by 5–10% |
| Katch-McArdle | Athletes, bodybuilders, known body fat % | Requires accurate body fat measurement |
For most people without body composition data, Mifflin-St Jeor is the recommended starting point. Treat any formula output as an estimate — individual metabolism varies by up to ±300 kcal/day from predicted values due to genetics, hormones, and medical conditions.
Use the free BMR calculator — enter your height, weight, age, and activity level to get your basal metabolic rate and daily calorie needs across all three formulas.
Calculate Your BMR →Frequently Asked Questions
How accurate is the BMR formula?
The Mifflin-St Jeor formula predicts BMR within 10% of measured values for roughly 80% of people in validation studies. It is least accurate for those with severe obesity and for highly trained athletes, whose body composition differs significantly from the population used to derive the formula.
Does BMR change with age?
Yes. BMR declines gradually after around age 30, primarily due to the age-related loss of lean muscle mass (sarcopenia). The Mifflin-St Jeor formula accounts for this through its age term. Each decade of age reduces BMR by roughly 50–100 kcal/day, independent of activity level.
What is the difference between BMR and RMR?
BMR is measured under strict conditions: lying still, fasted for 12 hours, in a thermoneutral environment after a full night's sleep. RMR (Resting Metabolic Rate) is measured under more relaxed conditions and is typically 10–20% higher. Most formulas technically estimate RMR, though the terms are used interchangeably in practice.
How do I calculate TDEE from BMR?
Multiply your BMR by an activity factor: 1.2 for sedentary, 1.375 for lightly active (1–3 days/week exercise), 1.55 for moderately active (3–5 days), 1.725 for very active (6–7 days hard training), and 1.9 for extremely active (physical job plus twice-daily training). The result is your estimated total daily calorie burn.
Evidence & Methodology
How This Page Is Grounded
Method
Documents the Mifflin–St Jeor and Harris–Benedict-style prediction equations and works through their inputs step by step.
Important limitation: Prediction equations estimate population averages and do not replace measured resting energy expenditure.
Primary Sources
Quality Checks
Sex-specific constants, units and worked examples are checked against the cited research.
See how sources are selected and corrections are handled in our Editorial & Calculation Methodology, and which automated checks this page has to pass in How We Test.