Katch-McArdle BMR Calculator
The Katch-McArdle formula calculates your Basal Metabolic Rate (BMR) from lean body mass rather than total body weight, making it the most accurate BMR equation for people who know their body fat percentage. Enter your weight, body fat percentage, and activity level to get your BMR, Total Daily Energy Expenditure (TDEE), and a daily calorie target matched to your goal.
Formula
Worked example
A person weighing 80 kg with 20% body fat has a lean body mass of 80 x 0.80 = 64 kg. BMR = 370 + (21.6 x 64) = 370 + 1382 = 1752 kcal/day. At a moderate activity level (x1.55), TDEE = 1752 x 1.55 = 2716 kcal/day.
What is the Katch-McArdle formula?
The Katch-McArdle equation was published in 1975 by Frank Katch and William McArdle as a way to calculate Basal Metabolic Rate (BMR) from lean body mass alone, without needing sex, age, or height. The formula is: BMR = 370 + (21.6 x lean body mass in kg). Because metabolically active muscle tissue is a primary driver of resting calorie burn, this approach is more accurate than equations that use total body weight for people who know their body composition. It is the preferred BMR formula for athletes, bodybuilders, and anyone who has measured their body fat percentage.
How lean body mass is calculated
Lean body mass (LBM) is everything in your body except fat: muscle, bone, organs, water, and connective tissue. If you know your body fat percentage, the calculation is straightforward: LBM = total weight x (1 - body fat % / 100). For example, an 80 kg person at 20% body fat has an LBM of 64 kg. If you do not have a body fat measurement, this calculator falls back to the Boer formula, which estimates LBM from weight, height, and sex. The Boer estimate is less precise than a direct body fat measurement, so using a measured body fat % (from a DEXA scan, hydrostatic weighing, or skinfold calipers) will give a more accurate BMR.
BMR vs TDEE: what is the difference?
BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest to keep vital functions running: breathing, circulation, cell repair, and temperature regulation. It is the floor of your calorie needs. TDEE (Total Daily Energy Expenditure) adds the calories burned through daily activity, exercise, and the thermic effect of food. TDEE = BMR x activity multiplier, where the multiplier ranges from 1.2 for a sedentary lifestyle to 1.9 for people with physically demanding jobs or twice-daily training. Most people aiming to manage their weight track TDEE, not BMR, because it reflects real-world calorie needs.
Choosing a calorie target for your goal
Once you have your TDEE, adjusting calories by 10-20% in either direction is a practical starting point. A 20% deficit (-20%) supports fat loss while preserving muscle when protein intake is adequate. A mild 10% deficit minimises muscle loss and is more sustainable for many people. Maintenance keeps weight stable. A 10% surplus supports lean muscle growth when combined with resistance training, while a 20% surplus accelerates bulk but carries more fat gain. These are starting points, not rules: weight change over 2-3 weeks of consistent eating is the most reliable calibration tool. If the scale is not moving as expected, adjust your intake by 100-200 kcal/day and reassess.
Activity level multipliers (Harris/Katch)
| Activity level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, 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 |
| Extra active | Physical job or twice-daily training | 1.9 |
Multiply your BMR by the factor for your typical week to get TDEE.
Frequently asked questions
Is the Katch-McArdle formula more accurate than Mifflin-St Jeor?
For people with a known body fat percentage, yes. Mifflin-St Jeor uses total body weight, height, age, and sex, which means it treats a pound of fat and a pound of muscle as equally metabolically active. Katch-McArdle uses only lean body mass, which is where virtually all resting metabolism occurs. Studies typically find Katch-McArdle is more accurate for athletes and individuals with above-average or below-average body fat. For the general population without body composition data, Mifflin-St Jeor is a reasonable alternative.
What body fat measurement method is most accurate?
DEXA (dual-energy X-ray absorptiometry) scanning is considered the gold standard for body composition measurement and is accurate to within about 1-2%. Hydrostatic (underwater) weighing is similarly precise. Air displacement plethysmography (Bod Pod) is also reliable. Skinfold calipers with a trained technician are practical and accurate to within 3-4% when done correctly. Consumer bioelectrical impedance scales are the least accurate, with errors of 5% or more common, and they are sensitive to hydration status.
Can I use this calculator without knowing my body fat percentage?
Yes. When you leave the body fat field blank, this calculator estimates lean body mass using the Boer formula, which uses your weight, height, and sex. The Boer estimate is less precise than a measured body fat percentage, so the BMR result will be less accurate, but it is still a useful starting point. If you have access to body composition testing, getting your body fat measured will noticeably improve the accuracy of the estimate.
Why does my BMR change if my body fat changes even if my weight stays the same?
Because lean body mass, not total weight, drives your resting metabolic rate. If your weight stays at 80 kg but your body fat drops from 25% to 20%, your lean mass increases from 60 kg to 64 kg. That extra 4 kg of metabolically active muscle raises your BMR by about 86 kcal/day (21.6 x 4). This is why resistance training and muscle retention are important during a calorie deficit: preserving lean mass prevents the drop in BMR that can make weight regain more likely.
What is a normal BMR range?
BMR varies considerably with body size and composition. A small woman might have a BMR of 1200-1400 kcal/day while a large muscular man might have a BMR above 2200 kcal/day. Typical ranges are roughly 1200-1600 kcal/day for women and 1500-2100 kcal/day for men, though athletes with high lean mass sit toward or above the top of these ranges. BMR tends to decline with age as muscle mass decreases, which is one reason calorie needs shift over a lifetime.
How should I use my TDEE to lose fat without losing muscle?
A moderate calorie deficit of 10-20% below TDEE, combined with high protein intake (1.6-2.2 g per kg of lean body mass) and progressive resistance training, is well-supported by research for minimising muscle loss during fat loss. Larger deficits speed fat loss in the short term but increase the risk of losing muscle and reducing BMR. Recalculate your TDEE every 4-6 weeks as your body weight and composition change, since both affect your calorie needs.