Muscle Mass Calculator: How Much Skeletal Muscle You Carry

Skeletal muscle is the muscle attached to your skeleton, the muscle you move with and the muscle strength training builds. MRI and CT scans can measure it directly, but they are costly research tools, so most people use a prediction equation. This calculator uses the one Lee and colleagues built from whole-body MRI scans (Lee et al. 2000), which needs only your sex, age, height and weight.

The result is the typical amount of muscle for an adult of your sex, age and size, not a measurement of your body. The sections below show how the number is calculated, how it compares with MRI averages for men and women, how it changes with age, how far off it can be and how to build more.

How to calculate skeletal muscle mass

The calculator uses the second, simpler model in Lee et al. (2000), published in the American Journal of Clinical Nutrition. The authors measured skeletal muscle with whole-body MRI in 244 adults aged 20 to 81, all with a BMI under 30, at research centers in New York and in Kingston, Canada. They then fitted the scans to body weight, height, sex, age and race.

Skeletal muscle (kg) = 0.244 × weight (kg) + 7.80 × height (m) + 6.6 × sex − 0.098 × age + group − 3.3Sex is 1 for men and 0 for women. Group is −1.2 for Asian adults, +1.4 for African American adults and 0 for white and Hispanic adults.

For the default man, a 35-year-old who is 5 ft 10 in and 180 lb, the arithmetic takes four steps.

1. Convert to metric. 180 lb × 0.4536 is 81.6 kg, and 5 ft 10 in is 70 inches, or 1.778 m.
2. Multiply. 0.244 × 81.6 is 19.9, and 7.80 × 1.778 is 13.9.
3. Add and subtract. Add 6.6 for a man, then take off 0.098 × 35 = 3.4 for age and the constant 3.3: 19.9 + 13.9 + 6.6 − 3.4 − 3.3 = 33.7 kg.
4. Convert back. 33.7 kg ÷ 0.4536 is 74.2 lb. Divided by his weight, 33.7 ÷ 81.6, that is 41.2 percent.

For a woman, leave out the 6.6. A 35-year-old woman who is 5 ft 5 in and 140 lb gets 21.6 kg (47.7 lb) of skeletal muscle, 34.1 percent of her weight.

The equation fits the scans well. It explained 86 percent of the differences in measured muscle between people (R² 0.86), with a standard error of 2.8 kg. The same paper gives a more accurate model, with an R² of 0.91 and a standard error of 2.2 kg, but it needs skinfold calipers and tape measurements of the arm, thigh and calf. This calculator uses the height-and-weight model because you can fill it in without any equipment.

The optional race or ethnicity term

Lee's equation includes race, recorded with the US federal categories the study used. With size, age and sex accounted for, Asian adults in the study had about 1.2 kg less skeletal muscle on average, and African American adults about 1.4 kg more, than white and Hispanic adults, who share the zero term. A group average says nothing about any one person, so the field is optional and starts at no adjustment. Either adjustment is at most half the equation's standard error. For the default man, the two options move 74.2 lb to 71.6 lb or 77.3 lb.

Skeletal muscle vs lean body mass

Skeletal muscle is one part of lean body mass, which also counts bone, organs, skin and body water. Our lean body mass calculator estimates that whole total with four published formulas. For a 5 ft 10 in, 180 lb man, its Boer formula gives 135.6 lb of lean mass, while this calculator gives a 35-year-old man of that size 74.2 lb of skeletal muscle, about 55 percent of it. So never compare a muscle number with a lean mass number, and check which one a smart scale reports before you compare it with anything.

What is a normal muscle mass percentage?

There is no single healthy target for skeletal muscle, but two MRI studies give the averages. Janssen and colleagues (2000) scanned 468 men and women aged 18 to 88. Lee's 244 adults are the group this calculator's equation was built on.

Lee's averages are higher as a share of weight, and his group left out anyone with a BMI of 30 or more. Muscle makes up a smaller share of a heavier body. In Janssen's data, muscle kept rising with body weight, but each extra kilogram of weight carried less muscle than the one before. Muscle also rose in a straight line with height, which is why the result card also shows a muscle index, your skeletal muscle in kilograms divided by your height in meters squared.

Muscle mass for women and men

Women carry less skeletal muscle than men, both in kilograms and as a share of weight: 21.0 against 33.0 kg, and 30.6 against 38.4 percent, in Janssen's study. The gap was larger in the upper body, about 40 percent, than in the lower body, about 33 percent. In the Lee equation, a woman of the same height, weight and age gets 6.6 kg (14.6 lb) less muscle than a man. The default woman above, 35 years old, 5 ft 5 in and 140 lb, comes out at 34.1 percent, close to the 33.1 percent average of the women in Lee's study.

Muscle mass by age

Muscle changes slowly through your 30s and 40s. In Janssen's MRI data, muscle as a share of body weight started to fall in the third decade of life, your 20s, while the amount of muscle held steady until the end of the fifth decade, around age 50. Most of the later loss came from the lower body.

The Lee equation carries this as its age term. At the same height and weight, each year takes 0.098 kg off the estimate, about 1 kg (2.2 lb) per decade. That term comes from comparing people of different ages in one study, not from following the same people over time. The table shows the default man and woman at five ages.

The table holds weight steady. If your weight climbs with age and the extra is fat, muscle's share of your weight falls faster than the table shows.

Sarcopenia and the muscle index

Muscle loss with age is called sarcopenia. In 4,449 adults aged 60 and over, Janssen and colleagues (2004) linked a skeletal muscle index at or below 8.50 kg/m² in men and 5.75 kg/m² in women, measured by bioimpedance, with a higher risk of physical disability. Those cut-offs were set for people over 60 with a different measuring method, so the calculator shows your muscle index without a label. In your 30s and 40s the useful step is to build muscle now, which the section on building muscle below covers. For how doctors diagnose it, see our summary of the sarcopenia research.

How accurate is this muscle mass calculator?

The calculator does the arithmetic exactly, so the question is how close the equation gets to an MRI scan. For most people the answer is within a few kilograms.

• In the adults it was built on. The equation explained 86 percent of the differences in measured muscle among the 244 adults, with a standard error of 2.8 kg (6.2 lb). That is about 9 percent of the average man's muscle in the study and 13 percent of the average woman's. By the usual meaning of a standard error, about two in three people land within that distance of their scan, which is the band the result card shows.
• In a half-sample test. Before pooling all 244 adults, the authors fitted the same kind of equation on half of them and tested it on the other 122. It explained 86 percent of the differences there too, with a standard error of 2.6 kg and no significant average bias.
• At a BMI of 30 or more. The final equation was then tested in 80 adults with obesity. It still tracked their scans (R² 0.79, standard error 3.0 kg), but it read about 2.3 kg high on average, roughly 10 percent, and the authors advise against using it at that weight. The calculator still shows a number above a BMI of 30, flags it and calls it an upper estimate.
• In a separate group. Al-Gindan and colleagues (2014) applied the equation to 197 other adults scanned at the same New York center, with BMIs from 15.7 to 36.4. It explained 75 percent of the differences in men and 63 percent in women, with standard errors of 2.9 and 2.1 kg, and read 2.8 kg high on average across both sexes. For 19 in 20 people it landed between 2.4 kg below and 8 kg above their scan. In men it read high for those with less muscle and low for those with more.

Two more limits matter. The study left out people in structured exercise programs, so the equation describes untrained adults and cannot see training. Two people of the same sex, age, height and weight get the same number even if one lifts and the other does not. It also will not show progress. Trade a pound of fat for a pound of muscle and your weight stays the same, so the estimate does too.

Worked examples: eight adults through the calculator

Each row runs the calculator with no race or ethnicity adjustment. The range is one standard error, 2.8 kg, either side of the estimate.

Two patterns stand out. Weight moves the estimate less than people expect, because each extra pound of body weight adds only about a quarter of a pound of muscle in the equation. That is why the 235 lb man has the most muscle in the table but the smallest share of his weight among the men. And ten years of age at the same size take off 2.2 lb, as the first two rows show.

How to build more muscle

The equation cannot see training, but your muscles respond to it. Three habits do most of the work.

• Strength train at least 2 days a week. That is the minimum in the Physical Activity Guidelines for Americans (Piercy et al. 2018), and dumbbells, bands or your own body weight all count. Our guide to building muscle at home lays out a full-body plan.
• Give it months. In a meta-analysis of 111 studies in healthy men, resistance training added about 1.5 kg (3.4 lb) of muscle mass on average, and the participants' age, weight and height did not explain differences in the gain (Benito et al. 2020).
• Eat enough protein. In a meta-analysis of 49 trials with 1,863 people, protein supplements added about 0.3 kg of fat-free mass on top of training, with no further gain above a total of about 1.6 g of protein per kg of body weight a day (Morton et al. 2018). The protein calculator turns that into grams for your weight.

Because the estimate only changes when your weight does, track your progress with something that can see muscle: the same tape measurements every four weeks, progress photos, or a DXA scan taken under the same conditions each time. Our article on how much muscle you can gain in a month explains how to measure small changes.

When to ignore this calculator

The Lee equation describes healthy, untrained adults of average build. In these cases, skip the number or ask a clinician for a measurement.

• A BMI of 30 or more. The equation reads about 2.3 kg high at that weight, and its authors advise against using it there. A DXA or bioimpedance measurement will serve you better.
• Regular lifters and athletes. The study left out people in structured exercise programs, so the estimate probably reads low for you.
• Pregnancy. Pregnancy changes weight and body water in ways the equation was not built for. Ask your obstetric care team.
• An eating disorder, now or in the past. Body composition numbers can work against recovery. Work with your treatment team.
• Conditions that change muscle or body fluid. The study left out people with medical conditions or medications known to affect body composition. Swelling from kidney, heart or liver disease, a muscle disease, an amputation or a long stretch of bed rest all break the link between your size and your muscle.
• Under 20 or over 81. The equation was built on adults aged 20 to 81, so the calculator stops outside that range.

Related reading

References

1. Lee RC, Wang Z, Heo M, Ross R, Janssen I, Heymsfield SB. "Total-body skeletal muscle mass: development and cross-validation of anthropometric prediction models." Am J Clin Nutr. 2000;72(3):796-803. doi:10.1093/ajcn/72.3.796, PubMed 10966902. The calculator uses model 2, the body weight and height equation (Equation 6).
2. Janssen I, Heymsfield SB, Wang ZM, Ross R. "Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr." J Appl Physiol. 2000;89(1):81-88. doi:10.1152/jappl.2000.89.1.81, PubMed 10904038
3. Al-Gindan YY, Hankey C, Govan L, Gallagher D, Heymsfield SB, Lean ME. "Derivation and validation of simple equations to predict total muscle mass from simple anthropometric and demographic data." Am J Clin Nutr. 2014;100(4):1041-1051. doi:10.3945/ajcn.113.070466, PMC6443297
4. Janssen I, Baumgartner RN, Ross R, Rosenberg IH, Roubenoff R. "Skeletal muscle cutpoints associated with elevated physical disability risk in older men and women." Am J Epidemiol. 2004;159(4):413-421. doi:10.1093/aje/kwh058, PubMed 14769646
5. Benito PJ, Cupeiro R, Ramos-Campo DJ, Alcaraz PE, Rubio-Arias JÁ. "A Systematic Review with Meta-Analysis of the Effect of Resistance Training on Whole-Body Muscle Growth in Healthy Adult Males." Int J Environ Res Public Health. 2020;17(4):1285. doi:10.3390/ijerph17041285, PMC7068252
6. Morton RW, Murphy KT, McKellar SR, Schoenfeld BJ, Henselmans M, Helms E, Aragon AA, Devries MC, Banfield L, Krieger JW, Phillips SM. "A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults." Br J Sports Med. 2018;52(6):376-384. doi:10.1136/bjsports-2017-097608, PMC5867436
7. Piercy KL, Troiano RP, Ballard RM, Carlson SA, Fulton JE, Galuska DA, George SM, Olson RD. "The Physical Activity Guidelines for Americans." JAMA. 2018;320(19):2020-2028. doi:10.1001/jama.2018.14854, PMC9582631

How this calculator differs from typical muscle mass calculators

Most online muscle mass calculators either relabel a lean body mass formula as muscle mass, which counts bone, organs and water and gives a number close to twice as large, or print a skeletal muscle figure with no word on the equation's limits. Few say that the result describes an untrained person of your size, or that the equation reads high above a BMI of 30.

This calculator runs the Lee equation as published, with its coefficients checked against the paper. It shows the standard error as a range on the result card, flags a BMI of 30 or more, and compares your result with the MRI averages for your sex. The race or ethnicity term is optional and off by default, and the accuracy section reports what the original authors and a separate validation study found, including where the equation reads high and where it reads low.