Muscle Mass Calculator

Your Muscle Mass Results

Skeletal Muscle Mass

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Lee (2000), MRI-validated — primary estimate.

Skeletal muscle tissue — not the same as lean body mass.

SMM Share — % of total body weight

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Population Benchmark

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Janssen (2000) population average SMM% for your sex (all ages).

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Body Composition Breakdown

Lean body mass—

Body fat mass—

Total body weight—

Body fat percentage—

SMM Index & FFMI

SMM Index (SMM ÷ height²)—

FFMI (lean mass ÷ height²)—

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SMM Index is used clinically to screen for sarcopenia (age-related muscle loss).

Muscle Mass Classification

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Body Composition

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Your SMM% vs Age-Group Norms

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Step-by-Step Solution

Age-Stratified Skeletal Muscle Mass Reference

Normative skeletal muscle mass as a percentage of total body weight, by age group and sex. Source: Janssen et al. (2000), Journal of Applied Physiology — 468 subjects. Your age group row is highlighted and marked "Your age group".
Age Group	Female SMM% Range	Male SMM% Range
18–29	28.4 – 39.8%	37.9 – 46.7%
30–39	25.0 – 36.2%	34.1 – 44.1%
40–49	24.2 – 34.2%	33.1 – 41.1%
50–59	24.7 – 33.5%	31.7 – 38.5%
60–69	22.7 – 31.9%	29.9 – 37.7%
70+	25.5 – 34.9%	28.7 – 43.3%
All Ages Average	30.6%	38.4%

Introduction

This Muscle Mass Calculator tells you how much skeletal muscle your body has. Skeletal muscle is the muscle you use to move, lift, and stand up. It is not the same thing as lean body mass, which also counts water, bones, and organs.

To use it, enter your sex, age, weight, height, and waist size. You can add your body fat percentage if you know it. If you leave that blank, the calculator estimates it from your waist and weight.

You will get your muscle mass in kilograms and pounds, your muscle as a percent of body weight, your SMM Index and FFMI, and a full body composition breakdown. The tool also compares your number to averages for people of your age and sex, so you can see if you are above, below, or right around normal.

The math uses the Lee (2000) formula, which was developed against whole-body MRI scans.¹ Its age norms come from Janssen (2000), an MRI study of 468 adults.² Every step of the work is shown on the page. This is an estimate, not a medical test, but it is a good way to track your progress over time.

How to Use Our Muscle Mass Calculator

Enter a few body details and the muscle mass calculator shows your skeletal muscle mass in kilograms and pounds, your muscle as a percent of body weight, your body fat and lean mass, your SMM Index and FFMI, and how you compare to people your age and sex.

Unit system: Pick Imperial (pounds, feet and inches) or Metric (kilograms and centimetres). Your numbers switch over for you, and results always show both units.

Sex: Choose Male or Female. The muscle mass formula uses a different value for each.

Age: Type your age in whole years, from 18 to 90. Muscle mass drops a little with age, so this changes your result and your comparison group.

Weight: Type your body weight. Use 30 to 600 lb in Imperial, or 13.6 to 272 kg in Metric. Weigh yourself in the morning for the best number.

Height: Enter feet and inches in Imperial, or centimetres in Metric (100 to 250 cm). Stand straight with no shoes when you measure.

Waist circumference: Measure the narrowest part of your waist, just above the belly button. Keep the tape level and snug, and breathe out first. Use 20 to 80 in, or 50 to 203 cm.

Body fat percentage (optional): If you know your body fat percent from a scan or caliper test, type it in (2 to 80%). Leave it blank and we estimate it from your waist and weight.

Press Calculate to see your results, charts, and the step-by-step math. Press Reset to start over.

What Is Muscle Mass?

Muscle mass is the weight of the muscle in your body. This tool looks at skeletal muscle mass (SMM), the muscles attached to your bones that move your arms, legs, and back. It does not count heart muscle or the smooth muscle inside your organs.

Muscle Mass vs. Lean Body Mass

People often mix up these two terms. Lean body mass is everything in your body that is not fat: muscle, bone, water, organs, and skin. Skeletal muscle mass is only the muscle part. So your muscle number will always be smaller than your lean mass number.

How Muscle Mass Is Estimated

The main math here comes from the Lee formula (2000), which was built by comparing real MRI scans of people to their height, weight, age, and sex.¹ It was developed in adults with a BMI under 30.¹ It adds 1.4 kg for African American adults and subtracts 1.2 kg for Asian adults.¹ It gives a close guess of skeletal muscle mass in kilograms without any scan. Body fat is worked out with the YMCA formula, which uses your waist size and weight. If you already know your body fat percent from a DEXA scan, calipers, or a smart scale, using that number makes the results more exact.

What the Numbers Mean

SMM in kg or lb: how much skeletal muscle you carry.
SMM percent: your muscle divided by your total weight. In an MRI study of 468 adults, men averaged 38.4% and women 30.6%.²
SMM Index (SMMI): muscle divided by height squared. Doctors use this to check for low muscle. In adults 60 and older, an index of 8.50 kg/m² or less in men, or 5.75 kg/m² or less in women, marked a high risk of physical disability.³ An index of 8.51 to 10.75 kg/m² in men, or 5.76 to 6.75 kg/m² in women, marked a moderate risk.³
FFMI: lean mass divided by height squared. Lifters use this to track progress.

Why Muscle Mass Matters

Muscle does more than make you look strong. It burns calories all day, helps control blood sugar, keeps your bones and joints safe, and makes daily jobs like carrying groceries or climbing stairs easier. People with more muscle often heal faster after sickness or surgery.

Muscle Loss With Age

Adults start losing muscle around age 30, losing roughly 3 to 8% every ten years.⁴ After 60, the loss speeds up.⁴ Big muscle loss is called sarcopenia, and it raises the risk of falls, breaks, and losing independence. This loss is not fixed, though. Strength training and enough protein slow it down or even reverse it.

How to Build or Keep Muscle

Lift weights at least 2 days a week, working all your main muscle groups.⁵
Eat enough protein. The International Society of Sports Nutrition puts most exercising people at 1.4 to 2.0 grams per kilogram of body weight a day.⁶
Sleep 7 to 9 hours.⁷ Muscle grows while you rest, not while you train.
Stay active and avoid sitting all day.
Do not crash diet. Cutting calories too fast burns muscle along with fat.

A Note on Accuracy

These results are estimates based on group averages, not a medical test. Very muscular athletes, people with a lot of body fat, and those with swelling or health conditions may get numbers that are off. MRI, CT, and DEXA scans are the gold standard. Use this tool to spot trends over time, and talk to a doctor if you think your muscle mass is low.

Formulas used

Skeletal Muscle Mass — Lee (2000) ¹

SMM = 0.244 \cdot W_{kg} + 7.80 \cdot H_{m} - 0.098 \cdot age + 6.6 \cdot S + R - 3.3 \quad (S = 1\ \text{male},\ 0\ \text{female};\ R = -1.2\ \text{Asian},\ 1.4\ \text{African American},\ 0\ \text{white or Hispanic})

Body Fat Percentage — YMCA estimate

BF\% = \frac{4.15 \cdot WC_{in} - 0.082 \cdot W_{lb} - C}{W_{lb}} \times 100 \quad (C = 98.42\ \text{male},\ 76.76\ \text{female})

Fat mass and lean body mass

FM = W_{kg} \cdot \frac{BF\%}{100}, \qquad LBM = W_{kg} \cdot \left(1 - \frac{BF\%}{100}\right)

SMM as percentage of body weight

SMM\% = \frac{SMM}{W_{kg}} \times 100

Skeletal Muscle Mass Index

SMMI = \frac{SMM}{H_{m}^2}

Fat-Free Mass Index

FFMI = \frac{LBM}{H_{m}^2}

Unit conversions to metric

W_{kg} = W_{lb} \times 0.45359237, \qquad H_{cm} = (ft \times 12 + in) \times 2.54

Frequently asked questions

Is muscle heavier than fat?

A pound of muscle and a pound of fat weigh the same. But muscle is more dense, so it takes up less room. That is why two people at the same weight can look very different. If you lift weights and the scale does not move, you may still be swapping fat for muscle.

How much muscle can you gain in a month?

It varies a lot from person to person. Beginners who lift hard and eat enough protein gain fastest, and gains slow down after the first year of training. Fast early changes on the scale are often water and food weight, not muscle.

Can you build muscle and lose fat at the same time?

Yes, but it works best for certain people:

Beginners who just started lifting
People with a lot of body fat to lose
People coming back after a long break

Trained lifters usually have to pick one goal at a time. To do both, keep protein high, lift weights, and cut calories only a little.

How many calories does a pound of muscle burn per day?

At rest, skeletal muscle burns about 13 calories per kilogram a day and fat tissue about 4.5.⁸ That is about 6 and 2 calories per pound. So gaining 10 pounds of muscle adds roughly 60 calories a day, not hundreds. The bigger win is that muscle lets you train harder and move more, which burns far more calories than the muscle itself.

How fast do you lose muscle if you stop working out?

Strength starts to drop after a few weeks off, and muscle size follows. Older adults lose it faster than young adults. Muscle you built before usually comes back quicker than it took to build.

What are the signs of low muscle mass?

Common signs include:

Feeling weak when you carry bags or open jars
Trouble standing up from a chair without pushing off
Walking slower than you used to
Losing weight without trying
Falling or feeling wobbly

If you notice these, see a doctor. Low muscle mass, called sarcopenia, can be treated with strength training and more protein.

Can you build muscle after 50 or 60?

Yes. Resistance training adds muscle and strength even in frail older people.⁴ Older adults may need more protein, about 1 gram per kilogram a day, than the 0.8 g/kg the Institute of Medicine recommends.⁴ Aim for muscle-strengthening work on 2 or more days a week.⁵

Does creatine actually increase muscle mass?

Yes. Creatine is one of the most popular sports supplements, and it raises the creatine stored in muscle, which improves high-intensity performance and training gains.⁹ It helps you do a few more reps, which builds more muscle over time. The usual maintenance dose is 3 to 5 grams a day.⁹

Does doing cardio make you lose muscle?

Normal amounts of cardio will not cost you muscle. Problems start with very long sessions, hard cardio every day, too few calories, or low protein. Keep cardio to moderate amounts, lift weights at least twice a week, and eat enough, and your muscle will stay.

What FFMI can you reach without steroids?

In one study of male athletes, the height-normalized FFMI of those who had not used steroids went up to a limit of 25.0, while many steroid users passed it.¹⁰ The FFMI shown here is not height-normalized, so compare with care. FFMI is a better way to compare lifters than BMI because it ignores fat weight.

Why does muscle mass drop as you get older?

Muscle mass falls by about 3% to 8% per decade after age 30, and faster after 60.⁴ Changes inside the muscle, lower hormone levels, and moving less all play a part. Lifting weights and eating protein at every meal slow it down.

How much protein do you need to keep muscle while dieting?

More than usual. The International Society of Sports Nutrition says resistance-trained people may need 2.3 to 3.1 grams per kilogram a day to keep lean mass while cutting calories.⁶ Its general advice is 20 to 40 grams of protein per dose, spread evenly every 3 to 4 hours.⁶ Keep lifting weights while you diet.

Sources