When weight comes off quickly, muscle is often lost along with the fat, and muscle is the tissue you most need to hold on to. Use this calculator to estimate your lean body mass and your skeletal muscle mass, and to see how your muscle level compares with established reference values.
Know your body fat percentage? Add it in the optional field and the lean mass figure will be based on your own body instead of a general prediction. If you do not have it, our body fat calculator can estimate it in a couple of minutes.
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Your result will show hereAdd your details, then select Calculate.
Estimated skeletal muscle mass
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Your result contains three numbers. They are connected, but each one answers a different question.
This is your total weight with the fat taken away. It includes your muscles, but also your bones, organs, connective tissue and the water your body holds, which is why it is always the bigger figure. When you supply your body fat percentage, the calculator works lean mass out from your own data. Without it, the figure comes from a formula based on population averages, which is less precise.
This narrows things down to the muscle attached to your skeleton, the kind that contracts when you move. It usually accounts for a little under half of lean body mass. Training builds it, while inactivity and crash dieting wear it away. Of the three figures, this one is most closely tied to your strength, your resting metabolism and your chances of staying mobile as you get older.
The index takes your muscle mass and scales it to your height, much as BMI does with body weight. That makes it fair to compare a tall person with a short one. Clinicians and researchers use it widely to screen for sarcopenia, the gradual muscle loss that comes with aging.
The cut-off points below are drawn from research led by Ian Janssen, which followed a large group of adults and found that a lower muscle index went hand in hand with a higher risk of physical disability. They are meant for screening and cannot diagnose a condition.
| Category | Men | Women | What it means |
|---|---|---|---|
| Normal range | Men10.76 and above | Women6.76 and above | Your muscle level is comparable with that of healthy younger adults. |
| Moderately low | Men8.51 – 10.75 | Women5.76 – 6.75 | Often labeled class I sarcopenia. People in this band are more likely to see their physical function decline over the years. |
| Severely low | Men8.50 and below | Women5.75 and below | Labeled class II sarcopenia. The risk of physical disability is markedly higher, so it is worth discussing with a healthcare provider. |
Skeletal muscle index is measured in kg/m². Cut-off points from Janssen and colleagues. These are screening categories, not a diagnosis.
Muscle preservation is easily the most overlooked part of medical weight loss, and arguably the most important. GLP-1 medication makes it much easier to bring the number on the scale down. Making sure that the weight coming off is fat takes deliberate effort.
Whenever weight falls quickly, part of the loss comes from lean tissue. In some clinical trials of GLP-1 medications, lean mass made up a sizeable portion of the total weight lost, especially when people ate far less and took no steps to protect their muscle.
Muscle burns energy even when you are resting. When you lose it, your body needs fewer calories each day, so the weight returns more easily and losing it again becomes tougher. There is a physical price as well. Strength, balance, bone health and the ability to live independently in later years all depend on how much muscle you keep.
Two habits have the best evidence behind them: eating enough protein and doing resistance exercise two or three times each week. Skip either one and a larger part of your weight loss is likely to come from muscle.
A bathroom scale shows how much you have lost, never what you have lost. Dropping weight while holding your muscle steady is a good result. Dropping weight while muscle falls with it is a warning sign, and each situation needs a different response.
Check your muscle mass regularly along with your weight while you are in treatment, and share both figures at your follow-up visits.
With a supervised program you get a personal protein goal, a strength training plan and regular body composition checks. That way the weight you lose is the weight you actually wanted gone.
