What is lean body mass?
Lean body mass is everything in your body that isn't fat: muscle, bone, organs, skin, and water. It equals your total weight minus your fat mass. For example, a 180 lb (81.6 kg) man with 20% body fat has about 144 lb (65.3 kg) of lean body mass.
Say you've been lifting for three months and the scale hasn't moved. Frustrating, right? But your jeans fit better and your bench press keeps going up. Most likely your lean body mass changed, and the scale can't see that on its own.
What does lean body mass include?
It includes every part of you except fat. Think of it this way:
Lean body mass = total weight minus fat mass
That "everything else" includes:
- Muscle
- Bone
- Organs, like your heart and liver
- Skin
- Water
You'll also see the term "fat-free mass." The two are often used as if they mean the same thing, and the formulas in this guide treat them that way.
Here's why it matters. Two people can weigh the same and have very different amounts of lean mass. A scale can't tell them apart. Lean body mass can.
How do you estimate your lean body mass?
Use your body fat percentage, or a height and weight formula like Boer or Hume. There's no easy way to weigh your muscles directly at home, so these are estimates. Here are three common methods, from simplest to most personal.
1. From your body fat percentage
If you know your body fat, this is the most direct method:
LBM = weight × (1 minus body fat as a decimal)
Take a man who weighs 180 lb (81.6 kg) with 20% body fat:
180 × 0.80 = 144 lb (65.3 kg) of lean mass.
Don't know your body fat? Our body fat calculator estimates it with a tape measure.
2. The Boer formula (1984)
P. Boer published this formula in the American Journal of Physiology in 1984. It only needs height and weight (weight in kg, height in cm):
- Men: 0.407 × weight + 0.267 × height minus 19.2
- Women: 0.252 × weight + 0.473 × height minus 48.3
3. The Hume formula (1966)
R. Hume published this one in the Journal of Clinical Pathology in 1966. It also uses only height and weight:
- Men: 0.32810 × weight + 0.33929 × height minus 29.5336
- Women: 0.29569 × weight + 0.41813 × height minus 43.2933
Do the lean body mass formulas agree?
Not always. They can be close for one person and far apart for another. Here are two example people run through all three methods. This is where it gets interesting.
Example A: a man, 5 ft 10 in (178 cm), 180 lb (81.6 kg), 20% body fat.
Example B: a woman, 5 ft 5 in (165 cm), 150 lb (68 kg), 30% body fat.
| Method | Example A (man) | Example B (woman) |
|---|---|---|
| From body fat % | 144 lb (65.3 kg) | 105 lb (47.6 kg) |
| Boer (1984) | 136 lb (61.5 kg) | 103 lb (46.9 kg) |
| Hume (1966) | 127 lb (57.6 kg) | 101 lb (45.9 kg) |
For the woman, the three methods land within about 4 lb (2 kg) of each other. For the man, they're 17 lb (8 kg) apart.
Why? The Boer and Hume formulas don't know how much you train. They guess based on an average person of your height and weight. The body fat method uses your actual measurements, so it can pick up extra muscle.
What this means for you: if you lift, the body fat method is usually the more personal choice. Whatever you pick, stick with the same method so your numbers stay comparable over time.
Our lean body mass calculator does the math for you in a few seconds.
Why is lean body mass worth knowing?
It can sharpen your calorie estimate, and it lets you compare muscle fairly across heights.
How does lean mass sharpen your calorie estimate?
The Katch-McArdle formula uses your lean mass instead of your total weight. Most calorie calculators use total weight. Here is the formula:
BMR = 370 + (21.6 × lean body mass in kg)
BMR, or basal metabolic rate, is roughly the calories your body burns at rest. For Example A, that's 370 + (21.6 × 65.3) = about 1,780 calories a day.
For comparison, the Mifflin-St Jeor formula (Mifflin and colleagues, 1990) gives about 1,760 calories for the same man at age 35. The two are close here. Katch-McArdle can be more personal because it uses your lean mass. But it's only as good as your body fat estimate. You can check your own resting burn with our BMR calculator.
How does FFMI let you compare yourself fairly?
Fat-free mass index (FFMI) adjusts lean mass for height. A 6 ft 4 in (193 cm) person will usually have more lean mass than someone who is 5 ft 6 in (168 cm). That doesn't mean they're more muscular for their size.
FFMI = lean mass (kg) ÷ height (m) squared
A 1995 study by Kouri and colleagues in the Clinical Journal of Sport Medicine also used a "normalized" FFMI. It adds 6.3 × (1.80 minus your height in meters) to adjust toward a common height.
For Example A: 65.3 ÷ (1.778 × 1.778) = 20.7, or 20.8 normalized.
Here's the fascinating part. In that study, natural athletes who had never used steroids topped out at a normalized FFMI of about 25.0. Mr. America winners from before the steroid era averaged 25.4. So a score in the low 20s leaves a lot of room to grow. Check yours with our FFMI calculator.
How do you build and keep lean body mass?
Lift weights regularly and eat enough protein. Lean mass responds well to both, and you control both.
1. Lift weights regularly. Strength training is the base. The protein research below was done in people doing resistance training, so the two work as a team.
2. Eat enough protein. A 2018 review by Robert Morton and colleagues in the British Journal of Sports Medicine found that gains in lean mass from strength training leveled off at about 1.6 g of protein per kg of body weight a day. The authors noted that up to 2.2 g/kg might help some people get the most out of training.
For Example A at 81.6 kg (180 lb), 1.6 g/kg works out to about 131 g of protein a day. Our protein calculator gives you a personal range.
How do you keep lean mass while cutting?
Keep your protein high when you cut calories. Losing fat without losing muscle is a common goal for gym-goers. The International Society of Sports Nutrition's 2017 position stand notes that higher protein intakes can help keep lean mass while you eat in a calorie deficit. So don't drop your protein when you drop calories.
If you have kidney disease, diabetes, or another health condition, talk to your doctor or a registered dietitian before you change your protein intake.
For more on training and eating for muscle, see our guide for gym-goers.
Is lean body mass worth tracking?
Yes. Based on what I've seen in the research, it's one of the most useful numbers a lifter can track. It explains why the scale sometimes stalls while you're clearly getting stronger. It also keeps the focus on building something, not just losing something.
I wouldn't obsess over the exact figure, though. Every home method is an estimate, and as the examples show, the formulas can disagree by a lot. The real value is in the trend. If your lean mass holds steady or climbs over a few months while your body fat drops, you're doing it right.
What's the short version?
Lean body mass is your weight minus your fat: muscle, bone, organs, and water. You can estimate it from your body fat percentage or with the Boer and Hume formulas. Track it the same way each time, pair it with FFMI for a height-fair view, and build it with steady strength training and about 1.6 g of protein per kg a day. Ready to see your number? Try our lean body mass calculator.