How to Calculate Lean Body Mass: 4 Formulas Compared
Lean body mass is total weight minus fat. Here are the four ways to calculate it, which formula is most accurate, and how to use your result for protein
Staring at the scale only tells you total gravity, not what you are actually made of. The problem with raw scale weight is that it masks reality: you can lose four pounds of pure fat, gain a pound of muscle, and the scale barely moves. So you assume nothing is happening, you panic, and you give up entirely.
We have all been there. But let's look at the actual numbers. You need to track lean body mass, not total weight. This metric separates the muscle you want to keep from the fat you want to lose. The math is actually pretty simple. Whether you are trying to calculate my lean body mass for a personal goal or calculate your lean body mass for a client, the formulas are identical. You can skip the math and calculate your lean body mass right now in seconds, or keep reading to see exactly how we figure it out.
Lean mass vs. muscle mass vs. fat-free mass
These three terms get thrown around interchangeably. They are not the same thing.
Lean body mass (LBM) is your total weight minus your fat. It includes muscle, bone, organs, connective tissue, and body water.
Muscle mass is just the skeletal muscle portion. It will always be a smaller number than your overall LBM. If you want to know more about this, check out our guide on muscle mass percentage.
Fat-free mass (FFM) excludes all fat. LBM technically includes the essential fat inside your organs and nervous system. In the real world, you can treat LBM and FFM as the exact same thing when calculating your FFMI.
The four methods, ranked by accuracy
There are multiple ways to find your lean mass. From the gold standard to the basic fallback:
- DEXA scan — The gold standard. An X-ray-based body scan that separates fat, lean tissue, and bone.
- Skinfold calipers — Pinch measurements at several sites. Accurate if done by a professional, wildly inaccurate if done by your buddy.
- Bioelectrical impedance (BIA) — Smart scales and handheld grips. Highly sensitive to how much water you drank or when you last ate.
- Height-and-weight formulas — Boer, James, and Hume. No body fat measurement needed, but they are just population average estimates.
| Method | Accuracy | What It Requires | Best For |
|---|---|---|---|
| DEXA Scan | Highest (Gold Standard) | Lab visit, X-ray scan | Professional athletes, exact tracking |
| Skinfold Calipers | High (if done right) | Calipers, trained professional | Consistent, accessible tracking |
| Smart Scales (BIA) | Medium to Low | Smart scale | Daily trend tracking at home |
| Formulas (Boer, etc.) | Lowest (Estimates) | Just height and weight | When you have zero equipment |
The first three give you a real body fat percentage. You plug that into the direct method below. The formulas are your fallback when you have absolutely nothing else. You can find more about your potential using our guide on maximum muscle potential.
The direct method
If you have a body fat percentage from a DEXA scan or a smart scale, the math is straightforward. Stop overthinking it.
LBM = Weight × (1 − BF% ÷ 100)
A 75 kg person at 18% body fat has 75 × (1 − 0.18) = 61.5 kg of lean mass. Done. This is undeniably the most accurate route because it skips the guesswork entirely.
The Boer formula, walked through
When you don't know your body fat, the Boer (1984) equation is the most reliable estimate for the vast majority of adults:
Boer (male): LBM = 0.407 × W + 0.267 × H − 19.2
Boer (female): LBM = 0.252 × W + 0.473 × H − 48.3
W is weight in kilograms and H is height in centimetres. Take an 80 kg man at 178 cm: 0.407 × 80 + 0.267 × 178 − 19.2 = 60.9 kg of lean mass. That leaves about 19.1 kg of fat, or roughly 24% body fat. It is that simple.
We also offer the James and Hume equations, but James often underestimates lean mass in heavier individuals. Stick to Boer unless you have a specific clinical reason. You can learn more about finding your ideal numbers in our ideal lean body mass guide.
Quick lean body mass calculator
Body fat % is optional — with it you get the more accurate direct estimate.
Estimates for tracking and planning — not medical advice.
Using lean mass to hit your protein targets
Protein targets scale better with lean mass than with total weight. You are not trying to feed your fat; you are trying to feed your muscle.
To preserve muscle during a calorie deficit, research from the National Institutes of Health shows you need 2.3–3.1 g of protein per kg of lean body mass. For the 60.9 kg example above, that is roughly 140–190 g of protein every single day.
You can use our calculator below to get your exact numbers, or read more in our protein per pound of lean body mass guide.
Protein target from lean body mass
A rough body fat estimate is fine — being off by a few percent barely moves the target.
- Select...
- Maintain
- Build muscle
- Cut (fat loss)
General guidance for healthy adults — not medical advice.
Frequently Asked Questions
How to calculate lean body mass? If you have a body fat percentage, you calculate it directly: Total Weight × (1 - Body Fat Percentage). If you don't know your body fat, you use predictive formulas like Boer based on your height, weight, and gender.
How to calculate my lean body mass? / How to calculate your lean body mass? Whether you want to calculate your personal lean body mass or a client's, the process is the same: strip away the fat mass from the total scale weight. You can use predictive math (like the Boer equation) if you lack testing equipment.
How do you calculate your lean body mass? / How do you calculate lean body mass? You calculate it by subtracting your fat mass from your total weight. If you do not have a body fat percentage, you can use validated predictive formulas like Boer or James based on your height and weight.
How to calculate lean mass? / How to calculate my lean mass? / How to calculate your lean mass? "Lean mass" is exactly the same as "lean body mass". You subtract your fat from your total weight.
How do I calculate my lean body mass? / How do I calculate lean body mass? / How can I calculate my lean body mass? You can either take a DEXA scan and subtract the fat percentage from your total weight, or use our free calculator above which applies clinical formulas to your height and weight.
How is lean body mass calculated? / How calculate lean body mass? If you have a body fat percentage from a scan, it is calculated directly: Total Weight × (1 - Body Fat Percentage). If you don't know your body fat, it is estimated using formulas.
How to calculate lean body mass formula?
The most common formula is the Boer formula. For men: LBM = 0.407 × W + 0.267 × H − 19.2. For women: LBM = 0.252 × W + 0.473 × H − 48.3 (where W is weight in kg, H is height in cm).
What is lean body mass calculator? / What is my lean body mass calculator? A lean body mass calculator is a tool that automatically applies formulas (like Boer, James, or Hume) to your height, weight, and gender to estimate your lean mass without needing a physical body fat test.
How to calculate lean body mass from body fat percentage? / How to calculate lean body mass with body fat percentage? This is the simplest and most accurate way (the Direct Method). Turn your body fat percentage into a decimal (e.g., 20% becomes 0.20). Subtract that from 1 to get your lean mass percentage (1 - 0.20 = 0.80). Multiply that by your total weight. If you weigh 200 lbs, 200 × 0.80 = 160 lbs of lean body mass.
How to calculate lean body mass in kg? / How to calculate lean body mass in lbs? / How to calculate lean body mass in pounds? The math works the exact same way regardless of the units. Our calculator handles both automatically. If you are doing the math manually, just remember to stick to one system: if you use pounds for weight, your result is in pounds. If you use kg, your result is in kg.
How to calculate body fat percentage and lean body mass? / How to calculate lean and fat mass? You can't easily calculate body fat percentage mathematically without physical measurements (like calipers or a DEXA scan). Once a test gives you your body fat percentage, calculating the lean mass is easy: simply subtract the fat weight from your total weight.
How does Whoop calculate lean body mass? Wearables like Whoop do not actually measure lean body mass directly. They estimate it using your profile inputs (height, weight, age, gender) combined with predictive algorithms and sometimes heart rate data to guess your body composition. For an accurate number, you still need a DEXA scan or proper caliper test.
How to calculate lean body mass reddit? The most common advice on Reddit for calculating lean body mass is to avoid smart scales (which are notoriously inaccurate) and either pay for a DEXA scan or use the US Navy Method (measuring your neck and waist) to estimate your body fat, then subtract that from your total weight.
The final reality check
Every formula here produces an estimate. Lean mass equations were built for specific populations and can be off if you are extremely tall, very muscular, or older. Use these numbers to guide your training and nutrition, not to make medical decisions. Stop guessing, do the math, and get back to work.
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