How Lean Body Mass is calculated

There are two common ways to estimate lean body mass: (1) using a body fat percentage (if you have it), or (2) using a height/weight formula (if you don’t). This page includes both — so you can choose the method that fits the data you have.

If you have body fat percentage, lean body mass is simply:

• LBM = Weight × (1 − BodyFat%/100)
• Fat Mass = Weight − LBM

Example: If you weigh 80 kg and your body fat is 20%, your lean mass is 80 × (1 − 0.20) = 64 kg. Your fat mass is 80 − 64 = 16 kg.

Many fitness calculators use height- and weight-based equations to estimate lean mass. Different formulas were developed on different populations, so they can disagree slightly — that’s normal. That’s why this tool shows multiple estimates and an “average” for a balanced snapshot.

• Boer (Male): LBM = 0.407×W + 0.267×H − 19.2
• Boer (Female): LBM = 0.252×W + 0.473×H − 48.3
• James (Male): LBM = 1.10×W − 128×(W²/H²)
• James (Female): LBM = 1.07×W − 148×(W²/H²)
• Hume (Male): LBM = 0.32810×W + 0.33929×H − 29.5336
• Hume (Female): LBM = 0.29569×W + 0.41813×H − 43.2933

In these equations, W is weight in kilograms and H is height in centimeters. If you enter US/Imperial units, the calculator converts them internally first.

LBM formulas are approximations. They work well for many people, but no single equation is perfect for every body type. Showing Boer, James, and Hume side-by-side helps you understand the “range” your estimate likely falls into — and the average is a practical summary that avoids over-trusting one equation.

LBM is often used as a “base number” for setting nutrition and training targets. For example, some athletes set protein goals relative to lean mass (not total mass), since lean tissue is more metabolically active. It can also help you evaluate whether weight loss is mostly fat loss or lean loss.

Examples you can copy

A male is 180 cm and 80 kg. The calculator outputs Boer/James/Hume and an average. If the average is about 64 kg, then his fat mass estimate (without body fat %) is about 16 kg (80 − 64). Lean % would be around 80%.

A female is 5 ft 6 in and 150 lb. The calculator converts to cm/kg, computes LBM, then converts back. If her lean mass is ~105 lb, then fat mass is ~45 lb.

Weight 200 lb, body fat 25% → LBM = 200 × (1 − 0.25) = 150 lb. Fat mass = 50 lb. Lean % = 75%.

If your body fat % comes from different tools (DEXA vs smart scale), your LBM may change even if your body is the same. For progress tracking, consistency beats perfection.

Save your result today. Re-check in 30 days. If your lean % improved, share the before/after screenshot with a short caption: “Lean % + LBM progress in 30 days 🔥”. People love measurable progress.

What this calculator does step-by-step

Here’s what happens after you click “Calculate” (no signup, no tracking needed):

• Step 1: Convert your inputs to metric units (kg and cm) if you typed US/Imperial.
• Step 2: Compute LBM using Boer, James, and Hume equations for your selected sex.
• Step 3: Compute the average of those three formulas (a balanced single estimate).
• Step 4: If you entered body fat %, compute a direct LBM estimate and show it too.
• Step 5: Choose a primary LBM for the headline result: if body fat % is present, use the direct method; otherwise use the formula average.
• Step 6: Compute fat mass and lean percentage from the primary LBM, then display all results.

Lean mass includes water, and water can shift quickly based on carbs, sodium, and hydration. That’s why short-term fluctuations happen — even with “no real muscle change.”

There’s no single “perfect” lean %. It depends on age, sex, genetics, sport, and goals. Instead of chasing a single number, use lean % to track direction over time: improving lean % typically means you’re gaining lean tissue, losing fat, or both.

For most people, every 2–4 weeks is plenty. Daily checks can be noisy because hydration swings can mask real progress. If you want a routine: pick the same day/time (like Saturday morning), measure consistently, and save the result.

Frequently Asked Questions

• What is Lean Body Mass (LBM)?

  Lean body mass is your total weight minus fat mass. It includes muscle, bone, organs, and body water.

• Is LBM the same as muscle mass?

  No. Muscle is part of LBM, but LBM also includes bones, organs, connective tissue, and water.

• Which formula should I trust (Boer vs James vs Hume)?

  They are all estimates. It’s normal for them to differ slightly. The average is a practical summary for most people.

• What’s the most accurate way to measure lean mass?

  Clinical methods like DEXA are more accurate, but formulas can still be useful for everyday tracking.

• Can my LBM change quickly?

  True muscle changes slowly, but LBM can swing due to water/glycogen changes. Track trends over weeks, not days.

• Why does my LBM drop during a diet?

  Some can be water/glycogen. If it keeps dropping and strength declines, you may be losing muscle — consider adjusting plan.

• Can I use this if I’m very muscular or extremely lean?

  Yes, but formulas can be less accurate at extremes. Use it as a rough estimate or consider a direct measurement method.