# Lean Body Mass Calculator | Forge > **Quick Answer:** The Forge Lean Body Mass Calculator tells you how much of your weight is working tissue: muscle, bone, organs, and water. Calculate it from your body fat percentage, or estimate it with the Boer, Hume, and James formulas. Includes FFMI with honest context. Free and ungated. Example: 200 lb at 20% body fat means 160 lb of lean mass. **dateModified:** 2026-08-09 --- ## Key Facts - **Two methods:** From a known body fat percentage, or from the Boer, Hume, and James formulas averaged together - **Formulas:** Boer (most cited in clinical work), Hume (lands close for most people), James (drifts when body fat runs high) - **FFMI:** Fat-free mass index, lean mass in kg divided by height in meters squared, with a normalized version adjusted to 1.8 m - **FFMI context:** A 1995 study of elite bodybuilders put 25 as the rough ceiling for drug-free men; most trained men land between 18 and 20 - **Units:** Imperial and metric - **Cost:** Free. No sign-up. --- ## How to Use It 1. Know your body fat percentage? Enter your weight and that percentage for the most direct answer. 2. No body fat number? Switch to the formula method and enter your sex, height, and weight. 3. Read your lean body mass, shown as the average of the three formulas. 4. Check your FFMI below it for a height-adjusted view of your muscle mass. Use your own trend over time for decisions. All three formulas were fit on general populations, so every output is an estimate. --- ## How the Math Works The body-fat method multiplies your weight by one minus your body fat fraction. At 200 lb and 20% body fat, lean mass is 200 x 0.8 = 160 lb. The formula method runs Boer, Hume, and James exactly as published, with weight in kilograms and height in centimeters, and averages the three. Averaging smooths out the weakest assumptions of each one. James was derived at moderate body fat levels and drifts when body fat runs high. FFMI divides lean mass in kilograms by height in meters squared, so tall and short lifters can compare muscle mass fairly. The normalized score adds 6.1 times the difference between 1.8 meters and your height. --- ## Frequently Asked Questions ### What is lean body mass? Your weight with the fat removed. Muscle makes up most of it, along with bone, organs, water, and skin. When lifters talk about getting bigger, lean body mass is the number they mean. Scale weight alone hides how much of you is working tissue. ### Which formula should I trust? Boer is the most cited in clinical work and holds up well across body types. Hume lands close for most people. James was derived at moderate body fat levels and drifts when body fat runs high. This calculator averages all three, which smooths out the weakest assumptions of each one. ### What is FFMI? The fat-free mass index divides your lean mass in kilograms by your height in meters squared, so tall and short lifters can compare muscle mass fairly. A 1995 study of elite bodybuilders put 25 as the rough ceiling for drug-free men, with most trained men landing between 18 and 20. The normalized version adjusts the score for heights far from 1.8 meters. ### Why does my lean mass drop when I diet? Some of that drop is water and glycogen leaving the muscles, and it comes back when you eat more. Real muscle loss happens when the deficit runs steep or protein runs low. Keeping protein high and training hard through a cut holds onto most of your lean mass. ### How do I increase lean body mass? Progressive overload with enough protein to support it. Most lifters add lean mass slowly after the beginner phase, a few pounds a year with consistent training. Programs that raise the weight or the reps week after week are what drive the gain. ### Is lean body mass the same as muscle mass? Muscle is the largest piece of lean body mass, sitting alongside bone, organs, and water. Skeletal muscle alone usually runs around half of total lean mass. When a formula estimates lean body mass, only part of the answer is contractile muscle tissue. --- ## Forge Builds the Muscle Side Forge is an AI personal trainer for iOS and Android. It programs progressive overload into an adaptive plan built around your goals, schedule, and equipment, and logs every set so you can watch the trend. You get 4 trainer personalities and a 24/7 trainer chat. Pricing: $19.99/month, $59.99/year, or $6.99/week. All plans include a 7-day free trial. - **App Store:** https://apps.apple.com/us/app/forge-ai-personal-trainer/id6758403402 - **Google Play:** https://play.google.com/store/apps/details?id=com.tendylabs.forge - **Website:** https://forgetrainer.ai --- *https://forgetrainer.ai/tools/lean-body-mass-calculator*