Lean Body Mass & Muscle Retention Calculator, Are You Keeping Your Muscle?
Your lean body mass is the muscle, bone and tissue that keeps your metabolism high and your strength intact. Answer a few quick questions to get your number from three validated formulas, a daily protein target, and a clear plan to hold onto your muscle while you lose fat.
Losing weight is easy to measure. Keeping your muscle is the part that decides whether the results last.
Most calculators just spit out a number. Calqulate.net averages the Boer, James and Hume formulas, turns your lean mass into a real protein target, and shows you how to protect that muscle while losing fat, on a GLP-1 or off it.
3 formulas
Boer · James · Hume
~30 sec
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What is your lean body mass?
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This shapes the plan we show you at the end.
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What is lean body mass, really?
Lean body mass, or LBM, is your total body weight minus all of your fat. It includes your muscle, bone, organs, skin, blood and water, so it represents the metabolically active tissue that actually keeps you running. When people talk about protecting their metabolism, this is the number they mean.
Unlike BMI, which cannot tell muscle from fat, lean body mass shows you the part of your weight worth keeping. Two people can weigh the same and have completely different lean mass, and that difference is what shows up in strength, energy, and how easily the weight stays off.
Why the scale misleads you
- Cannot tell muscle from fat
- Hides muscle loss during a diet
- Gives no metabolic insight
Why lean mass tells the truth
- Tracks the tissue that burns calories
- Shows if you are losing fat or muscle
- Predicts whether results last
To see your fat side of the picture, pair this with the Body Fat Calculator.
Muscle retention: the number that decides if the weight stays off
Signs you are keeping muscle
- Losing weight but strength is holding or rising
- Hitting a real protein target most days
- Lifting or resistance training 2 to 3 times a week
- Losing at or under about 1 percent of body weight a week
- Energy steady, clothes looser but you still feel solid
Signs you are losing muscle
- Scale dropping fast but you feel weaker or flatter
- Barely eating, protein has fallen off a cliff
- No strength training at all
- Losing more than about 1 percent of body weight a week
- Looking softer or more drawn even as weight drops
Lean body mass on a GLP-1
GLP-1 medications like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) are extremely good at taking weight off. The catch is that when weight comes off fast, a big share of it can be lean muscle rather than fat. Research on rapid weight loss suggests that, unprotected, roughly 25 to 40 percent of the loss can be lean mass.
That is why your lean body mass matters more than ever on these drugs. Losing muscle slows your metabolism and makes the weight easier to regain the moment you taper off. Enough protein, resistance training, and a steady pace of loss keep most of your muscle where it belongs.
Want to know your split right now? Use the GLP-1 Body Composition Tracker to find out if you are losing fat or muscle on Ozempic, Wegovy, Mounjaro or Zepbound.
How to protect and build your lean body mass
Whether you are cutting fat or gaining muscle, the levers are the same three, in priority order:
1. Eat enough protein
Aim for 1.6 to 2.2 g per kg of body weight. Protein supplies the amino acids that repair and build lean tissue. Spread it across the day, and never let a low-appetite day drop you to zero.
2. Train with resistance
Lift 2 to 3 times a week to keep muscle in a deficit, or 3 to 5 times to build it. Compound moves like squats, rows and presses with progressive overload do the heavy lifting.
3. Control your pace
When losing fat, target around 1 percent of body weight a week or less. Faster loss tilts the ratio toward muscle. When building, a small surplus of 200 to 500 kcal is plenty.
The formulas behind your number
We average three validated prediction formulas so you get a realistic range instead of false precision. Each was built on a different population, and each uses only your weight, height and sex.
Boer formula (best for average adults)
Men: LBM = 0.407 x weight(kg) + 0.267 x height(cm) - 19.2
Women: LBM = 0.252 x weight(kg) + 0.473 x height(cm) - 48.3
Hume formula (clinical standard)
Men: LBM = 0.32810 x weight(kg) + 0.33929 x height(cm) - 29.5336
Women: LBM = 0.29569 x weight(kg) + 0.41813 x height(cm) - 43.2933
James formula (for muscular builds)
Men: LBM = 1.1 x weight(kg) - 128 x (weight / height)squared
Women: LBM = 1.07 x weight(kg) - 148 x (weight / height)squared
If you enter a body fat percentage, we switch to the direct method, lean body mass = weight x (1 minus body fat), which is more accurate than any formula.
What is a healthy lean body mass percentage?
Your lean mass as a share of body weight depends on sex, age and training. Use this as a rough guide, not a verdict.
| Group | Typical lean mass | Notes |
|---|---|---|
| Men 20 to 39 | 75 to 85% | Peak muscle years |
| Women 20 to 39 | 65 to 75% | Higher essential fat |
| Men 40 to 59 | 70 to 80% | Natural decline begins |
| Women 40 to 59 | 60 to 70% | Hormonal shifts |
| Adults 60+ | Falls about 1% a year | Without resistance training |
| Athletes, any age | 80 to 90% | Elite body composition |
Why lean mass is your insurance policy
Muscle is metabolically active, so holding onto it keeps your daily calorie burn higher. That makes maintenance far more forgiving after a diet or after you taper off a GLP-1. Someone who lost 30 pounds of mostly fat has a very different metabolism than someone who lost 30 pounds with a big chunk of muscle mixed in.
Lean mass also protects you with age. Muscle loss starts around 30 and speeds up after 50, and it is the quiet driver behind lost strength, slower metabolism and frailty later on. Protein and lifting are the only proven ways to hold the line.
Keep your muscle where you can see it
A single lean-mass reading is a snapshot. What protects your results is logging your weight, body composition and protein over time. Calqulate Vitals keeps all of that on one timeline and checks the protein in a real meal for free, with the lean-mass trend and dose-timing views on Premium.
- Log weight and body composition in seconds
- Everything you log on one timeline
- Find out how much protein is really in your meal
- Lean-mass trend over time (Premium)
Related Body Composition Calculators
Explore the rest of our body composition tools — each one adds a different piece of the picture.
Sources and references
This page is built from public clinical guidance and peer-reviewed research. Always confirm decisions with a licensed clinician.
- 1.Boer P. Estimated lean body mass as an index for normalization of body fluid volumes (1984)
- 2.Hume R. Prediction of lean body mass from height and weight (1966)
- 3.NEJM: STEP 1 trial of semaglutide for weight management
- 4.NEJM: SURMOUNT-1 trial of tirzepatide
- 5.NIH: Sarcopenia and age-related loss of muscle mass
Frequently Asked Questions
Frequently Asked Questions
Medical Disclaimer: This tool is for informational purposes only and provides an educational estimate, not a body-composition measurement or medical advice. Lean mass includes water and can shift with hydration. Always consult a qualified healthcare provider about your health decisions, especially if you have underlying conditions.
Tool Information
June 2026
Dr. Jaydeep Sanghani
Meet Akabari
Dr. Jaydeep Sanghani
MBBS, MD, DNB(Anaesth.), PDCC(CCM), DrNB(CCM)
AIIMS Bhubaneswar · AIIMS Rishikesh
Critical care specialist and anesthesiologist with advanced training from AIIMS. Reviews health calculators at Calqulate to ensure medical accuracy and evidence-based standards.
About the authors
Meet Akabari
·Co-Founder, Calqulate.net