What Is Fat-Free Mass (FFM)?
Fat-free mass (FFM) is total body weight minus extractable fat. It includes skeletal muscle, bone, organs, connective tissue, and body water — not muscle alone. In research and clinical settings, FFM is often measured or estimated from body fat percentage; in fitness tools, the same math is applied when you enter weight and BF%.
FFM vs Lean Body Mass (LBM)
Term
Definition
On this site
Term
Definition
On this site
Term
Definition
On this site
| Term | Definition | On this site |
|---|---|---|
| Fat-free mass (FFM) | All mass except extractable fat | Primary result when body fat % is entered |
| Lean body mass (LBM) | Often includes essential fat (~2–5% men, ~8–12% women in classic definitions) | Boer comparison row + link to LBM hub for height/weight formulas |
| Skeletal muscle mass | Muscle tissue only — a subset of FFM | Not calculated — cannot be isolated reliably from BF% alone |
Fitness articles often use LBM and FFM interchangeably. When derived from body fat %, this calculator reports FFM using the standard formula below — and shows a Boer LBM estimate for comparison. Boer can differ by several kilograms from BF%-based FFM because it ignores your measured composition, not only because of essential-fat definitions.
How This Calculator Works
Inputs
Sex, height, weight
Body fat
DEXA, BIA, calipers, Navy, etc.
FFM
weight × (1 − BF%)
Context
FFMI, Boer LBM, BMI
Plan
RMR preview + TDEE links
DEXA: 80 kg at 16% body fat
Lab-reported body fat percentage.
- FFM = 80 × (1 − 0.16) = 67.2 kg
- Fat mass = 80 − 67.2 = 12.8 kg
- FFMI (180 cm) ≈ 20.7 kg/m²
Result: Primary FFM — same math for all BF% sources
US Navy circumferences
Field estimate when lab data unavailable.
- Navy formula → estimated BF%
- FFM = weight × (1 − Navy BF%/100)
- Wider error than DEXA — use for trends
Result: Convenient field method, not lab precision
FFM Formula Explained
Fat-free mass from body fat %
FFM (kg) = weight (kg)
× (1 − body fat % / 100)
Fat mass (kg) = weight (kg) − FFM
Same math for DEXA, BIA, calipers,
hydrostatic, Bod Pod, manual, or
Navy-estimated body fat %.- kg
- Body weight in kilograms
- BF%
- Body fat percentage
Fat-free mass index (FFMI)
FFMI = FFM (kg) ÷ height (m)² Example: 67.2 kg FFM, 180 cm tall height = 1.80 m FFMI = 67.2 ÷ (1.80)² ≈ 20.7 kg/m² For context only — not a medical diagnosis.
- FFM
- Fat-free mass in kilograms
- m
- Height in meters
Body Composition Assessment Methods
Method
Typical use
Limitation
Method
Typical use
Limitation
Method
Typical use
Limitation
Method
Typical use
Limitation
Method
Typical use
Limitation
| Method | Typical use | Limitation |
|---|---|---|
| DEXA | Research/clinical reference | ±1–3% BF under ideal conditions; not perfect for individuals |
| Hydrostatic / Bod Pod | Lab reference | Protocol-dependent; access limited |
| Skilled calipers | Field tracking | Operator skill affects results |
| BIA (smart scales) | Home trends | Hydration shifts readings |
| US Navy circumferences | Military field estimate | Wider error than lab; convenient |
All methods feed the same FFM formula once you have a body fat % — the difference is measurement quality, not the math. Do not compare DEXA and Navy results as if they use different FFM equations.
Why Fat-Free Mass Matters
Metabolically active tissue drives resting energy more than stored fat. Katch-McArdle and Cunningham estimate resting calories from lean/FFM-based inputs. Protein for gym goals on this site uses total body weight (ISSN/Morton ranges) — clinical dosing sometimes references FFM/LBM separately.
Katch-McArdle preview
Lean mass (kg) = weight (kg)
× (1 − body fat % / 100)
BMR = 370 + (21.6 × lean mass kg)- LBM
- From FFM or BF% path
Cunningham preview
RMR = 500 + (22 × lean mass kg) Lean mass can be entered directly, or: lean mass = weight (kg) × (1 − body fat % / 100)
- LBM
- Lean mass in kg
How to Increase Fat-Free Mass
Sustainable FFM gain combines progressive resistance training, adequate protein (~1.6 g/kg/day per Morton et al. 2018 for many lifters), sleep, and often a modest surplus — not unlimited overeating. Scale weight and FFM change slowly; this calculator does not predict muscle vs fat partitioning. See the Calorie Surplus Calculator for lean-bulk planning.
FFM vs FFMI vs BMI
Metric
What it reflects
Limitation
Metric
What it reflects
Limitation
Metric
What it reflects
Limitation
| Metric | What it reflects | Limitation |
|---|---|---|
| FFM (kg) | Absolute fat-free tissue mass | Depends on BF% measurement quality |
| FFMI | FFM normalized for height | Context only — not diagnostic |
| BMI | Weight vs height | No composition — two people same BMI differ in FFM |
Accuracy and Limitations
DEXA and hydrostatic weighing are common reference methods. Body fat from DEXA may be within roughly ±1–3% under ideal scan conditions, but FFM derived from any single BF% estimate remains an approximation — especially with BIA or Navy field methods. O'Neill et al. (2023) reminds us that resting-energy equations also vary by athlete population once composition is known.
TDEE estimate error comes from two stacked layers — and the second is usually bigger in practice.
Layer 1: BMR formula error
Mifflin-St Jeor predicts resting metabolic rate within ~10% for roughly 82% of non-obese adults and ~70% of obese adults (Frankenfield et al., 2005). That is ±150–200 kcal for many people.
Layer 2: Activity multiplier error
Picking one activity bucket too high adds ~200–400 kcal/day. Most people remember gym time but underestimate desk hours. Take our Activity Level Quiz if unsure.
Common Mistakes
- Mixing measurement methods — DEXA vs Navy vs scale BIA are not interchangeable as ground truth.
- Equating FFM and LBM — use the LBM Calculator for height/weight anthropometric estimates.
- Using RMR as TDEE — multiply by activity in REE or RMR tools.
Myths vs Facts
Myth
This calculator measures your muscle mass.
Evidence-based view
It estimates total fat-free mass from body fat % — muscle is only part of FFM.
Myth
DEXA FFM is always exact.
Evidence-based view
DEXA is a reference method with protocol limits; treat as high-quality estimate, not infallible truth.
Myth
FFMI diagnoses steroid use or health status.
Evidence-based view
FFMI normalizes leanness for height — descriptive context only.
Myth
You need a separate FFMI calculator.
Evidence-based view
This page shows FFMI inline when height and FFM are available.
Suggested next steps
Metabolic Age Calculator
Composition-aware age-equivalence
Lean Body Mass Calculator
Boer, James, Hume when BF% unknown
Katch-McArdle Calculator
BMR/TDEE from body fat %
Cunningham Calculator
RMR from lean mass
RMR Calculator
Resting hub + goals
REE Calculator
7 equations + TDEE
Deficit Calculator
Fat loss presets
Surplus Calculator
Lean bulk presets
Macro Calculator
Protein, fat, carbs
Activity Level Quiz
TDEE multiplier
TDEE Calculator (Home)
Site overview
Frequently Asked Questions
Common questions about the fat-free mass calculator.
Research & References
Each citation below supports a specific claim on this page. We explain relevance so you can verify the science yourself.
- McArdle WD, Katch FI, Katch VL — Exercise Physiology: Energy, Nutrition, and Human Performance. Lippincott Williams & Wilkins, 7th edition, 2010.Textbook reference for the lean-body-mass-based Katch-McArdle resting energy estimate.
- Jager R, Kerksick CM, Campbell BI, et al. — International Society of Sports Nutrition Position Stand: Protein and Exercise. J Int Soc Sports Nutr. 2017;14:20, 2017.Supports 1.6–2.2 g/kg/day protein ranges for many exercising adults — basis for protein and macro guidance.
- Morton RW, Murphy KT, McKellar SR, et al. — A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376-384, 2018.Meta-analysis finding ~1.6 g/kg/day as an inflection point for muscle gain — supports protein calculator ranges.
- O'Neill JER, Corish CA, Horner K — Accuracy of Resting Metabolic Rate Prediction Equations in Athletes: A Systematic Review with Meta-analysis. Sports Med. 2023;53(12):2373-2398, 2023.Athlete systematic review and meta-analysis — several common equations including Mifflin-St Jeor and Owen differed significantly from measured RMR in pooled athlete data; lean-mass equations (e.g., Cunningham 1980) and Ten-Haaf performed differently by population, with no single best equation for all athletes.
- Cunningham JJ — A reanalysis of the factors influencing basal metabolic rate in normal adults. Am J Clin Nutr. 1980;33(11):2372-2374, 1980.Primary source for the Cunningham equation (500 + 22 × lean body mass kg). Cunningham’s paper labels the output BMR; the 1980 reanalysis of Harris-Benedict (1919) data found LBM as the single predictor, with sex and age adding little once LBM was included.
- de Boer P — Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol. 1984;247(4 Pt 2):F632-F636, 1984.Primary source for the Boer (1984) sex-specific anthropometric lean body mass equations — widely used in pharmacology and clinical normalization.
- James WPT — Research on obesity: a report of the DHSS/MRC group. HM Stationery Office, London. ISBN 0114500347, 1976.DHSS/MRC report containing the sex-specific quadratic LBM equations commonly cited as James (1976) in nutrition and clinical tools; some pharmacology sources attribute the quadratic form to work cited within the report (e.g., Eddy).
- Hume R — Prediction of lean body mass from height and weight. J Clin Pathol. 1966;19(4):389-392, 1966.Classic linear sex-specific anthropometric lean body mass prediction — older than Boer/James; useful for comparison but often diverges from modern DEXA cohorts.
- Hodgdon JA, Beckett MB — Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center Technical Report 84-11, 1984; companion report 84-29 for women, 1984.IOM/NAP summary documents the Hodgdon & Beckett (1984) U.S. Navy circumference body-fat equations (NHRC reports 84-11 and 84-29) used when Navy body fat % is selected in this calculator.