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

Fat-free mass (FFM)

Definition

All mass except extractable fat

On this site

Primary result when body fat % is entered

Term

Lean body mass (LBM)

Definition

Often includes essential fat (~2–5% men, ~8–12% women in classic definitions)

On this site

Boer comparison row + link to LBM hub for height/weight formulas

Term

Skeletal muscle mass

Definition

Muscle tissue only — a subset of FFM

On this site

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

1

Inputs

Sex, height, weight

2

Body fat

DEXA, BIA, calipers, Navy, etc.

3

FFM

weight × (1 − BF%)

4

Context

FFMI, Boer LBM, BMI

5

Plan

RMR preview + TDEE links

DEXA: 80 kg at 16% body fat

Lab-reported body fat percentage.

  1. FFM = 80 × (1 − 0.16) = 67.2 kg
  2. Fat mass = 80 − 67.2 = 12.8 kg
  3. 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.

  1. Navy formula → estimated BF%
  2. FFM = weight × (1 − Navy BF%/100)
  3. 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

DEXA

Typical use

Research/clinical reference

Limitation

±1–3% BF under ideal conditions; not perfect for individuals

Method

Hydrostatic / Bod Pod

Typical use

Lab reference

Limitation

Protocol-dependent; access limited

Method

Skilled calipers

Typical use

Field tracking

Limitation

Operator skill affects results

Method

BIA (smart scales)

Typical use

Home trends

Limitation

Hydration shifts readings

Method

US Navy circumferences

Typical use

Military field estimate

Limitation

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

FFM (kg)

What it reflects

Absolute fat-free tissue mass

Limitation

Depends on BF% measurement quality

Metric

FFMI

What it reflects

FFM normalized for height

Limitation

Context only — not diagnostic

Metric

BMI

What it reflects

Weight vs height

Limitation

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

U.S. Navy Body Fat Calculator

Dedicated circumference BF% hub

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.

What is fat-free mass (FFM)?
Fat-free mass is total body weight minus extractable fat. It includes muscle, bone, organs, connective tissue, and water — not muscle alone. When you enter body fat %, FFM = weight × (1 − BF%/100).
What is the fat-free mass formula?
FFM (kg) = weight (kg) × (1 − body fat % / 100). Fat mass = weight − FFM. The same formula applies whether BF% comes from DEXA, BIA, calipers, hydrostatic, Bod Pod, manual entry, or the US Navy method.
Is fat-free mass the same as lean body mass?
Not strictly. FFM excludes extractable fat. Lean body mass (LBM) in classic definitions may include a small amount of essential fat. Fitness tools often conflate the terms; this calculator reports FFM from BF% and shows a Boer LBM estimate for comparison.
Is FFM the same as muscle mass?
No. Skeletal muscle is a subset of FFM. This calculator does not output a separate muscle mass number — anthropometric or BF%-based tools cannot isolate muscle reliably.
How does this fat-free mass calculator work?
Enter sex, height, weight, and body fat % (or Navy circumferences). The calculator computes FFM, fat mass, FFMI, Boer LBM comparison, protein ranges on total weight, and RMR previews from Cunningham and Katch-McArdle.
Which body fat method should I use?
Use the most accurate source you have: DEXA or lab when available, skilled caliper tracking, or Navy circumferences as a field estimate. Home BIA scales are useful for trends but shift with hydration.
How does the DEXA / lab scan option work?
Enter the body fat % from your DEXA, BIA report, hydrostatic test, or Bod Pod summary. The calculator applies the standard FFM formula — the source label documents how BF% was obtained, not a different equation.
How does the US Navy body fat option work?
It estimates body fat % from neck, waist, and hip (women) circumferences plus height, then calculates FFM. It is a field estimate — wider error than lab methods but convenient.
What is FFMI?
Fat-free mass index = FFM (kg) / height (m)². It normalizes leanness for height. Shown when height is entered — for context only, not diagnosis.
What is a healthy fat-free mass?
There is no single healthy FFM number for everyone — it depends on sex, height, training status, and goals. Use FFMI and body fat context rather than absolute FFM targets from generic tables.
Why show Boer LBM comparison?
Boer (1984) estimates LBM from height and weight without measured BF%. Comparing it to your BF%-based FFM shows how anthropometric regressions can diverge from composition-based results — see the Lean Body Mass Calculator for full formula comparison.
How accurate is FFM from body fat %?
Accuracy depends on body fat measurement quality. DEXA may be within roughly ±1–3% BF under ideal conditions; Navy and BIA have wider individual error. FFM inherits that uncertainty.
Is DEXA the gold standard?
DEXA is a common reference method for research and clinical composition — not infallible for every individual. Hydrostatic weighing and MRI are also used as references depending on context.
Can women use this calculator?
Yes. Enter sex, measurements, and body fat % or Navy circumferences. Pregnancy mode shows a disclaimer only — composition estimates are unreliable during pregnancy.
Can teenagers use this calculator?
Users under 18 see a pediatric caution. Adolescent composition needs professional context — this tool is educational only.
Should athletes use Navy or BIA?
Muscular physiques can skew field estimates. Prefer lab methods (DEXA, skilled calipers consistently) when available; enable athlete mode for the caution flag.
Why do body fat methods differ?
DEXA, BIA, calipers, Navy, and hydrostatic measure or estimate fat through different protocols. The same person can get different BF% values — do not treat them as interchangeable without understanding method error.
How do I use FFM for Katch-McArdle or Cunningham?
Your FFM approximates the lean mass input for those equations. Open the Katch or Cunningham calculators with your weight and body fat %, or use the RMR preview rows on this page.
Why show RMR but not full TDEE?
This site has dedicated RMR, REE, Maintenance, Surplus, and Deficit calculators with seven-equation engines and activity multipliers. Duplicating TDEE here would fragment calibration guidance.
How much protein do I need based on FFM?
This site uses ISSN/Morton-aligned ranges on total body weight for fitness goals — not FFM-only targets. Clinical dosing sometimes references FFM/LBM; that is a separate context.
What do the goal options mean?
Fat loss, muscle gain, performance, and maintenance are interpretation hints only — not prescriptions. Each links to relevant Deficit, Surplus, RMR, or Maintenance tools.
How is this different from the Lean Body Mass Calculator?
The LBM hub focuses on Boer/James/Hume height-weight formulas plus optional BF%. This FFM hub is body-fat-first — when BF% is known or estimated, FFM is the primary result.
How is this different from the Katch-McArdle calculator?
Katch estimates BMR/TDEE from lean mass. This tool estimates FFM itself from body composition inputs, with FFMI and education, before you open Katch for calories.
Does BMI measure fat-free mass?
No. BMI is weight divided by height squared — it does not measure composition. This page shows BMI alongside FFM for context only.
Can I calculate total body water from FFM?
Not reliably from basic inputs on this page. TBW estimation requires validated clinical equations and context — omitted here to avoid false precision.
Can I calculate skeletal muscle mass here?
No. SMM cannot be estimated reliably from weight and BF% alone. Competitor tools that show precise muscle mass often overstate what the inputs support.
How often should I recalculate FFM?
Every 4–8 weeks during fat loss or muscle gain, or after ~3–5 kg scale change. Re-measure body fat when using lab or field methods.
Why is essential fat mentioned?
Strict LBM definitions may include small essential fat stores that FFM excludes. That definitional gap is usually smaller than differences from using Boer height/weight estimates instead of measured body fat %.
What units are supported?
Metric (kg, cm) and imperial (lb, ft/in). Navy circumferences accept inches or centimeters.
Can I share my results?
Yes — use Copy, Print, or Share actions for a text summary or URL where supported.
Does this store my data?
Calculations run in your browser. Last inputs may save to local storage on this device — nothing is sent to a server.
Is this medical advice?
No. Informational tool for fitness and nutrition awareness only — not for chemotherapy dosing, ICU nutrition, or other clinical decisions without measured composition.
Where do the equations come from?
FFM from standard composition math; Navy from Hodgdon & Beckett (1984); Boer LBM comparison from Boer (1984); RMR previews from Cunningham (1980) and Katch-McArdle. See references on this page.

Research & References

Each citation below supports a specific claim on this page. We explain relevance so you can verify the science yourself.

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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).
  8. 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.
  9. 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.