Example 1
- 70 kg · 170 cm · 15% BF
BMI 24.2 · FFM 59.5 kg · FFMI 20.6
BMI and muscle mass with a normal BMI: FFMI shows mass after fat is removed.
BMI and Muscle Mass
Enter weight and height to get Quetelet BMI (WHO). Add body-fat percent to see lean mass and FFMI. Example: 70 kg, 170 cm, and 15% → BMI 24.2, 59.5 kg lean, FFMI 20.6. Plain BMI does not say how much of the mass is muscle.
Result
Enter weight and height on the left, then Calculate. Everything else is optional.
Step 1 is enough for BMI. In step 2, enter a body-fat percent from the U.S. Navy method, a scale, or DXA to see FFMI. Step 3 covers calorie needs only.
How it works
BMI24.2
FFM59.5 kg
FFMI20.6
The calculator starts with Quetelet’s index — the same adult BMI WHO uses: mass in kilograms divided by height in meters squared. At 70 kg and 170 cm the result is 24.2. That ratio alone does not separate muscle from fat.
When you enter a body-fat percent, the calculator subtracts estimated fat from mass and divides the remainder by the same meters squared. At 70 kg, 170 cm, and 15% lean mass is 59.5 kg and FFMI is 20.6. That percent must come from another method: Navy, skinfolds, bioimpedance, or DXA.
Age, sex, and activity do not enter BMI or FFMI. Only after you enter age do Mifflin–St Jeor BMR and TDEE = BMR × PAL appear. Those are separate calorie estimates, not a reading of body composition.
The separate BMI calculator shows only the WHO ratio. The dedicated FFMI calculator also normalizes to 1.80 m. Here BMI is shown next to lean mass so a muscular person’s higher BMI is not read automatically as excess fat.
Example: the same person at 92 kg, 180 cm, and 12% body fat has BMI 28.4 (WHO “overweight”) and FFMI 25.0. BMI alone suggests excess mass, while fat-free mass index describes a build typical of strength training — which is why BMI can mislead for muscular people.
Formula
BMI = kg / (m)² — Adolphe Quetelet; 18.5–24.9 bands are the WHO adult convention
FFM = kg × (1 − BF% / 100)
FFMI = FFM / (m)²
Optional: BMR (Mifflin–St Jeor), TDEE = BMR × PAL. Body-fat percent comes from the value you enter — the calculator does not estimate it on its own.
How to use
- Enter weight and height (metric: 70 kg and 170 cm). With US units the fields take pounds and inches; quick examples convert those same kilograms and centimeters so BMI stays 24.2.
- Optionally enter body-fat percent, for example 15, to see lean mass and FFMI.
- Optionally enter age and choose sex and activity if you want BMR and TDEE.
- Click Calculate. At 70 kg, 170 cm, and 15% the result is BMI 24.2, 59.5 kg lean, and FFMI 20.6.
- Compare with 92 kg / 180 cm / 12%: BMI 28.4 with FFMI 25.0 — a higher BMI need not mean higher fat.
BMI beside lean mass
First Quetelet’s ratio, then — after you enter body-fat percent — lean mass and FFMI. At 70 kg, 170 cm, and 15% the result is BMI 24.2, FFM 59.5 kg, and FFMI 20.6.
- BMI
- Body Mass Index: kg / m². At 70 kg and 170 cm the result is 24.2. It does not separate muscle from fat.
- FFM
- Fat-free mass: kg × (1 − BF%/100). At 70 kg and 15% that is 59.5 kg. The percent comes from the field you enter.
- FFMI
- Fat-Free Mass Index: FFM / m². 59.5 / 1.70² ≈ 20.6. An index after removing fat, not a DXA scan.
- Mifflin–St Jeor
- Optional BMR after age, sex, weight, and height. TDEE multiplies BMR by PAL. It does not change BMI or FFMI.
Examples
Example 2
- 92 kg · 180 cm · 12% BF
BMI 28.4 · FFM 81.0 kg · FFMI 25.0
BMI says “overweight” while FFMI is high — a common muscular pattern.
Example 3
- 100 kg · 175 cm · 18% BF
BMI 32.7 · FFM 82.0 kg · FFMI 26.8
High BMI and high FFMI together: the WHO ratio alone does not describe body composition.
Related calculators
Frequently asked questions
What is a BMI and muscle mass calculator?
It shows Quetelet BMI from weight and height, and after you enter body-fat percent it also shows lean mass and FFMI. At 70 kg, 170 cm, and 15% fat, BMI is 24.2, lean mass 59.5 kg, and FFMI 20.6. Not a DXA scan and not plain adult BMI without fat percent.
Does BMI account for muscle?
No. BMI = kg ÷ m² and does not separate muscle from fat. Body-fat percent on this page unlocks lean mass and FFMI. At 70 kg and 15% fat, lean mass is 59.5 kg.
Is BMI accurate if you have muscle?
BMI still rises with muscle because it uses total mass. A muscular person can land in overweight WHO bands. Adding body-fat percent here shows lean mass and FFMI beside that BMI — context, not a medical override.
Is there a BMI that includes muscle mass?
Not in the WHO BMI formula. FFMI is the lean-mass counterpart: lean kg ÷ m². This page computes both when you enter body-fat percent. A dedicated FFMI calculator is linked nearby.
How is BMI different from FFMI?
BMI divides total mass by height squared. FFMI divides fat-free mass by height squared. At 70 kg, 170 cm, and 15% fat, BMI is 24.2 and FFMI is 20.6.
How is this different from a plain BMI calculator?
It uses the same Quetelet BMI (mass / height²), but after a body-fat percent it also shows lean mass and FFMI. At 70 kg, 170 cm, and 15% you see BMI 24.2 next to 59.5 kg lean and FFMI 20.6 — so a higher BMI is not read as “more fat” by default.
When do lean mass and FFMI appear?
Only after you enter a body-fat percent between 0 and 70. The calculator does not estimate BF% from girths — that measurement is on the Navy method page. Without a percent you get BMI alone, and after age also BMR and TDEE.
Which formulas are used?
BMI = kg / m² (Adolphe Quetelet; WHO adult convention). Lean mass = kg × (1 − BF%/100). FFMI = lean mass / m². Optionally BMR via Mifflin–St Jeor and TDEE = BMR × PAL from the activity list.
When do BMR and TDEE show?
When you enter age. BMR then uses Mifflin–St Jeor (weight, height, age, sex), and TDEE multiplies BMR by the selected activity factor. BMI itself does not need those fields.
Does a high BMI in a muscular person mean obesity?
Not necessarily. BMI divides total mass by height² and does not separate muscle from fat. An athlete with low BF% can land in the “overweight” band. Compare FFMI after entering body-fat percent — still an estimate, not a diagnosis.
Do US units change the inputs?
Yes. With US units weight is in pounds and height in inches; the calculator converts them internally to kg and meters. On-page examples (70 kg, 170 cm) stay metric. Body-fat percent has no mass unit.
Is the result a medical measurement?
No. BMI is a WHO screening convention. FFMI depends on the BF% you enter from another method. Not a DXA scan, not a clinical verdict, and not dietary advice.
Knowledge sources
BMI is Quetelet’s ratio (WHO). FFMI and lean mass need a BF% you enter — not a DXA scan.
- Wikipedia — body mass index
- WHO — obesity and overweight fact sheet
- MedlinePlus — body mass index
- Wikipedia — fat-free mass index
Page updated October 29, 2026.