FFMI calculator

Type weight, height, and body-fat percent. The calculator subtracts fat, then divides the rest by meters squared. Eighty kilograms, 180 cm, and 15% is FFMI 21.0 and 68 kg lean.

FFMI = FFM / m². You type body-fat percent yourself. A girth estimate is on the US Navy body fat page. BMI keeps the whole mass and does not subtract fat.

Inputs

Mass, height, body-fat %

Result

Weight (kg), Height (cm), Body fat (%) and Age (years, optional, for TDEE). FFMI shows up here.

How it works

FFMI starts with fat-free mass: kilograms times (1 minus body-fat percent over 100). Then you divide that mass by height in meters squared. Eighty kilograms, 180 cm, and 15% give 68.0 kg lean and FFMI 21.0. BMI at the same weight and height is 24.7, because the whole load stays in the numerator. Sixty kilograms, 165 cm, and 25% is 45.0 kg lean, FFMI 16.5, and BMI 22.0.

People ask about FFMI because BMI mixes muscle with fat. This calculator subtracts fat only when you supply the percent. It does not guess that percent from waist or neck. Girths belong on the Navy page. A bad percent ruins FFMI even when the scale is honest. The calculator clamps BF to 0-95% so the formula never gets a negative lean mass.

Sex is optional and only picks an orientation band: 17, 20, and 22 for men, 14, 17, and 19.5 for women. Without sex the band stays hidden. Age and activity, when you fill them together with sex, add Mifflin and TDEE from the whole mass, not from the lean kilograms. FFM goes only into the index.

The US switch converts pounds and inches with 0.453592 and 2.54. Ninety kilograms, 185 cm, and 18% is 73.8 kg lean and FFMI 21.6. Seventy kilograms, 175 cm, and 20% is FFMI 18.3. Eighty-five kilograms, 178 cm, and 12% is FFMI 23.6: a high number at a low percent and a large mass, still a sketch, not a label.

Lorentz proposes one mass from height and sex, with no body-fat percent. At 180 cm in a man, Lorentz is 72.5 kg. Here 80 kg and 15% give FFMI 21.0. Different question, different number. Daily protein does not use this index either.

More pairs from the calculator: 55 kg, 160 cm, and 28% is FFMI 15.5. 95 kg, 190 cm, and 16% is 22.1. 75 kg, 170 cm, and 22% is 20.2. Type the three numbers, click Calculate. Treat 21.0 as class arithmetic, not as a steroid verdict. FFMI uses lean mass after BF%.

How to use

  1. Type mass, height, and body-fat percent. Get the percent from Navy, a skinfold, a scale with electrodes, or DXA. This calculator does not guess it.
  2. Add sex if you want an orientation band. Age and activity add a TDEE sketch from the whole mass.
  3. Click Calculate. At 80 kg, 180 cm, and 15% you will see 68.0 kg FFM and FFMI 21.0, with BMI 24.7 beside them.
  4. Compare 60 kg, 165 cm, and 25%: FFMI drops to 16.5, because the numerator is 45 kg, not the full sixty.
  5. The 17-22 band in men and 14-19.5 in women is rough. It depends on the percent you typed, and that percent can be off.

Formula

FFM = kg × (1 - BF% / 100)

FFMI = FFM / m²

BF clamped to 0-95%. Not BMI. Body-fat % must come from another method.

FFM and FFMI after subtracting fat

Lean mass first, then divide by meters squared. 80 kg, 180 cm, and 15% body fat is 68 kg FFM and FFMI 21.0.

FFM
Fat-free mass: kg × (1 − BF% / 100). At 80 kg and 15% that is 68 kg. Body-fat percent must come from another method; this calculator does not invent it.
FFMI
Fat-Free Mass Index: FFM / m². 68 / 1.80² = 21.0. Not BMI from total weight; 80 kg at 180 cm without subtracting fat would be a different ratio.
BF%
The body-fat percent field, clamped to 0-95%. At 15% of 80 kg you keep 68 kg lean, and only that remainder goes into FFMI.

Examples

Example 1

  • Mass: 80 kg
  • Height: 180 cm
  • Body fat: 15%

FFMI 21.0

80 kg, 180 cm, and 15% gives FFM 68.0 kg and FFMI 21.0, BMI 24.7.

Example 2

  • Mass: 60 kg
  • Height: 165 cm
  • Body fat: 25%

FFMI 16.5

60 kg, 165 cm, and 25% gives FFM 45.0 kg and FFMI 16.5, BMI 22.0.

Example 3

  • Mass: 90 kg
  • Height: 185 cm
  • Body fat: 18%

FFMI 21.6

90 kg, 185 cm, and 18% gives FFM 73.8 kg and FFMI 21.6, BMI 26.3.

Example 4

  • Mass: 70 kg
  • Height: 175 cm
  • Body fat: 20%

FFMI 18.3

70 kg, 175 cm, and 20% gives FFM 56.0 kg and FFMI 18.3, BMI 22.9.

Example 5

  • Mass: 85 kg
  • Height: 178 cm
  • Body fat: 12%

FFMI 23.6

85 kg, 178 cm, and 12% gives FFM 74.8 kg and FFMI 23.6, BMI 26.8.

Example 6

  • Mass: 55 kg
  • Height: 160 cm
  • Body fat: 28%

FFMI 15.5

55 kg, 160 cm, and 28% gives FFM 39.6 kg and FFMI 15.5, BMI 21.5.

Example 7

  • Mass: 95 kg
  • Height: 190 cm
  • Body fat: 16%

FFMI 22.1

95 kg, 190 cm, and 16% gives FFM 79.8 kg and FFMI 22.1, BMI 26.3.

Example 8

  • Mass: 75 kg
  • Height: 170 cm
  • Body fat: 22%

FFMI 20.2

75 kg, 170 cm, and 22% gives FFM 58.5 kg and FFMI 20.2, BMI 26.0.

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Frequently asked questions

How do 80 kg, 180 cm, and 15% become FFMI 21.0?

You divide 68 kg lean by 1.80 squared, which is 3.24. 68 / 3.24 ≈ 21.0. BMI at that weight is 24.7, because fat stays in the numerator.

Does the calculator guess body-fat percent like Navy?

No. Percent is a field. Navy from girths lives on its own page. A bad % makes a bad FFMI even with a good scale reading.

How is FFMI different from BMI?

BMI divides the whole mass by m². FFMI divides mass after fat is removed. At 15% BF the two numbers split.

Why is there a sex field if the formula ignores it?

Only for an orientation band. Without sex the band stays hidden. The 17, 20, 22 marks in men and 14, 17, 19.5 in women are not a diagnosis.

Does FFMI 23.6 at 85 kg, 178 cm, and 12% mean steroids?

It is arithmetic at a low percent and a large mass. A “very high” band is rough. Training history and a DXA reading say more than one number.

Does Lorentz replace FFMI at 180 cm?

No. Lorentz at 180 cm in a man is 72.5 kg, with no %. Here 80 kg and 15% give FFMI 21.0. Different inputs, different result.

Where do I get the percent for the BF field?

Navy, a skinfold, a scale with electrodes, or DXA. The “e.g. 15” placeholder does not enter the formula until you type a number.

Does TDEE in this calculator run from FFM?

No. It runs from the whole weight, with Mifflin, when age, sex, and PAL are complete. FFM goes only into FFMI.

What is FFMI at 60 kg, 165 cm, and 25%?

FFM 45.0 kg and FFMI 16.5. At 70 kg, 175 cm, and 20%, FFMI is 18.3.

Do pounds change 16.5 for the same person?

No, if you convert the same 60 kg and 165 cm with 0.453592 and 2.54. A different pound and inch pair is a different FFMI.

Knowledge sources

FFMI is fat-free mass over m². You type body-fat percent yourself.

Page updated in 2026.