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FFMI Calculator

See how much lean mass you carry for your height (your FFMI), plus a height-adjusted version, from height, weight and body fat.

Level: Grades 6–7

  • Common Core standard 6.RP.A.3.cFind a percent of a quantity as a rate per 100 (e.g., 30% of a quantity means 30/100 times the quantity); solve problems involving finding the whole, given a part and the percent.
  • Common Core standard 6.EE.A.2.cEvaluate expressions at specific values of their variables. Include expressions that arise from formulas used in real-world problems. Perform arithmetic operations, including those involving whole-number exponents, in the conventional order when there are no parentheses to specify a particular order (Order of Operations).
Printable worksheet

FFMI Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Units

From a body fat estimate such as tape measurements, calipers or a scan.

Result

Fat-free mass index (FFMI)

21.2 kg/m²

Your fat-free mass index is 21.2 kg/m² (21.3 normalized to a height of 1.8 m). It depends on the body fat estimate you entered.

Normalized FFMI (to 1.8 m)
21.3 kg/m²
Fat-free mass
147.6 lb
Fat mass
32.4 lb

See the step-by-step solution ↓

Step-by-step solution

  1. Change your height to centimeters, because the formula uses metric units (1 in is exactly 2.54 cm)

    (5×12+10)×2.54=177.8 cm

  2. Change your weight to kilograms (1 lb is exactly 0.45359237 kg)

    180 lb×0.453592=81.647 kg

  3. Take the fat out of your weight to find your fat-free mass: muscle, bone, organs and water

    81.647 kg×(1−18%100)=66.95 kg

  4. Change your height to meters, the unit the index uses

    177.8 cm100=1.778 m

  5. Divide your fat-free mass by your height times itself, so people of different heights can be compared

    66.951.7782=21.18 kg/m²

  6. Adjust to a set height of 1.8 m: add 6.1 for every meter you are shorter, or take it off if taller

    21.178+6.1×(1.8−1.778)=21.31 kg/m²

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

Please note: This calculator gives a general screening estimate, not a diagnosis. Talk to a qualified healthcare professional about your individual health.

What this calculator does

Find out how much lean mass you carry for your height. The fat-free mass index (FFMI) works like BMI. BMI divides your whole body weight by your height squared (height times itself). FFMI divides only the part of your weight that isn't fat: muscle, bone, organs and water.

Two people with the same BMI can have very different FFMIs. One may carry more muscle and the other more fat. That's why the index is popular with people interested in strength training and body composition (what your body is made of).

Enter your height, weight and an estimate of your body fat percentage. The calculator works out your fat-free mass. Then it gives both the plain FFMI and the normalized FFMI. The normalized one adjusts the figure to a set reference height, so taller and shorter people can be compared more fairly. It's a number that describes you, not a health assessment.

Where it's used

Why this math matters outside the classroom:

  • Fat-free mass. At 80 kg and 15% body fat, fat-free mass is 80 × 0.85, which is 68 kg.
  • Evaluating FFMI. Dividing 68 kg by a height of 1.80 m squared gives an FFMI of 68 ÷ 3.24, which is about 21.0.
  • Height adjustment. The normalized FFMI adds 6.1 × (1.8 − height), so a 1.70 m lifter's score rises by 0.61.
  • Strength training. Lifters track FFMI over a training block to see whether the weight they gained came with more lean mass.
  • Sports science. Sports science studies use FFMI to compare muscularity across athletes of different heights.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.c, 6.EE.A.2.c. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose US or metric units.
  2. Enter your height and current weight.
  3. Enter your body fat percentage. If you don't know it, the body fat calculator can estimate it from tape measurements.
  4. Read your FFMI, normalized FFMI, fat-free mass and fat mass.

What the inputs mean

Weight
Weigh yourself at the same time each day, ideally in the morning before eating. Your weight goes up and down through the day.
Body fat
FFMI is only as good as this number. Different methods can disagree by several percentage points. Those include calipers, tape formulas, bioimpedance scales (scales that send a tiny current through you) and scans. A 5-point change in body fat moves the FFMI noticeably. If you're tracking change, use the same method each time.

Formula and method

Fat-free mass (kg) = weight (kg) × (1 − body fat % ÷ 100)
FFMI = fat-free mass (kg) ÷ height (m)²
Normalized FFMI = FFMI + 6.1 × (1.8 − height in m)

US inputs are converted exactly (1 in = 2.54 cm, 1 lb = 0.45359237 kg) before the math starts. The first line takes the fat out of your body weight. The second divides what's left by your height squared, the same way BMI does.

The normalized version comes from a 1995 study of muscularity by Kouri and colleagues. They noticed that FFMI tends to rise slightly with height, even among people of similar build. So they added a simple straight-line correction to a reference height of 1.8 m. If you're exactly 1.8 m tall, both figures are the same. If you're shorter, the normalized value is higher. If you're taller, it's lower.

Worked example

A person is 175 cm tall, weighs 80 kg and has an estimated body fat of 15%.

  1. Fat-free mass: 80 × (1 − 0.15) = 68 kg, so fat mass is 12 kg.
  2. Height squared: 1.75² = 3.0625 m².
  3. FFMI: 68 ÷ 3.0625 = 22.2 kg/m².
  4. Normalized FFMI: 22.204 + 6.1 × (1.8 − 1.75) = 22.204 + 0.305 = 22.5 kg/m².

Result: The FFMI is 22.2 kg/m² and the normalized FFMI is 22.5 kg/m².

Understanding the result

A higher FFMI means more lean mass for a given height. It leaves fat out of the picture. So it changes when you gain or lose muscle, not just when your weight changes. That makes it a useful number to track during a training program.

Values differ between men and women and across ages. Published reference figures come from particular study groups. So this calculator reports the number without calling it good or bad. If you want to know what a healthy body composition looks like for you, a healthcare or sports-medicine professional can help.

Most of the uncertainty comes from the body fat figure. So small changes in FFMI from one measurement to the next may be measuring error, not real change.

Assumptions and limitations

  • The body fat percentage entered is a reasonable estimate of your true body fat.
  • Fat-free mass is everything that isn't fat. That includes bone, organs and water as well as muscle.
  • The height adjustment uses the published straight-line correction to 1.8 m, and applies it at any height.
  • It inherits all the error in the body fat estimate.
  • It doesn't separate muscle from other lean tissue. So changes in how much water your body holds can shift it.
  • It isn't designed for children, pregnancy or people with conditions that change body water or bone mass.
  • It's a number that describes your build, not a measure of health, fitness or strength.

Frequently asked questions

How is FFMI different from BMI?

BMI uses total weight, so it can't tell muscle from fat. FFMI uses only fat-free mass. So a muscular person with a high BMI may have a high FFMI. Someone with the same BMI but more fat will have a lower one.

Should I look at FFMI or normalized FFMI?

The normalized figure is better for comparing people of different heights. It removes FFMI's slight tendency to rise with height. For tracking your own progress, either works, since your height doesn't change.

Where do I get my body fat percentage?

Common options are a tape-measure formula, skinfold calipers, a bioimpedance scale or a body composition scan. They vary in accuracy, so stick with the same one.

Why do I get a different result from another tool?

Tools may round height or weight differently or use a different reference height. Some ask for lean mass directly. Check that the body fat percentage and units match.

How this calculator is checked

Method: FFMI = fat-free mass ÷ height²; normalized FFMI = FFMI + 6.1 × (1.8 − height in m), as described by Kouri et al. (1995). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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