Free FFMI calculator

This FFMI calculator turns your height, weight and body fat percentage into three numbers: lean body mass, fat-free mass index (FFMI) and normalized FFMI. It doubles as a lean body mass calculator, and it can estimate lean mass even if you do not know your body fat. Below you will find the formulas, worked examples, charts by height, and what the research does and does not say about the famous “25 limit”.

How to use the FFMI calculator

Pick US or metric units, choose male or female, then enter your height and weight. Add your body fat percentage if you know it. The results update as you type, and the link in your address bar saves your inputs so you can bookmark or share them.

• Height: feet and inches, or centimeters.
• Weight: pounds or kilograms. Weigh yourself in the morning for the most repeatable number.
• Body fat %: from a DEXA scan, skinfold calipers, a smart scale or a clinic test. Use the same method each time you check.
• Sex: only changes two things. It picks the right Boer formula when you leave body fat blank, and it picks the right reference point for the result text.

The short line under the big number compares you with published reference values. It is not a grade or a health check.

If you leave body fat blank, the calculator switches to an estimate. That estimate comes from the Boer formula, which predicts lean mass for an average person of your height and weight. It cannot see muscle, so treat it as a rough start.

What is FFMI?

FFMI, or fat-free mass index, is your fat-free mass in kilograms divided by your height in meters squared. It works like BMI, except it only counts the parts of your body that are not fat: muscle, bone, organs and water.

The idea is simple. Two people can weigh the same and be the same height, yet one carries far more muscle. BMI gives them the same score. FFMI does not, because it strips out fat before it divides by height.

Kouri and colleagues defined FFMI this way in their 1995 study in the Clinical Journal of Sport Medicine: fat-free body mass in kg times height in meters to the power of minus 2. That is the same as dividing by height squared.

Schutz, Kyle and Pichard used the same index in 2002 to build reference values from 5,635 healthy Swiss adults.

What is fat-free mass?

Fat-free mass is your total body weight minus all of your fat. If you weigh 180 lb and 27 lb of that is fat, your fat-free mass is 153 lb.

How to calculate FFMI

To calculate FFMI, find your lean mass from your weight and body fat, then divide it by your height in meters squared. There are three steps, plus an optional fourth.

1. Lean mass (kg) = weight in kg × (1 − body fat % ÷ 100)
2. Height in meters squared = height in m × height in m
3. FFMI = lean mass ÷ height in meters squared
4. Normalized FFMI = FFMI + 6.3 × (1.80 − height in m)

Unit conversions you will need: 1 lb = 0.4536 kg, 1 inch = 2.54 cm, and 100 cm = 1 m.

Worked example in US units

Take a man who is 5 ft 10 in, weighs 180 lb and has 15% body fat.

• Height: 70 in × 2.54 = 177.8 cm = 1.778 m
• Height squared: 1.778 × 1.778 = 3.161
• Weight: 180 × 0.4536 = 81.6 kg
• Lean mass: 81.6 × (1 − 0.15) = 69.4 kg, which is 153 lb
• Fat mass: 81.6 − 69.4 = 12.2 kg, which is 27 lb
• FFMI: 69.4 ÷ 3.161 = 22.0
• Normalized FFMI: 22.0 + 6.3 × (1.80 − 1.778) = 22.0 + 0.14 = 22.1

He sits above the 18.9 median for young men in the Schutz data, and well below the 25 mark from the Kouri study.

Worked example in metric units

Now a woman who is 165 cm, weighs 62 kg and has 25% body fat.

• Height squared: 1.65 × 1.65 = 2.7225
• Lean mass: 62 × 0.75 = 46.5 kg
• Fat mass: 62 − 46.5 = 15.5 kg
• FFMI: 46.5 ÷ 2.7225 = 17.1
• Normalized FFMI: 17.1 + 6.3 × (1.80 − 1.65) = 17.1 + 0.95 = 18.0

Her FFMI of 17.1 is above the 15.4 median for young women in the Schutz study.

What is normalized FFMI?

Normalized FFMI adjusts your score to what it would be if you were 1.80 m (about 5 ft 11 in) tall. Kouri et al. added this step because taller people tend to score a little lower on plain FFMI than shorter people with a similar build.

The normalized FFMI formula is: FFMI + 6.3 × (1.80 − height in meters). If you are shorter than 1.80 m, the adjustment adds a little. If you are taller, it takes a little away. At exactly 1.80 m, it does nothing.

A note on the number 6.3. The published abstract of Kouri et al. (1995) gives the correction as 6.3 × (1.80 m − height). Many online calculators use 6.1 instead. This calculator follows the abstract. The difference between the two is tiny, as the table shows.

Height	Adjustment with 6.3	Adjustment with 6.1
155 cm (5 ft 1 in)	+1.57	+1.52
160 cm (5 ft 3 in)	+1.26	+1.22
165 cm (5 ft 5 in)	+0.95	+0.92
170 cm (5 ft 7 in)	+0.63	+0.61
175 cm (5 ft 9 in)	+0.32	+0.31
180 cm (5 ft 11 in)	0.00	0.00
185 cm (6 ft 1 in)	−0.32	−0.31
190 cm (6 ft 3 in)	−0.63	−0.61
195 cm (6 ft 5 in)	−0.94	−0.91

Even at 155 cm, the two versions differ by only 0.05. Your body fat reading will move your result far more than that.

Use normalized FFMI when you compare yourself with the Kouri numbers, since that is what the study reported. Use plain FFMI when you compare yourself with the Schutz reference values, since those were not height-normalized.

FFMI chart by height and lean mass

This FFMI chart shows plain (not normalized) FFMI for common heights and lean masses. Find your lean mass on the left and your height across the top. If you do not know your lean mass, use the calculator first.

Lean mass	5 ft 4 in	5 ft 6 in	5 ft 8 in	5 ft 10 in	6 ft 0 in	6 ft 2 in	6 ft 4 in
110 lb	18.9	17.8	16.7	15.8	14.9	14.1	13.4
120 lb	20.6	19.4	18.2	17.2	16.3	15.4	14.6
130 lb	22.3	21.0	19.8	18.7	17.6	16.7	15.8
140 lb	24.0	22.6	21.3	20.1	19.0	18.0	17.0
150 lb	25.7	24.2	22.8	21.5	20.3	19.3	18.3
160 lb	27.5	25.8	24.3	23.0	21.7	20.5	19.5
170 lb	29.2	27.4	25.8	24.4	23.1	21.8	20.7
180 lb	30.9	29.1	27.4	25.8	24.4	23.1	21.9
190 lb	32.6	30.7	28.9	27.3	25.8	24.4	23.1
200 lb	34.3	32.3	30.4	28.7	27.1	25.7	24.3

The same chart in metric units:

Lean mass	160 cm	165 cm	170 cm	175 cm	180 cm	185 cm	190 cm
45 kg	17.6	16.5	15.6	14.7	13.9	13.1	12.5
50 kg	19.5	18.4	17.3	16.3	15.4	14.6	13.9
55 kg	21.5	20.2	19.0	18.0	17.0	16.1	15.2
60 kg	23.4	22.0	20.8	19.6	18.5	17.5	16.6
65 kg	25.4	23.9	22.5	21.2	20.1	19.0	18.0
70 kg	27.3	25.7	24.2	22.9	21.6	20.5	19.4
75 kg	29.3	27.5	26.0	24.5	23.1	21.9	20.8
80 kg	31.2	29.4	27.7	26.1	24.7	23.4	22.2
85 kg	33.2	31.2	29.4	27.8	26.2	24.8	23.5
90 kg	35.2	33.1	31.1	29.4	27.8	26.3	24.9

Some cells, such as 200 lb of lean mass at 5 ft 4 in, would be extremely unusual. The chart is maths, not a list of real people.

How much lean mass puts you at the median, or at 25?

This table flips the question. For each height, it shows the lean mass for a plain FFMI of 18.9 (the Schutz median for young men) and for a normalized FFMI of 25 (the top of the Kouri non-user group).

Height	Lean mass for FFMI 18.9	Lean mass for normalized FFMI 25
5 ft 4 in	110 lb (50 kg)	139 lb (63 kg)
5 ft 6 in	117 lb (53 kg)	150 lb (68 kg)
5 ft 8 in	124 lb (56 kg)	161 lb (73 kg)
5 ft 10 in	132 lb (60 kg)	173 lb (79 kg)
6 ft 0 in	139 lb (63 kg)	186 lb (84 kg)
6 ft 2 in	147 lb (67 kg)	199 lb (90 kg)
6 ft 4 in	155 lb (70 kg)	212 lb (96 kg)

To turn lean mass into body weight, divide by (1 − body fat % ÷ 100). A 5 ft 10 in man with 173 lb of lean mass at 12% body fat would weigh about 197 lb.

What is a good FFMI?

A good FFMI is one that is moving in the direction you want. As a reference, the median FFMI for people aged 18 to 34 in the Schutz et al. (2002) study was 18.9 for men and 15.4 for women. The authors also found little change with age in men and a modest rise in women.

That gives you two useful anchors:

• Below the median: you carry less lean mass than the typical young adult your height. That is common for people who have never lifted, and it is where a lot of home gym beginners start.
• Above the median: you carry more lean mass than typical. Regular strength training tends to push people here over time.

Average FFMI for men

The average FFMI for men sits near the Schutz median of 18.9 for the 18 to 34 group. That study used bioelectrical impedance cross-checked against DEXA in Swiss adults, so other populations may differ. For trained men, the Kouri sample is the closest guide: athletes who had never used steroids ranged up to a normalized FFMI of 25.

FFMI for women

The median FFMI for women aged 18 to 34 in the Schutz study was 15.4. The 1995 Kouri study that produced the 25 figure looked only at men. That means there is no well-studied “natural limit” for women, and you should not apply the male cutoff to a female result. This calculator compares women’s results with the 15.4 median and stops there.

Is FFMI 25 the natural limit?

No study has shown that 25 is a hard ceiling. It is the highest normalized FFMI that Kouri et al. saw among the non-users in one 1995 sample of male athletes. It is a useful rough marker, not a rule.

Here is what the study actually reported:

• Sample: 157 male athletes, 83 who had used anabolic-androgenic steroids and 74 who had not.
• Non-users: normalized FFMI values extended up to a “well-defined limit” of 25.0.
• Pre-steroid bodybuilders: 20 Mr. America winners from 1939 to 1959 had an estimated mean normalized FFMI of 25.4.
• Users: many easily exceeded 25.0, and some exceeded 30.
• Authors’ caution: the findings “must be regarded as preliminary”. They suggested FFMI as an initial screening measure, not proof.

A few limits stand out. The non-user group was 74 men. All the subjects were male. The Mr. America average was itself slightly above 25, so some men from before steroids were in wide use sat past the line. And FFMI depends on a body fat estimate, which carries its own error.

So what does the natural FFMI limit mean for you? If your normalized FFMI comes out above 25, the most likely explanation is a body fat reading that is too low. Re-test with a better method before you read anything into it. A high number is not evidence that anyone used anything. Accusing a person of steroid use based on a calculator is unfair and unsupported by this research.

FFMI vs BMI

BMI uses your whole weight. FFMI uses only your lean weight. That one difference is why BMI can label a muscular person “overweight” while FFMI shows they simply carry more muscle.

The CDC says BMI “does not distinguish between fat, muscle, and bone mass” and calls it a screening measure, not a diagnosis. Its adult categories are: under 18.5 underweight, 18.5 to under 25 healthy weight, 25 to under 30 overweight, and 30 or more obesity.

Here is a neat fact: BMI splits cleanly into two parts. BMI equals FFMI plus fat mass index (FMI), where FMI is fat mass divided by height squared. Schutz et al. reported median FMI values of 4.0 for men and 5.5 for women.

Go back to the 5 ft 10 in, 180 lb man at 15% body fat:

Measure	Formula	His number
BMI	weight ÷ height²	81.6 ÷ 3.161 = 25.8
FFMI	lean mass ÷ height²	69.4 ÷ 3.161 = 22.0
FMI	fat mass ÷ height²	12.2 ÷ 3.161 = 3.9

By BMI alone, he falls in the CDC “overweight” band. Split the number apart and most of it is lean mass, while his fat mass index is just under the male median. That is the gap FFMI fills. It is still not a health test. For health questions, a doctor will look at much more than any single index.

FFMI without body fat percentage: the Boer formula

If you do not know your body fat, the calculator estimates lean body mass with the Boer (1984) formula. Boer built it as a way to scale body fluid volumes to body size, published in the American Journal of Physiology. The coefficients are printed in a 2018 paper in BioMed Research International that compared Boer with the older James formula.

• Men: lean body mass (kg) = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2
• Women: lean body mass (kg) = 0.252 × weight (kg) + 0.473 × height (cm) − 48.3

Using the 5 ft 10 in, 180 lb man again: 0.407 × 81.6 + 0.267 × 177.8 − 19.2 = 61.5 kg, or about 136 lb. That gives an FFMI of 19.5.

Compare that with the 69.4 kg (22.0 FFMI) from his 15% body fat reading. The Boer estimate is almost 8 kg lower. Why? Because Boer predicts an average build for a given height and weight. It has no way to know that he lifts.

That is the key limit of any lean body mass calculator that uses only height and weight. It is fine for the job Boer designed it for, and fine as a rough first guess for an untrained adult. It is useless for judging how muscular you are, because a muscular person and a person with more fat get the same answer. Get even a rough body fat reading and your FFMI becomes far more meaningful.

For the woman at 165 cm and 62 kg, Boer gives 0.252 × 62 + 0.473 × 165 − 48.3 = 45.4 kg. That is close to the 46.5 kg from her 25% body fat reading, which fits the idea that Boer works best for average builds.

How accurate is FFMI?

The FFMI formula is exact. The weak link is your body fat percentage. Every method you can use at home or in a clinic is an estimate, and the errors carry straight into your FFMI.

How body fat error moves your FFMI

For the 5 ft 10 in, 180 lb man, each one-point change in body fat % moves his FFMI by about 0.26.

Body fat reading	Lean mass	FFMI
10%	73.5 kg (162 lb)	23.2
15%	69.4 kg (153 lb)	22.0
20%	65.3 kg (144 lb)	20.7

A 10-point spread in body fat readings changes his FFMI by about 2.6. That is the gap between “clearly above average” and “near the 25 mark”.

Comparing body fat methods

Different methods can disagree by several points on the same person on the same day.

• DEXA: a low-dose X-ray scan, often used as the reference in research. It is still an estimate, but it is usually the most consistent option you can book.
• Bioelectrical impedance (BIA): smart scales and handheld devices. In a 2024 study of 23 male college students in the Journal of Functional Morphology and Kinesiology, BIA read body fat 4.4 points lower than DXA on average, with wide limits of agreement.
• A-mode ultrasound: in that same study, it read 9.6 points lower than DXA on average. The authors said both methods had limited potential to measure body fat accurately compared with DXA, especially in people with higher body fat.
• Skinfold calipers: depend heavily on the skill of the person measuring and on which equation is used.

What does a 4.4-point gap do? If our example man’s true body fat were 19.4% but a scale read 15%, his FFMI would show 22.0 instead of about 20.8. Low body fat readings push FFMI up. That is one reason a surprising score above 25 deserves a second test.

Who FFMI does not fit well

• Teens: the reference values cited here are for adults.
• Older adults: Schutz found stable FFMI with age in men, but fat mass rose a lot with age. Compare yourself with people your own age where you can.
• Non-European populations: the Schutz values come from Swiss Caucasian adults.
• People with fluid changes: pregnancy, some medications and some medical conditions change body water, which BIA reads as lean mass.

How to read and act on your result

Treat your FFMI as a progress marker, not a grade. The number matters most when you track it the same way over months.

1. Pick one body fat method and stick to it. Same device, same time of day, same conditions. A trend from one method beats single readings from three.
2. Check every couple of months, not daily. Lean mass changes slowly, and day-to-day readings swing with food and water.
3. Watch lean mass, not just FFMI. Your height does not change, so FFMI only moves when lean mass moves. The lean mass line in the calculator is the one to follow.
4. Pair it with strength numbers. If your lifts climb and your lean mass climbs, the trend is real. Log your top sets with the one rep max calculator and compare lifters of different sizes with the DOTS calculator.
5. Eat enough protein. Building lean mass needs protein. The protein calculator gives you a daily target in grams.

Common FFMI mistakes

• Using a scale’s body fat reading as gospel. Home BIA scales can be several points off, and the error goes straight into your FFMI.
• Comparing plain FFMI with the 25 figure. Kouri reported normalized FFMI. Compare like with like.
• Reading a Boer estimate as your real FFMI. It is an average for your size, not a measure of your muscle.
• Treating 25 as proof of drug use. The study authors called it preliminary and a screening idea. It is not evidence about any individual.

Metric and UK units

The calculator takes centimeters and kilograms directly. If you know your weight in stone, convert it first: 1 stone is 14 lb, which is about 6.35 kg.

A worked UK example: 12 st 12 lb is (12 × 14) + 12 = 180 lb, or 81.6 kg. At 5 ft 10 in and 15% body fat, that is the same man as above, with an FFMI of 22.0.

In metric, FFMI is simply kilograms of lean mass divided by meters squared, so no conversion is needed.

Building lean mass in a home gym

Lean mass goes up with steady, progressive strength training and enough food and protein. You do not need a big commercial gym to do it. A small setup covers the basics.

• Adjustable dumbbells let you add load in small steps for presses, rows, lunges and curls. See the best adjustable dumbbells and whether adjustable dumbbells are worth it.
• A squat rack or power rack lets you squat and bench heavy with safety arms. Start with the best squat racks.
• A sturdy bench turns dumbbells into a full upper-body setup. Compare options in the best weight bench guide.
• Plates let you add weight a little at a time, which is the core of progressive overload. See the best weight plates.

Starting from scratch? The home gym setup guide covers space and what to buy first. To judge how hard each set felt, the RPE calculator turns effort into a load estimate.

Safety notes

FFMI is a body composition estimate, not a health test. It cannot diagnose anything, and a high or low number is not a medical finding.

• If you have a heart, kidney or metabolic condition, are pregnant, or take medication, talk to a doctor before starting a new training or diet plan.
• Do not chase a lower body fat reading to raise your FFMI. Very low body fat can harm health, and the number is not worth it.
• Warm up, use safety arms or spotter bars when you lift heavy alone, and add weight gradually.
• If you have a history of disordered eating or feel anxious about body numbers, it is fine to skip body metrics altogether. Strength and how you feel are good markers too.