FFMI calculator: how much muscle you carry for your height

How the FFMI calculator works

FFMI, the fat-free mass index, shows how much muscle you carry for your height. BMI counts your whole weight, so a lifter with broad shoulders can read as overweight. FFMI takes the fat out first, so it measures what you built in the gym.

It takes the fat out of your weight

Fat-free mass = weight × (1 − body fat). FFMI = fat-free mass ÷ height in metres². The woman already in the calculator is 165 cm tall, weighs 62 kg and has 24% body fat. Her fat-free mass is 47.1 kg, so her FFMI is 17.3.

Height gets a correction

Taller people score a little lower on FFMI for the same build. Kouri and colleagues (1995) added a correction to a height of 1.8 m: normalised FFMI = FFMI + 6.1 × (1.8 − height in metres).

The calculator compares men on the normalised value, because the men’s limit of 25 comes from Kouri’s normalised figures. It compares women on the raw value, because the studies of women report raw values. At 165 cm the correction would add 0.9 and move a woman up a level the studies do not support. You see both numbers in the result.

Body fat can come from a tape measure

If you do not know your body fat, the calculator estimates it from your neck, waist and, for women, hips. It uses the US Navy method (Hodgdon and Beckett, 1984). Its typical error is 3 to 4 percentage points against lab methods. It tends to read too high in very lean people and too low at high body fat.

Body fat is the weak link. For the woman above, every point of body fat moves her FFMI by 0.23. A DXA scan gives a closer figure.

There are 6 levels for each sex

The bottom and top lines come from studies. The lines in between are our own split of the range between them:

• Average starts where the normal range starts in Kyle and colleagues (2003): FFMI of 16.7 to 19.8 in men and 14.6 to 16.8 in women with a normal BMI.
• The men’s top line is Kouri and colleagues (1995). None of the 74 men in their study who did not use steroids scored over 25.
• The women’s top line is Harty and colleagues (2019). In 372 female college athletes measured by DXA, only 1 in 40 scored over 23.9. Their average was 18.8, in Athletic.

A worked example

The woman in the calculator has an FFMI of 17.3: Athletic. To reach Very muscular she needs about 4.5 kg more fat-free mass. She has trained for 1 to 3 years, so she can expect 0.5% to 1% of her body weight a month. That is about 8 to 15 months of steady training.

A man of 180 cm and 80 kg with 15% body fat has 68 kg of fat-free mass and an FFMI of 21. At 1.8 m the normalised value is the same. That is Athletic, about 3.5 kg from Very muscular, or 5 to 9 months at 1 to 3 years of training.

How long the next level takes

The calculator uses a coaching rule of thumb from Alan Aragon, not a study result. It assumes steady training and enough food:

• less than a year of training: 1% to 1.5% of body weight a month
• 1 to 3 years: 0.5% to 1%
• more than 3 years: 0.25% to 0.5%

Women and men use the same rates. Roberts and colleagues (2020) found that both gain a similar share of muscle from training. A lighter body already gives fewer kilograms a month.

What FFMI cannot tell you

• Whether someone is natural. A man over 25 is rare without drugs, but it proves nothing. Male college athletes reached about 28 in Currier and colleagues (2019), and female athletes passed 25 in Blue and colleagues (2019). The most common reason for a very high result here is a body fat figure that is too low.
• Exactly how much muscle you have. Fat-free mass also counts water, bone and organs. Training adds mostly muscle to it, so the result calls the gain muscle.
• How you look or how healthy you are. Two people with the same FFMI can look very different.
• Much about teenagers. The levels come from adults, and a body under 18 is still growing.

How to measure your neck, waist and hips

Measure in the morning, standing relaxed, with the tape flat on your skin. Do not pull it tight or suck in.

• Neck: just below the Adam’s apple, with the tape sloping slightly down at the front.
• Waist: men at the navel, women at the narrowest point.
• Hips: around the widest part of your bottom, with your feet together.

Measure each one twice, and use the average.

Other calculators

To see how strong you are for your sex, age and body weight, try the strength calculator. To estimate your one-rep max from any set, use the 1RM calculator.

Sources

• Kouri EM, Pope HG, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine, 1995.
• Kyle UG, Schutz Y, Dupertuis YM, Pichard C. Body composition interpretation: contributions of the fat-free mass index and the body fat mass index. Nutrition, 2003.
• Harty PS and colleagues. Upper and lower thresholds of fat-free mass index in a large cohort of female collegiate athletes. Journal of Sports Sciences, 2019.
• Currier BS and colleagues. Fat-free mass index in a diverse sample of male collegiate athletes. Journal of Strength and Conditioning Research, 2019.
• Blue MNM and colleagues. Fat-free mass index in a diverse sample of female collegiate athletes. International Journal of Exercise Science: Conference Proceedings, 2019.
• Hodgdon JA, Beckett MB. Prediction of percent body fat for US Navy men and women from body circumferences and height. Naval Health Research Center, 1984.
• Roberts BM, Nuckols G, Krieger JW. Sex differences in resistance training: a systematic review and meta-analysis. Journal of Strength and Conditioning Research, 2020.
• Alan Aragon’s rule of thumb for muscle gain by training age, from his research review.