FFMI Calculator: Your Fat-Free Mass Index, Normalized for Height

FFMI stands for fat-free mass index. It is the amount of fat-free mass you carry relative to your height, in kilograms per square meter. It is calculated the way BMI is, with one change: only the fat-free part of your body weight is counted.

What Is Fat-Free Mass?

Fat-free mass is everything in the body that is not fat. That means skeletal muscle, and also bone, organs, connective tissue, and water. It is found by subtraction: body weight minus fat mass. A person who weighs 180 lb at 15% body fat has 27 lb of fat and 153 lb of fat-free mass.

Fat-free mass is often used as a stand-in for muscle, but the two are not the same. Water is a large part of it, and water changes from day to day.

Why Not Use BMI?

BMI divides total weight by height squared, so it cannot tell fat from muscle. A population study in the journal Nutrition (Kyle et al., 2003) concluded that BMI alone cannot show how much of body weight is fat-free mass and how much is fat. FFMI and the fat mass index split BMI into those two parts. The BMI calculator, with its limitations spelled out shows the combined figure.

The FFMI formula is fat-free mass in kilograms divided by height in meters squared. This calculator runs it in your browser. Every step depends on the body fat percentage, which you can enter or have estimated from tape measurements. Fat-free mass is body weight multiplied by (1 − body fat percentage ÷ 100).

Here is the full calculation for a man who is 5 ft 10 in, weighs 180 lb, and has 15% body fat.

1. Convert to metric: 180 lb × 0.4536 = 81.65 kg. 70 in × 0.0254 = 1.778 m.
2. Fat-free mass: 81.65 × (1 − 0.15) = 69.40 kg, which is 153.0 lb.
3. Height squared: 1.778 × 1.778 = 3.161.
4. FFMI: 69.40 ÷ 3.161 = 21.95, shown as 22.0.
5. Normalized FFMI: 21.95 + 6.3 × (1.80 − 1.778) = 21.95 + 0.14 = 22.09, shown as 22.1.

Enter the same details above and the calculator returns the same 153.0 lb, 22.0, and 22.1.

Normalized FFMI is FFMI with a height correction added: 6.3 × (1.80 − your height in meters). It expresses your figure as if you were 1.8 m tall, which is about 5 ft 11 in.

The correction comes from Kouri and colleagues (Clinical Journal of Sport Medicine, 1995), who used it to express every athlete’s value at the height of a 1.8 m man. In practice it is small. At 5 ft 6 in it adds about 0.8. At 6 ft 4 in it subtracts about 0.8. At exactly 1.8 m it changes nothing.

The correction was derived from a sample of 157 male athletes, so applying it to women or to people who do not train is an extrapolation. The reference ranges on this page are for plain FFMI, so the calculator compares your plain FFMI against them.

There is no clinical cutoff for a good FFMI. What exists is reference data: what FFMI looks like across a large group of healthy people. The data used here comes from a Swiss cohort.

The median values come from Schutz, Kyle, and Pichard (International Journal of Obesity, 2002), who measured 5,635 apparently healthy adults, 2,986 men and 2,649 women, with bioelectrical impedance. The ranges come from a second analysis of the same cohort (Kyle et al., 2003), which described its participants as aged 15 to 98 and reported the FFMI values that correspond to a normal BMI. The average FFMI in men did not differ with age in that data, and in women it rose modestly.

How to Read the Range

The range describes people at a normal BMI. It is not a target, and a figure outside it is not a diagnosis. Three things commonly put an FFMI above it:

• Resistance training. Years of lifting add fat-free mass, which is the reason FFMI is tracked by people who train.
• A higher BMI. A larger body carries more fat-free mass as well as more fat. The 2003 analysis reported separate FFMI values for each BMI category for that reason.
• A body fat figure that is too low. Underestimating body fat by a few points inflates FFMI directly.

An FFMI below the range can reflect low fat-free mass or a body fat figure that is too high. Evaluating a deficit in fat-free mass belongs with a clinician, not a calculator. The study population was white and Swiss, so the ranges may fit other populations less well.

No. An FFMI of 25 is the highest value observed in one group of 74 athletes in a 1995 study. It has never been shown to be a ceiling on what a human body can reach.

The figure comes from “Fat-free mass index in users and nonusers of anabolic-androgenic steroids” (Kouri, Pope, Katz, and Oliva, 1995). Here is what that paper reports:

• The sample was 157 male athletes: 83 who had used anabolic-androgenic steroids and 74 who had not.
• Among the 74 non-users, normalized FFMI values “extended up to a well-defined limit of 25.0.”
• The authors also estimated normalized FFMI for 20 Mr. America winners from 1939 to 1959. Their mean was 25.4, which is above 25.
• The authors described their own findings as preliminary.

So the often-quoted 25 is an observation about one sample of men, measured once, in a study its own authors called preliminary. The paper’s second sample averaged above it. The study included no women, so it says nothing about a figure for women.

What an FFMI Above 25 Does and Does Not Mean

An FFMI cannot tell you, or anyone else, what a person has taken. This calculator does not attempt to, and a high figure should not be read that way. FFMI is built on a body fat estimate, and an estimate that is three or four points too low is enough to move an ordinary trained physique past 25. Height, frame, and a higher BMI all move the figure too. If your result is 25 or more, the first thing to check is the body fat number.

The U.S. Navy method estimates body fat percentage from a tape measure and your height. It needs a neck and a waist measurement for men, and neck, waist, and hip measurements for women. This calculator offers it for anyone who does not have a body fat figure.

The equations were published by J. A. Hodgdon and M. B. Beckett of the Naval Health Research Center in 1984, in two technical reports: Report No. 84-11 for men and Report No. 84-29 for women. Each one predicts body density from measurements in centimeters:

• Men: density = −0.19077 × log10(waist − neck) + 0.15456 × log10(height) + 1.0324.
• Women: density = −0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height) + 1.29579.
• Both: body fat percentage = 100 × (4.95 ÷ density − 4.50), the Siri equation.

Where to Measure

The reports define each site. The neck is measured just below the larynx, with the tape sloping slightly downward toward the front. For men, the waist is measured at the level of the navel. For women, it is measured at the narrowest point of the abdomen, about midway between the bottom of the breastbone and the navel, and the hips are measured at their widest point. Keep the tape level and snug without compressing the skin.

How Accurate Is the Navy Method?

The men’s equation was built from 602 Navy men aged 18 to 56, with underwater weighing as the reference. It had a correlation of 0.90 with that reference and a standard error of 3.52 percentage points of body fat. The women’s equation was built from 214 Navy women, with a correlation of 0.85 and a standard error of 3.72 points. A standard error of about 3.5 points means roughly one estimate in three is off by more than that. Both samples were active-duty personnel.

FFMI is exactly as accurate as the body fat percentage behind it. Height and weight are easy to measure well. Body fat is not, and FFMI inherits all of that error.

The effect is easy to quantify. One percentage point of body fat moves FFMI by your BMI divided by 100. For the man in the worked example above, at a BMI of 25.8, each point of body fat is worth 0.26 FFMI. If his body fat came from the Navy method, its standard error of 3.52 points is about 0.9 FFMI in either direction. A result of 22.0 is better read as roughly 21 to 23.

What FFMI Cannot Tell You

FFMI does not distinguish muscle from the rest of fat-free mass, does not measure strength or function, and does not diagnose anything. DXA scans, impedance scales, calipers, and tape-measure equations can each give a different body fat figure for the same person on the same day. FFMI is most useful as a number you track over time with one consistent method, where the direction of change means more than the value.

During weight loss on a GLP-1 medication, the scale cannot tell you whether the weight coming off is fat or fat-free mass. Two measurements of weight and body fat can, at least roughly. That is the most practical use of FFMI for someone who is not a bodybuilder.

What the Trials Measured

Fat-free mass falls during almost any substantial weight loss. In the body composition substudy of the SURMOUNT-1 trial of tirzepatide (Look et al., Diabetes, Obesity and Metabolism, 2025), 160 participants had DXA scans at the start and at week 72. With tirzepatide, body weight fell 21.3%, fat mass fell 33.9%, and lean mass fell 10.9%. Of the weight lost, about 75% was fat and 25% was lean mass, and the proportion was the same in the placebo group, which lost far less weight. Several of that paper’s authors are employees of the drug’s manufacturer.

Results vary across studies. A 2024 review of lean body mass changes with GLP-1-based therapies (Neeland, Linge, and Birkenfeld) found that lean mass made up 40% to 60% of the weight lost in some trials and about 15% or less in others. It also made a point that matters for reading any FFMI: lean mass is not just muscle. It includes organs, bone, fluids, and the water held in fat tissue. The review describes the muscle changes seen so far as in line with what is expected for the weight lost, while noting that older age and more severe disease raise the concern about sarcopenia.

These trials studied FDA-approved medications. Compounded medications were not studied in them, and results vary from person to person.

How to Track It with This Calculator

1. Take a starting measurement. Record your weight and a body fat figure, from whichever method you can repeat.
2. Repeat it the same way. Use the same method, the same device or tape sites, and similar conditions, such as the same time of day.
3. Enter both. Put the current figures in the main fields and the earlier ones under “Earlier measurement.”
4. Read the split. Once at least 5 lb (about 2.3 kg) has been lost, the calculator shows what share of it was fat and what share was fat-free mass.

A Worked Example, and Its Error

Someone who is 5 ft 10 in goes from 230 lb at 32% body fat to 200 lb at 28%. Fat mass went from 73.6 lb to 56.0 lb, and fat-free mass from 156.4 lb to 144.0 lb. Of the 30 lb lost, 17.6 lb was fat and 12.4 lb was fat-free mass: 59% and 41%. FFMI fell from 22.4 to 20.7.

Now suppose the second body fat reading was two points high, and the true figure was 26%. Fat-free mass would then be 148.0 lb, the loss of fat-free mass 8.4 lb, and its share 28%. A two-point error, which is well inside what consumer methods produce, moved the answer from 41% to 28%. That is why a single pair of readings is a rough guide, and why the trend across several measurements deserves more weight than any one of them.

A falling FFMI during a large weight loss is expected and is not by itself a sign of a problem. If the numbers concern you, bring them to the physician managing your treatment. For more background, read our guide to muscle loss on GLP-1 medications.

Protocol MD prescribes compounded semaglutide and compounded tirzepatide to patients a physician determines are appropriate candidates. Compounded medications are not FDA-approved. The FDA-approved semaglutide and tirzepatide medications carry a boxed warning for thyroid C-cell tumors and are contraindicated in people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2.

Enough protein and resistance training are the two measures current guidance names for holding on to fat-free mass during weight loss. The 2025 joint advisory on nutritional priorities to support GLP-1 therapy for obesity (Mozaffarian et al., American Journal of Clinical Nutrition) names preserving muscle and bone through resistance training and an appropriate diet as a priority, and notes that protein targets of 1.2 to 1.6 grams per kilogram a day have been proposed during active weight reduction.

The protein calculator (grams per day and per meal) sets a daily protein range from your weight, activity, and goal, and the macro calculator (protein, carbs, and fat in grams) builds a full day around a protein-first split. Our guide to how many calories to eat on a GLP-1 covers the calorie side, and the weight loss percentage calculator puts a change in scale weight in context.

Protocol MD is a physician-guided telehealth practice. An FFMI is one description of body composition. Whether any treatment is appropriate depends on your history, your labs, and your goals, and only a licensed physician can make that call after reviewing them.

Protocol MD sells compounded semaglutide and compounded tirzepatide and therefore has a commercial interest in this topic. For patients a physician determines are appropriate candidates, Protocol MD prescribes compounded semaglutide or compounded tirzepatide, prepared by a licensed compounding pharmacy. Compounded medications are not FDA-approved. They are not generic equivalents of, and are not the same product as, the brand-name medications. Nothing in this calculator depends on taking a medication, and FFMI is the same calculation for anyone.