FFMI Calculator
Find your fat-free mass index and Kouri's height-normalized FFMI from height, weight and body fat, and see how close you are to the 25 natural limit.
FFMI Calculator: with the default inputs, ffmi is 22.
From DEXA, calipers, a bioimpedance scale or the body fat calculator. FFMI is only as good as this number.
Fat-free mass ÷ height².
- Normalized FFMI
Adjusted to a height of 1.8 m (Kouri 1995). This is the figure the ranges and the 25 limit refer to.
- 22.1
- Range
- Upper natural range
- Fat-free mass
- 153 lb
- Fat mass
- 27 lb
- Fat-free mass at normalized FFMI 25
The lean mass that would put you on Kouri's natural maximum at your height.
- 173.3 lb
- Lean mass below the 25 line
Positive means you are under 25; negative means above it.
- 20.3 lb
Assumptions
- Fat-free mass is weight × (1 − body fat %). The result is only as accurate as the body-fat figure you enter.
- Normalized FFMI uses Kouri et al.'s 1995 correction to a height of 1.8 m.
- The 25 limit is the maximum observed among 74 steroid-free male athletes in that study; the 18, 20 and 22 divisions are widely used conventions, not study cut points. All ranges are guidelines for men.
| Normalized FFMI | Range from here | Fat-free mass(lb) | Body weight at 15% fat(lb) |
|---|---|---|---|
| 18 | Average | 124.5 lb | 146.5 lb |
| 20 | Above average | 138.5 lb | 162.9 lb |
| 22 | Upper natural range | 152.4 lb | 179.3 lb |
| 25 | Above Kouri's natural maximum | 173.3 lb | 203.9 lb |
25 is the highest normalized FFMI among the steroid-free men in Kouri et al. (1995). 18, 20 and 22 are common rule-of-thumb divisions, not cut points from the study.
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How this is worked out
The formula
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 (m)) Kouri et al. (1995): no steroid-free athlete in the sample exceeded a normalized FFMI of 25.
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Height
- A number.in ft, in, cm, m · from 3 to 9 · defaults to 5.8333
- Weight
- A number.in lb, kg, st · from 40 to 1000 · defaults to 180
- Body fat
- From DEXA, calipers, a bioimpedance scale or the body fat calculator. FFMI is only as good as this number.a percentage · from 2 to 70 · defaults to 15
What you get back
- FFMImain answer
- Fat-free mass ÷ height².
- Normalized FFMI
- Adjusted to a height of 1.8 m (Kouri 1995). This is the figure the ranges and the 25 limit refer to.
- Range
- Fat-free mass
- Fat mass
- Fat-free mass at normalized FFMI 25
- The lean mass that would put you on Kouri's natural maximum at your height.
- Lean mass below the 25 line
- Positive means you are under 25; negative means above it.
What this assumes
- Fat-free mass is weight × (1 − body fat %). The result is only as accurate as the body-fat figure you enter.
- Normalized FFMI uses Kouri et al.'s 1995 correction to a height of 1.8 m.
- The 25 limit is the maximum observed among 74 steroid-free male athletes in that study; the 18, 20 and 22 divisions are widely used conventions, not study cut points. All ranges are guidelines for men.
About this calculator
Fat-free mass index is BMI with the fat taken out: your lean mass — muscle, bone, organs and water — divided by your height squared. Where BMI cannot tell a lifter from someone carrying extra fat, FFMI can, which is why it is the number people reach for when they ask how muscular they are, or how close they are to the "natty limit".
Where the numbers come from
The index and the limit both come from Kouri, Pope, Katz and Oliva (1995), who measured 157 male athletes — 83 anabolic steroid users and 74 non-users — with skinfold body fat. They also found that FFMI drifts upward with height, so they added a correction to express everyone at a common height of 1.8 m: normalized FFMI = FFMI + 6.1 × (1.8 − height in metres). Among the non-users, no one's normalized FFMI exceeded 25, while many of the users went well past it. That is the origin of the 25 "natural limit".
How to use it
Enter height, weight and a body-fat percentage. Body fat matters most: every point of error moves fat-free mass by about 1% of body weight, so a DEXA scan or careful calipers beats a bathroom scale. The calculator gives raw FFMI, normalized FFMI, your lean and fat mass, and how much lean mass would put you on the 25 line at your height.
Reading the result
The ranges here are guidelines, read against the normalized figure:
- Under 18 — below average for a man
- 18–20 — average
- 20–22 — above average; typical of men who train
- 22–25 — upper natural range; years of serious training
- Over 25 — above every steroid-free athlete in Kouri's sample
Only the 25 line comes from the study. The 18/20/22 divisions are rules of thumb that have grown up around Kouri's natural-athlete distribution, not cut points the paper defines. The chart plots your fat-free mass against height with each range drawn at every height, the same way the classic BMI chart works.
Caveats
Kouri's sample was 74 drug-free men in 1995, with skinfold estimates of body fat. It was never meant as a lie detector, and later reviews of the data argue that some natural lifters with exceptional genetics or decades of training can sit a little above 25. A reading over 25 is far more often a body-fat estimate that is too low. The sample contained no women, so there is no equivalent established natural limit for women and the ranges above should not be applied to them directly. FFMI also ignores where the muscle is and how strong it is — it is a ratio, not a verdict.
Frequently asked questions
▸What is the natural FFMI limit?
A normalized FFMI of about 25. In Kouri et al. (1995), none of 74 steroid-free male athletes exceeded it, while many steroid users did. It is the edge of one sample, not a physiological law.
▸What is the difference between FFMI and normalized FFMI?
Normalized FFMI adds 6.1 × (1.8 − height in metres) to adjust everyone to a height of 1.8 m, because taller people tend to have slightly higher raw FFMI. At 1.8 m (5'11") the two are identical.
▸What is a good FFMI?
For men, roughly 20–22 normalized is typical of someone who trains, and 22–25 reflects years of serious lifting. These divisions are rules of thumb; only the 25 line comes from Kouri's study.
▸Does FFMI work for women?
The formula works for anyone, but the ranges and the 25 limit come from an all-male sample. There is no comparable published natural limit for women, so compare a woman's FFMI to her own earlier readings rather than to these bands.
▸Can FFMI prove someone uses steroids?
No. A normalized FFMI over 25 is unusual for a natural lifter but not impossible, and body-fat estimates are often a few points too low, which inflates FFMI. It is a screening heuristic, not evidence.
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