FFMI
calculator
Your fat-free mass index — the muscle-side answer to BMI. It scales your lean mass to your height, so it separates muscularity from fatness in a way BMI can't. Enter weight, height, and a body-fat percentage and this page returns your FFMI and the height-normalized version, plus two things most FFMI tools leave out: the error band your body-fat number carries into the result, and an honest read on the famous ~25 "natural limit." Imperial or metric.
Fat-free mass index (FFMI) is your fat-free mass divided by your height in metres squared — the same shape as BMI, but with the fat stripped out first, so it measures how much lean mass you carry for your height. For a 200 lb (90.7 kg) man at 6'0" (183 cm) and 12% body fat, fat-free mass is 176 lb (79.8 kg) and FFMI is 23.9; height-normalized to Kouri's 1.8 m reference it's 23.7. You need a body-fat percentage to compute it — FFMI has no height-and-weight-only form — and the number is only as firm as that input, which is the part the reference tables gloss over.
How the math works
FFMI takes three inputs — weight, height, and body-fat percentage — and runs two short steps. First, split weight into fat-free mass and fat mass using the body-fat percentage. Then divide the fat-free part by height squared, exactly as BMI divides total weight by height squared. The height-normalized version adds a correction so people of different heights compare on one scale. Written out:
The calculator converts pounds and feet-inches to kilograms and metres with the exact 1 lb = 0.45359237 kg and 1 in = 2.54 cm factors, then runs the four lines above. The two indices — FFMI for lean mass and FMI for fat mass — are from VanItallie et al. (1990), who proposed height-normalizing each separately as a nutritional-status measure. The 1.8 m normalization and the 6.1 coefficient come from Kouri et al. (1995), who added it so a 5'6" and a 6'4" athlete could be compared on the same number. The correction does nothing at exactly 1.8 m (5'10.9"), adds to FFMI for shorter people, and subtracts for taller ones.
The pieces, and where each comes from
| Quantity | Formula | Source |
|---|---|---|
| FFMI | FFM / H² | VanItallie 1990 |
| FMI | fat mass / H² | VanItallie 1990 |
| Normalized FFMI | FFMI + 6.1 × (1.8 − H) | Kouri 1995 |
| ~25 ceiling | normalized FFMI drug-free upper bound | Kouri 1995 (men) |
Worked example
Take a 6'0" (183 cm; 72 in) man at 200 lb (90.7 kg) and 12% body fat. Split off the fat, then divide by height squared:
- Fat-free mass: 200 × (1 − 0.12) = 176.0 lb (79.8 kg); fat mass 24.0 lb (10.9 kg).
- FFMI: 79.8 kg ÷ 1.8288² = 79.8 ÷ 3.34 = 23.9 kg/m².
- Normalized FFMI: 23.9 + 6.1 × (1.8 − 1.8288) = 23.9 − 0.18 = 23.7 kg/m².
- Fat-mass index: 10.9 kg ÷ 3.34 = 3.3 kg/m².
That's the headline the category charts miss. This is a genuinely lean, muscular 200 lb lifter — the same body BMI flags as "overweight" at 27.1 — and his normalized FFMI still sits below the ~25 line. The natural ceiling is higher than it sounds; most people quoting "25" have never computed how much muscle at what body fat it actually takes to get there.
Run a 5'5" (165 cm) woman at 150 lb (68.0 kg) and 31.1% body fat and the arithmetic is identical: fat-free mass 103.4 lb (46.9 kg), FFMI = 46.9 ÷ 1.651² = 17.2. The normalized value and the 25 ceiling don't apply — both were derived from a male sample, so this page computes her FFMI and stops there rather than grading her against a men's line.
The number is only as precise as your body fat
FFMI is linear in the body-fat term, so it hands back every bit of that input's error. Because FFMI = weight × (1 − body fat/100) / height², a body-fat estimate off by a few points shifts FFMI by exactly weight(kg) × error/100 / height². For the 200 lb (90.7 kg), 6'0" man, a 3.5-point body-fat error — the standard error a Navy-tape or skinfold estimate carries against hydrostatic weighing — moves FFMI by 90.7 × 0.035 ÷ 3.34 = 0.95, close to a full point. His "23.9" is really a band of 22.9–24.8.
That band is why chasing FFMI to one decimal place off a bathroom-scale body-fat reading is false precision. Two points of body-fat error — routine for field methods — is enough to move you a full FFMI point, and near the 25 line that's the difference between "impressive but ordinary" and "over the natural ceiling." The calculator prints the band above the point estimate for exactly this reason: if the low and high ends straddle a number you care about, your body-fat measurement isn't precise enough to settle it. A DXA or hydrostatic reading narrows the band; a visual estimate widens it.
When this calculator is wrong
The "25 FFMI natural ceiling" is an epidemiological bound from one 1995 male sample, not a biological wall — and it's only as trustworthy as the body-fat number you feed it. Kouri et al. (1995) measured fat-free mass index in 157 male athletes and found that drug-free athletes rarely exceeded a normalized FFMI of about 25, while anabolic-steroid users clustered above it. That "25" became the popular natural-limit line, and as a population flag it's useful. But it's an upper bound from a single sample of men, not a hard cap on an individual: it was derived on men, so reading a woman's number against the male 25 line is unsupported, and documented drug-free outliers do sit above it. And because FFMI inherits the body-fat error, a body-fat estimate off by 3.5 points moves the number about a full point for a 200 lb (90.7 kg) lifter — enough to cross 25 on measurement noise alone.
The counter-case is the general lifting population, where the ~25 line is a fair screen: the large majority of drug-free trainees, even advanced ones, land well below it, so a normalized FFMI comfortably under 25 built from a lab-grade body-fat measurement is a reasonable "natural-range muscularity" read. The ceiling misleads when it's treated as a lie-detector on one person, applied to a woman, or computed from a scale-estimated body-fat number. FFMI describes how much lean mass you carry for your height; it was never built to certify how you got it.
What to do with the result
Use FFMI as a height-fair way to track lean mass over time, not as a grade. Its real advantage over the scale and over BMI is that it factors out height and fat: two lifters at the same bodyweight but different heights, or the same person at two body fats, get numbers you can actually compare. Hold the method constant — same body-fat measurement approach each time — and watch the trend. A normalized FFMI drifting up at a stable or falling body fat is muscle being added; the absolute value matters less than the direction.
If you're using FFMI to sanity-check a physique against the "natural" question, do it with a real body-fat measurement and read the band, not the point. And remember what the number is downstream of: lean mass is the input to the protein target (often set per pound of lean mass) and to Katch-McArdle BMR. If you've measured body fat well enough to trust your FFMI, you've already got the number those calculators want.
Common questions
- How do I calculate my FFMI?
- Split your weight into fat-free mass and fat using your body-fat percentage — fat-free mass = weight × (1 − body fat/100) — then divide the fat-free part by your height in metres squared. A 200 lb (90.7 kg) man at 6'0" (183 cm) and 12% body fat has 176 lb (79.8 kg) of fat-free mass and an FFMI of 79.8 ÷ 1.8288² = 23.9. You need a body-fat number; there's no height-and-weight-only version of FFMI.
- What is a good FFMI?
- There's no single "good" number — FFMI is a description, not a target, and any category label you've seen comes from aggregators rather than a primary source. The two defensible anchors: a lean, muscular 200 lb / 6'0" man computes to about 23.9, and Kouri (1995) found drug-free male athletes rarely exceeded a normalized 25. So low-20s already reflects a lot of muscle at a low body fat, and the mid-20s sits near the drug-free ceiling. The honest use is your own trend over time, not where you land in a chart.
- What is the highest natural FFMI?
- The widely cited figure is a normalized FFMI of about 25, from Kouri et al. (1995), who found few drug-free athletes above it in a sample of 157 men. Treat it as an approximate upper bound, not a hard wall: it came from one male sample, documented drug-free outliers sit higher, and because FFMI inherits your body-fat error a few points of measurement noise can push the number across 25 on its own. It's a flag, not a verdict.
- Is FFMI better than BMI?
- For muscular people, yes — that's the whole point. BMI can't tell muscle from fat, so it labels a lean 200 lb (90.7 kg), 6'0" (183 cm) lifter at 12% body fat "overweight" (BMI 27.1). FFMI factors the fat out first, so it reflects lean mass instead of penalising it. The trade-off is that FFMI needs a body-fat measurement BMI doesn't, and it carries that measurement's error. BMI is a free population screen; FFMI is a body-composition index you have to earn with a body-fat number.
- Does FFMI work for women?
- The FFMI calculation itself is identical for women — fat-free mass over height squared. What doesn't transfer is the interpretation: the 1.8 m normalization, the 6.1 coefficient, and the ~25 ceiling all come from Kouri's 1995 sample of men, and none of it was validated for women. So a woman's FFMI is a valid description of her lean mass, but reading it against the male "25" line isn't supported. This page computes the number for women and deliberately withholds the natural/enhanced verdict.
- Why do I need my body-fat percentage for FFMI?
- Because FFMI is built on fat-free mass, and the only way to separate fat-free mass from total weight is a body-fat percentage. Unlike the lean-body-mass formulas (Boer, James, Hume), which estimate lean mass from height and weight alone, FFMI has no such shortcut — feed it a body fat and it's exact; without one it can't be computed. Use a measured number (tape, calipers, or a scan), and remember the ±3.5-point field error rides along into the result.