What this calculator does
If you already know your body fat percentage, from a smart scale, a DEXA scan, calipers or the tape-measure body fat calculator, this splits your weight into fat mass and lean mass. It then adjusts for height with the fat mass index (FMI) and fat-free mass index (FFMI). If you enter a target body fat, it shows the weight you'd be at that percentage if your lean mass stayed the same.
To estimate lean mass from height and weight alone, with no body fat number, use the lean body mass calculator instead.
How it's calculated
Lean mass = weight − fat mass
FMI = fat mass (kg) ÷ height (m)² · FFMI = lean mass (kg) ÷ height (m)²
Weight at target = lean mass ÷ (1 − target % ÷ 100)
Lean mass here means everything that isn't fat: muscle, bone, organs, water and so on.
Worked example
A man who weighs 180 lb (81.65 kg) at 22% body fat and is 5 ft 10 in (1.778 m):
Lean mass = 81.65 − 17.96 = 63.68 kg (140.4 lb)
FMI = 17.96 ÷ 1.778² = 5.7 kg/m² · FFMI = 63.68 ÷ 1.778² = 20.1 kg/m²
At a target of 15% with the same lean mass, he'd weigh 63.68 ÷ 0.85 = 74.93 kg, about 165.2 lb, so about 14.8 lb of fat lower.
Fat mass index cut-points
Kelly, Wilson and Heymsfield (2009) built FMI cut-points from US NHANES DEXA scans, choosing them so they matched the share of young adults in each WHO BMI class:
| FMI (kg/m²) | Men | Women |
|---|---|---|
| Excess fat (like a BMI of 25+) | above 6 | above 9 |
| Obesity (like a BMI of 30+) | 9 or more | 13 or more |
Because FMI looks at fat itself, it can tell apart two people with the same BMI but very different amounts of muscle.
Limitations
- The answer is only as good as the body fat number. Home scales and tape methods can be off by several percentage points, which moves fat mass by several pounds.
- Target weight assumes lean mass doesn't change. In real weight loss some lean mass is often lost too, and strength training can add some.
- The FMI cut-points came from DEXA scans. A number from another method may not line up exactly.
- The cut-points are for adults, not children or teens.
Sources
- Kelly TL, Wilson KE, Heymsfield SB. Dual energy X-ray absorptiometry body composition reference values from NHANES. PLoS ONE 2009;4(9):e7038. — fat mass index and its cut-points.