Enter your height, body weight, estimated body fat percentage, wrist/ankle bone-frame measurements and shoulder/waist tape to compute your height-normalised Fat-Free Mass Index (Kouri et al., 1995), a rough frame estimate from Casey Butt’s formula, and the shoulder-to-waist gap to a 1.618 V-Taper proportion.
Core Equation & Guard-Rail
FFMI = Lean Mass (kg) ÷ Height (m)² · Normalised FFMI = FFMI + 6.1 × (1.80 − Height m) · V-Taper Target = 1.618 × Waist
FFMI was designed in 1995 to screen male athletes for steroid use, and it has not been validated as a general guide for women or for most other groups. Casey Butt’s frame formula comes from bodybuilder-era data. Treat both as rough general fitness estimates, never as a limit on what you can build, and never as a medical or body-composition assessment. Body fat percentage estimates are approximate too.
Methodology, Guides & Coaching
21.8
70.5 kg lean mass
82.9 kg
At 12% body fat, a rough guide
129.4 cm
Target shoulder tape
Trained Athletic Lean Mass (20.0 to 21.9). Casey Butt's frame formula gives about 73 kg of lean mass at 12% body fat for your wrist and ankle size. Treat it as a rough guide, not a limit: plenty of athletes sit above it.
A rough general fitness estimate. It is not a limit on what you can build and not a medical assessment.
The link carries the numbers you entered in the part of the address after the #, which is never sent to a server. Share it only with people you are happy to show those numbers to.
FFMI = Lean Mass (kg) ÷ Height (m)² · Normalised FFMI = FFMI + 6.1 × (1.80 − Height m) · V-Taper Target = 1.618 × Waist
FFMI was designed in 1995 to screen male athletes for steroid use, and it has not been validated as a general guide for women or for most other groups. Casey Butt’s frame formula comes from bodybuilder-era data. Treat both as rough general fitness estimates, never as a limit on what you can build, and never as a medical or body-composition assessment. Body fat percentage estimates are approximate too.
General fitness and training estimates only, labelled EST. Not medical advice or a clinical diagnosis.
Verified Literature
Fat-free mass index (FFMI) scales lean mass to height. This study used it to compare male athletes who did and did not use anabolic steroids, so it is a rough reference point rather than a target.
Kouri, E.M., Pope, H.G. Jr, Katz, D.L. & Oliva, P. "Fat-free mass index in users and nonusers of anabolic-androgenic steroids." Clinical Journal of Sport Medicine, 1995;5(4):223-228. doi:10.1097/00042752-199510000-00003
View the sourceWaist-to-height ratio earns its place on your card, this review of 31 studies found it a sharper screening signal than BMI or waist alone.
Ashwell, M., Gunn, P. & Gibson, S. "Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis." Obesity Reviews, 2012;13(3):275-286. doi:10.1111/j.1467-789X.2011.00952.x
View the sourceWe show BMI exactly as the World Health Organization defines it, using the same adult cut-offs public health uses worldwide.
World Health Organization. "Obesity and overweight" (fact sheet). Geneva: World Health Organization, last updated 8 December 2025.
View the sourceDeep-Dive Training Guides
Automatic Tracking & Adaptation
Inside Pelaris, you never need to punch numbers into a spreadsheet. Every logged set, run and check-in updates your estimates automatically and adapts next week’s sessions.
Unlike BMI (which divides total body weight by height squared), Fat-Free Mass Index (Kouri et al., 1995) uses only your lean mass and normalises to a 1.80m reference stature so you can track muscular development independent of body fat.
Wrist and ankle circumferences reflect skeletal frame breadth independently of muscle. People with thicker bone frames tend to carry more lean mass at the same height, which is the idea behind Casey Butt’s frame formula. It is a rough guide and plenty of athletes sit above it.
A V-Taper proportion is built from two directions: widening the upper silhouette through lateral deltoid and upper-back training, while looking after your waist through nutrition and conditioning. 1.618 is one popular aesthetic benchmark, not a health target, and every frame is different.