Body composition assessment
Weight tells you only how much you weigh, while body composition tells you what that weight is made of. The calculator breaks body mass down into fat and fat-free parts, estimates muscle mass (FFMI, SMM, SMI), fat distribution (waist-to-height, waist-to-hip, body shape) and the ABSI index — all in one report, with no lab tests.
What body composition is
The simplest model splits body mass into two parts. Fat mass — all of the body's lipids. Fat-free mass — muscle, bone, water and organs. Fat comes in two forms: essential (part of cell membranes, needed for hormones, its share is stable) and storage — subcutaneous and visceral (inside the abdomen, the most dangerous).
That's why weight — and even BMI — is misleading: neither tells muscle from fat. Two people with the same weight can have completely different body composition, and different health. This calculator brings the key composition metrics together in one report; for an in-depth look at individual indices there are dedicated tools (links below).
Body composition calculator
Enter sex, age, height, weight and circumferences (waist and hips are required; chest is optional), and pick your fitness level. The calculation runs on the server; your data is not stored.
Muscle mass: FFMI, SMM and SMI
Muscle is the "engine" of metabolism and your main defense against age-related frailty. We assess it in three ways:
- FFMI (fat-free mass index) — fat-free mass relative to height² (an analog of BMI, but for "lean" mass). Normalized FFMI adjusts the value to a height of 1.8 m for comparison.
- SMM (skeletal muscle mass) — an estimate of muscle tissue specifically; it depends on your level of training.
- SMI (skeletal muscle index) — SMM relative to height². A low SMI is a marker of sarcopenia (muscle loss), which progresses after age 50 without strength training.
Want to assess fat alone using two validated methods (U.S. Navy and BMI-based)? Use the body-fat percentage calculator.
ABSI — the body shape index
ABSI (A Body Shape Index) is a metric that isolates the contribution of abdominal fat to body shape, "stripping" waist circumference of the influence of height and BMI. Its distinctive feature: a high ABSI is linked to elevated risk independent of weight and BMI — so it catches risk even in people with a normal weight. The result is expressed as a z-score relative to people of the same sex and age: near zero is typical, above +1 means more abdominal fat than average.
Body composition and aging
For the pace of aging, what matters is not weight but the balance of fat and muscle. Excess fat sustains chronic inflammation and insulin resistance, while muscle loss (sarcopenia) lowers metabolism and functional independence. Maintaining muscle mass and a moderate fat share is one of the most reliable investments in longevity. To estimate biological age more precisely, use blood tests (PhenoAge).
How to improve body composition
The core principle is to lose fat while preserving muscle: a moderate calorie deficit, enough protein, strength training at the base and cardio for the heart, plus sleep and stress management. A step-by-step review of lifestyle is in the metabolic age from lifestyle calculator.
Frequently asked questions
How accurate are the calculations without bioimpedance?
This is a formula-based estimate from anthropometry. It reflects trends and changes over time well, but is less accurate than instrument methods (bioimpedance, DEXA). To track progress, measure yourself under the same conditions each time.
Why is fitness level needed?
It determines the muscle share of fat-free mass: trained people have more muscle at the same "lean" mass. This refines the SMM and SMI estimates.
How does this calculator differ from "body-fat %", WHR and WtHR?
What is "essential" mass?
It is the residual body mass excluding fat and skeletal muscle: bone, organs, water, connective and cardiac tissue. It is relatively stable and serves as a reference value.
