Formula, in Body Composition

BMI Formula

BMI is mass divided by height squared. It's a 200-year-old screening tool — useful as a fast triage signal, but blind to muscle mass, fat distribution, and bone density. Treat the number as the start of a conversation, not the verdict.

3 variables. Published Mar 12, 2026. Updated May 10, 2026. By AI Fit Hub.

Education, not medical advice. The result comes only from the published formula and your inputs. How we check our formulas.

Formula

BMI = mass_kg / (height_m)^2

Variables

BMI

Body Mass Index

Index value in kg/m². WHO cutoffs: <18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, ≥30 obese. Does not distinguish fat from lean mass.

mass_kg

Body mass

Body weight in kilograms. Use morning weight after voiding, before food. To convert from pounds, divide by 2.205.

height_m

Height

Standing height in meters (cm divided by 100). Measure barefoot, heels together, looking forward.

Step by step

1

Convert height from centimeters to meters by dividing by 100.

178 cm → 1.78 m.
2

Square the height in meters.

1.78² = 3.1684 m².
3

Divide body mass in kilograms by the squared height.

80 kg / 3.1684 m² = 25.25 kg/m².
4

Compare against the WHO band the value lands in. Note that the bands assume average bone density and trained-muscle-free physiques — they over-flag muscular athletes.

25.25 lands just inside 'overweight'. For a recreationally trained adult that often just means denser tissue.

Worked example

80 kg adult at 178 cm

Weight (kg)

Height (cm)

178

BMI = 80 / (1.78)² = 80 / 3.1684 = 25.25 kg/m²

BMI 25.25 — top edge of normal, bottom edge of overweight. Combine with waist-to-hip ratio or body-fat measurement for an honest read.

Common variations

BMI for children and teens uses age-and-sex-specific percentile curves, not the adult cutoffs.

BMI Prime (BMI / 25) compresses the index around the upper-normal boundary — useful for population studies.

Asian-Pacific WHO cutoffs apply lower overweight (23) and obese (27.5) thresholds based on cardiometabolic risk data.

Run the numbers next

Body Composition Body Fat Percentage Calculator
Estimate body fat percentage using the U.S. Navy circumference method.
Body Composition Waist-to-Hip Ratio Calculator
Calculate waist-to-hip ratio and assess body composition using WHO guidelines.

Sources

Body mass index (BMI) , World Health Organization
Appropriate body-mass index for Asian populations , The Lancet