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.
Formula
Copy the exact expression or work through it step by step below.
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)
80
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
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Sources & References
- Body mass index (BMI) — World Health Organization
- Appropriate body-mass index for Asian populations — The Lancet