Breakdown
How it works
WHO categories: under 18.5 underweight; 18.5–24.9 healthy; 25–29.9 overweight; 30+ obese (classes I–III at 30, 35, 40). For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background, UK NICE guidance uses lower thresholds of 23 (overweight) and 27.5 (obese) because health risks rise at lower BMI.
Formula
Worked example
175 cm, 75 kg: BMI 24.5 — healthy. Healthy range for 175 cm: 56.7–76.3 kg.
Frequently asked questions
Is BMI accurate?
It misclassifies muscular people as overweight and can under-flag older adults who have lost muscle. Waist circumference (under half your height) and waist-to-hip ratio add useful context.
What BMI is used for children?
Children use age- and sex-specific centile charts, not the adult categories. Do not use this calculator for under-18s.
Does BMI change with age?
The adult thresholds are the same at every age, though some evidence suggests slightly higher BMI (up to 27) is not harmful over 65.