Free BMI Calculator.

Body Mass Index (BMI) is a simple screening measure that divides your weight by the square of your height (kg ÷ m², or 703 × lb ÷ in²). It's used by the NHS and the World Health Organization as a quick, free way to flag whether someone's weight is likely to be in a healthy range for their height — not as a diagnosis on its own.

18.5–24.9 is classed as a healthy weight for most adults, using the same WHO thresholds the NHS uses. Below 18.5 is underweight, 25–29.9 is overweight, and 30 and above is obese, split further into Class I (30–34.9), Class II (35–39.9) and Class III (40+). The chart on this page shows every range at once.

No — BMI can't tell the difference between muscle and fat, so very muscular people (athletes, bodybuilders) are often classed as “overweight” or “obese” despite having low body fat. It's also not designed for use under 18, during pregnancy, or for older adults who've lost muscle mass, all of whom need a different way of assessing healthy weight. Treat a BMI result as a starting point for a conversation with a GP or pharmacist, not a verdict.

The standard 18.5–24.9 range was developed mostly from data on white European populations. NHS guidance notes that people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background can face increased health risks at a lower BMI, and some NHS services use adjusted thresholds for these groups (around 23 for overweight and 27.5 for obese, instead of 25 and 30). This calculator uses the standard WHO scale; if this applies to you, it's worth mentioning to a GP or pharmacist alongside your result.

No — your height and weight are calculated entirely inside this browser tab, on your device. There's no server involved at all, so nothing you type is ever seen, copied or kept by Altto.

No. There's no upload or storage involved. See the Privacy Policy for details.