How to use: BMI (Body Mass Index) is a screening tool to categorize weight status. Select your age group (Adult or Child) and gender for accurate interpretation. Enter your height in feet and inches, then your weight in pounds or kilograms. The calculator shows your BMI number, category (Underweight, Normal, Overweight, Obese), and associated health risks. BMI ranges: Underweight <18.5, Normal 18.5-24.9, Overweight 25-29.9, Obese ≥30. Note that BMI doesn't distinguish muscle from fat, so athletes may show higher BMI despite being healthy.

Children (2 to 17): adult BMI ranges do not apply to children; anyone aged 18 or over should use Adult mode. Child mode asks for the child's age and compares their BMI with percentiles for age and sex from the US CDC 2000 growth charts. This uses the US CDC 2000 growth charts and US categories (overweight from the 85th percentile, obesity from the 95th). The NHS uses UK growth charts with different cut-offs (overweight from the 91st, very overweight from the 98th), so the category here can differ from the NHS calculator. For a UK assessment, use the NHS calculator or speak to your GP. Below the 5th percentile is underweight, 5th to below the 85th is a healthy weight, 85th to below the 95th is overweight, and the 95th or above is obesity; a BMI at or above 120% of the 95th percentile, or a BMI of 35 or more, is severe obesity. This is a screening guide, not a diagnosis. This calculator may not be suitable during pregnancy, for children with an eating disorder, or for children with a growth or medical condition that affects growth. In those cases, speak to a GP first. Otherwise, speak to your GP, practice nurse, health visitor or school nurse about your child's weight, or use the NHS BMI calculator for children and teenagers.