BMI Calculator
Calculate your Body Mass Index using metric or imperial measurements, with UK NHS and US CDC category guidelines.
How to Use the BMI Calculator
- 1Choose your preferred measurement system: Metric (centimetres and kilograms) or Imperial (feet, inches, and pounds).
- 2Enter your height and weight in the fields provided. Imperial users enter feet and inches separately.
- 3Click Calculate BMI to see your result instantly.
- 4Your BMI value is displayed with a colour-coded category badge (blue, green, amber, or red).
- 5The BMI scale bar shows where your value falls on the range from 10 to 40, and the healthy weight range for your height is listed below.
Frequently Asked Questions
What is BMI and how is it calculated?
Body Mass Index (BMI) is a value derived from your weight and height. For metric units it is calculated as weight (kg) divided by height (m) squared. For imperial units the formula is weight (lbs) × 703 divided by height (inches) squared. The result is dimensionless and used to classify weight status.
What are the BMI categories?
According to NHS and CDC guidelines: below 18.5 is Underweight, 18.5–24.9 is Normal weight, 25.0–29.9 is Overweight, and 30.0 or above is Obese. Some organisations use additional sub-categories within the Obese range.
Is a high BMI always bad?
Not necessarily. BMI does not distinguish between fat, muscle, and bone mass. Athletes and people with high muscle mass may have a high BMI while being in excellent health. Conversely, someone with a 'normal' BMI may have high body fat. Always consult a doctor for a full health assessment.
What is a healthy BMI for adults in the UK and US?
Both the NHS (UK) and CDC (US) define a healthy BMI range as 18.5 to 24.9 for adults. Note that different thresholds may apply for certain ethnic groups — for example, some Asian populations are assessed using slightly lower cut-off values.
How do I convert between metric and imperial?
The calculator converts internally. 1 inch = 2.54 cm, so 1 foot = 30.48 cm. 1 pound = 0.453592 kg. If you switch modes, simply re-enter your measurements in the new unit system and recalculate.