What is BMI (Body Mass Index)?

Body Mass Index (BMI) is a simple, widely used measure that estimates whether your weight is in a healthy range for your height. It is calculated by dividing your weight in kilograms by the square of your height in metres (kg/m²). Because it needs only two everyday measurements, BMI is used by doctors, the NHS, and health organisations worldwide as a quick screening tool for underweight, healthy weight, overweight, and obesity.

How is BMI calculated?

The formula is the same everywhere; only the units differ. In metric units, BMI = weight (kg) ÷ height (m)². For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which falls in the healthy range. In imperial units, BMI = (weight in pounds ÷ height in inches²) × 703. The calculator above handles both systems automatically, so you can simply enter your figures and read the result.

BMI categories for adults

BMI range	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obese

These ranges apply to most adults aged 18 and over. They are the same for men and women, but they do not apply to children, who are assessed using age- and sex-specific BMI percentile charts.

Why BMI matters

BMI is a useful first indicator of weight-related health risk. A BMI in the overweight or obese range is associated with a higher likelihood of conditions such as type 2 diabetes, high blood pressure, heart disease, and certain cancers. A BMI in the underweight range can signal undernutrition or an underlying health issue. Tracking your BMI over time can help you see whether lifestyle changes are moving you toward a healthier range.

Limitations of BMI

BMI is a screening tool, not a diagnosis, and it has well-known limitations. It does not distinguish between muscle and fat, so very muscular people such as athletes can have a high BMI while carrying little fat. It also does not show where fat is stored — abdominal fat carries more health risk than fat on the hips and thighs, which is why measurements like waist-to-hip ratio and body fat percentage are useful alongside BMI. Age, ethnicity, and pregnancy can also affect how BMI should be interpreted. Use it as one piece of the picture, and speak to a healthcare professional for personalised advice.

Frequently asked questions

What is a healthy BMI?

For most adults, a BMI between 18.5 and 24.9 is considered healthy.

Is BMI accurate for everyone?

No. BMI is less reliable for athletes, older adults, pregnant women, and some ethnic groups. It is best used as a general guide rather than a definitive measure of health.

How often should I check my BMI?

Checking every few months is plenty for most people. BMI changes slowly, so frequent measurement is unnecessary unless you are actively trying to gain or lose weight.

Disclaimer: This calculator provides estimates for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making health decisions.