BMI calculator

Calculate your Body Mass Index instantly. Mix and match metric and imperial units — no need to convert first.

kg, lbs or stone for weight
cm, m or ft/in for height
Instant BMI category result
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What is BMI?

Body Mass Index (BMI) is a simple number calculated from your height and weight. It gives a quick indication of whether you are in a healthy weight range for your height. BMI is used by GPs and healthcare professionals worldwide as a routine screening tool — not a diagnosis.

To check your measurements in different units before calculating, use our unit converter.

BMI categories

The NHS and World Health Organization use these standard BMI ranges for adults:

Category	BMI range
Underweight	Below 18.5
Normal weight	18.5 – 24.9
Overweight	25 – 29.9
Obese	30 and above

What your result means

Underweight (below 18.5): Your weight may be too low for your height. This can affect energy levels and bone health. A GP can help assess whether this is a concern.

Normal weight (18.5–24.9): This is generally considered a healthy range for most adults. A balanced diet and regular physical activity can help you stay there.

Overweight (25–29.9): Slightly above the healthy range. Small, sustainable changes to diet and activity levels can bring meaningful improvements.

Obese (30 and above): Associated with increased risk of conditions such as type 2 diabetes and heart disease. A healthcare professional can help you identify the right next steps.

Limitations of BMI

BMI is a useful starting point, but it has well-known limitations:

Muscle mass: Athletes and people with high muscle mass may register a high BMI despite having very little body fat.
Age: Healthy BMI thresholds can differ for older adults and children. BMI ranges for children are age- and sex-specific — our adult calculator is not suitable for under-18s. You can verify your exact age using our age calculator.
Ethnicity: Some ethnic groups face higher health risks at lower BMI values. NICE recommends lower thresholds for South Asian, Chinese, and Black African populations.
Body shape: BMI does not measure where fat is distributed. Waist circumference is often a better indicator of cardiovascular risk.

This tool is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional for a personal assessment.

Frequently asked questions

What is a healthy BMI?

For most adults, a BMI between 18.5 and 24.9 is considered healthy. Below 18.5 is underweight; 25–29.9 is overweight; 30 or above is obese. These thresholds are set by the NHS and World Health Organization.

Is BMI an accurate measure of health?

BMI is a useful screening indicator, but it is not a complete picture of health. It does not account for muscle mass, fat distribution, age, or ethnicity. Use it alongside other indicators and professional advice.

Does BMI work for athletes?

Not always. Athletes with significant muscle mass may have a high BMI despite very low body fat. In these cases, body fat percentage or waist-to-hip ratio gives a more accurate picture.

Should I be worried about my BMI?

A single BMI reading is not cause for alarm. It is one of many health indicators. If your result falls outside the healthy range, consider speaking to your GP for a fuller assessment rather than making drastic changes based on BMI alone.

How is BMI calculated?

BMI = weight (kg) ÷ height (m)². For example, someone weighing 70 kg at 1.75 m has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.

What is the difference between overweight and obese?

Overweight means a BMI of 25–29.9; obese means 30 or above. Both categories carry increased health risks compared to the normal range, but obesity is associated with significantly higher risk of chronic conditions.

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