BMI calculation
The standard BMI formula is the same for all adults. Age is only used for extra recommendations.
Visual result
Choose your height and weight – the result updates automatically

BMI categories (WHO)

BMI Category Risk
< 16Severely underweightVery high
16 – 18.4UnderweightElevated
18.5 – 24.9Healthy weightMinimal
25 – 29.9OverweightElevated
30 – 34.9Obesity class IHigh
35 – 39.9Obesity class IIVery high
≥ 40Obesity class IIICritical

What is BMI and why does it matter?

Body Mass Index is one of the most widely used health indicators in the world, helping to quickly assess the balance between weight and height

The basics and the boundaries

Body Mass Index (BMI) is a simple calculation devised in 1832 by Belgian mathematician Adolphe Quetelet. The formula: BMI = weight (kg) ÷ height² (m). Today it's used by the NHS, WHO, GPs and researchers worldwide as a first-line tool for classifying weight.

Healthy range

A BMI of 18.5–24.9 is considered ideal for adults. This range statistically carries the lowest risk of heart disease, diabetes and other chronic conditions.

Its limitations

BMI doesn't distinguish muscle from fat. Athletes can have a high BMI from muscle mass, while older adults can have a "normal" BMI yet carry excess fat.

International standard

WHO BMI categories are used worldwide. Some countries (e.g. across Asia) apply lower thresholds to reflect differences in body composition and disease risk.

Frequently asked questions about BMI

Answers to the most common questions about Body Mass Index

BMI is a number obtained by dividing your weight in kilograms by the square of your height in metres. Formula: BMI = weight (kg) ÷ height² (m). It's a quick, widely-used way to check whether your weight falls within a healthy range for your height.
According to NHS and WHO guidance, a healthy BMI for adults is 18.5–24.9. This is the range associated with the lowest statistical risk of chronic disease. Below 18.5 indicates underweight, 25–29.9 overweight, and 30+ obesity.
Using the formula: BMI = weight (kg) ÷ height (m)². Example: someone weighing 70 kg at 175 cm tall: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 – a healthy weight. Our calculator does this automatically.
The standard BMI formula and thresholds are the same for men and women. Biologically, women tend to carry 6–11% more body fat than men at the same BMI. That's why sex is used here for tailoring recommendations, not for changing the formula.
For under-18s, BMI is interpreted differently – it's compared against age- and sex-specific growth percentiles rather than fixed thresholds. This tool is built for adults. If your child is under 18, speak to a GP or health visitor.
Your ideal weight range is derived from the healthy BMI band (18.5–24.9). Formula: minimum weight = 18.5 × height², maximum weight = 24.9 × height². For someone 170 cm tall, that's 53.5–72.0 kg. The calculator shows this range automatically.
BMI is a quick screening tool, but it has limits: it doesn't separate muscle from fat, and it doesn't account for fat distribution (abdominal fat is riskier). Athletes often have a high BMI due to muscle mass. For a fuller picture, doctors also use waist circumference, body fat percentage or a full health check.
You lower BMI by reducing body weight through a balanced diet (fewer calories, more vegetables, less processed food), regular physical activity (cardio plus strength training) and good sleep and stress management. Aim for 0.5–1 kg per week – losing weight too fast can cost you muscle mass.

Everything you need to know about BMI

Body Mass Index (BMI) is a simple mathematical measure that quickly indicates whether a person's weight is proportionate to their height. Formula: BMI = weight (kg) ÷ height² (m). It's widely used in clinical practice, epidemiological research and public health monitoring across the UK and beyond.

Our free BMI calculator lets you instantly work out your Body Mass Index, see which category you fall into, and get personalised recommendations. Simply enter your height and weight – the calculator does the rest and displays your result visually.

According to World Health Organisation (WHO) and NHS guidance, the healthy BMI range for adults is 18.5–24.9. It's worth knowing that some countries, particularly in Asia, apply lower thresholds (overweight from 23) to reflect different body composition and disease risk patterns.

It's important to understand BMI's limitations: it can't tell the difference between muscle and fat. Professional athletes often have a BMI above 25 despite very low body fat. Older adults may have a "normal" BMI while still carrying excess fat tissue. For a more precise assessment, GPs also use waist circumference, waist-to-hip ratio, or full body composition analysis.

Across the UK, as in much of Europe, overweight and obesity remain a growing public health concern. NHS data shows a majority of UK adults are currently overweight or living with obesity. Monitoring your BMI regularly and acting early can help prevent chronic conditions such as type 2 diabetes, cardiovascular disease and certain cancers.

Checking your BMI regularly is a quick, simple way to keep an eye on your health. Combined with a balanced diet, regular exercise and routine health checks, a BMI calculator can be a genuinely useful tool for managing your own wellbeing. We always recommend speaking to your GP or a registered dietitian for tailored advice.