NHS Obesity & BMI Calculator
Check your Body Mass Index and weight category using NHS guidelines. Enter your height and weight to instantly see your BMI, healthy weight range, and what it means for your health.
Calculate your BMI & weight category
Enter your height, weight, age and sex. We’ll calculate your BMI, show your NHS weight category, your healthy weight range, and how much weight you’d need to gain or lose to reach a healthy BMI.
Your details
All calculations follow NHS BMI guidelines for adults (18+)
Used for waist-to-height ratio context. Standard BMI thresholds are the same for both sexes.
This calculator is for adults aged 18 and over. For children use a separate age-adjusted BMI tool.
Example: 5 ft 9 in = 175 cm
Example: 11 st 11 lb = 75 kg
Measure around your natural waist (just above the belly button). Used to assess central obesity risk alongside BMI.
Your BMI Result
Based on NHS guidelines for adults (18+)
Enter your height and weight, then click Calculate my BMI to see your result, weight category, and healthy weight range.
Understanding BMI categories
The NHS uses BMI ranges to classify weight status. Here’s what each category means and the health considerations associated with it.
Underweight
A BMI below 18.5 may indicate insufficient body fat or muscle. Can be associated with nutritional deficiencies, weak bones, and a weakened immune system.
Healthy Weight
A BMI of 18.5–24.9 is associated with the lowest risk of weight-related health problems. Maintaining this range through balanced diet and regular activity is recommended.
Overweight
A BMI of 25–29.9 increases the risk of conditions including type 2 diabetes, high blood pressure, and heart disease. Modest weight loss can significantly reduce these risks.
Obese (Class I–III)
A BMI of 30 or above is classified as obese. Significantly increases risk of type 2 diabetes, heart disease, stroke, and certain cancers. NHS weight management support is available.
Full BMI classification table
The complete NHS and WHO BMI classification for adults, including all three obesity classes and their associated health risks.
| Category | BMI Range | NHS Classification | Health Risk | Action |
|---|---|---|---|---|
| Underweight | Below 18.5 | Underweight | Moderate – nutritional deficiency | Speak to a GP |
| Healthy | 18.5 – 24.9 | Healthy weight | Low | Maintain weight |
| Overweight | 25.0 – 29.9 | Overweight | Increased | Lose some weight |
| Obese I | 30.0 – 34.9 | Obese class I | High | NHS weight management |
| Obese II | 35.0 – 39.9 | Obese class II | Very high | Medical support recommended |
| Obese III | 40.0 + | Severely obese (class III) | Extremely high | Specialist referral |
What BMI doesn’t tell you
BMI is a useful screening tool but it has well-known limitations. Understanding these helps you put your result in the right context.
Muscle vs fat
BMI cannot distinguish between muscle and fat. Athletes and very muscular people often have a high BMI but low body fat. A muscular person may be classified as overweight despite being in excellent health.
Ethnicity matters
People of South Asian, Chinese, and other Asian backgrounds face higher health risks at a lower BMI. The NHS recommends lower thresholds (23 for overweight, 27.5 for obese) for people of Asian ethnicity.
Age & older adults
In older adults (65+), a slightly higher BMI may be protective against frailty and osteoporosis. The relationship between BMI and health risk changes with age, particularly above 70.
Waist circumference
Where you carry weight matters as much as how much you carry. Central obesity (fat around the abdomen) is a stronger predictor of cardiovascular risk than BMI alone. A waist above 88cm (women) or 102cm (men) indicates high risk.
Pregnancy
BMI is not a reliable health indicator during pregnancy. Weight gain during pregnancy is expected and healthy. Pregnant women should use guidance from their midwife or GP rather than a standard BMI calculator.
Children & teenagers
Standard adult BMI thresholds do not apply to children and young people under 18. The NHS uses age- and sex-adjusted BMI centile charts for under-18s. Use a separate child BMI tool for anyone under 18.
Calculations aligned with NHS guidelines
This BMI calculator uses the standard NHS formula: BMI = weight (kg) ÷ height² (m²). Results and categories match those published by the NHS and World Health Organization (WHO) for adults aged 18 and over.
We also include waist circumference context, healthy weight ranges, and the weight change needed to reach a healthy BMI — helping you understand not just your number, but what it means in practice.
- ✓NHS-aligned BMI formula and categories
- ✓Metric and imperial unit support
- ✓Visual BMI gauge showing your position
- ✓Healthy weight range for your height
- ✓Waist circumference risk assessment
- ✓No data stored — runs entirely in your browser
NHS BMI calculator FAQs
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