BMI Calculator: Check Your Body Mass Index
Work out your BMI in seconds using metric or imperial units, see your UK weight category — including the adjusted NICE thresholds for some ethnic backgrounds — and understand what your result means.
Part of our guide to Overweight and Obesity: Causes, Health Risks and Treatment in the UK.
Key fact: BMI (body mass index) is your weight in kilograms divided by your height in metres squared. For most UK adults, 18.5–24.9 is the healthy range — but NICE recommends lower thresholds (23 for overweight, 27.5 for obesity) for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background.
Calculate your BMI
For adults aged 18 and over. Not suitable for children, pregnant women or very muscular athletes.
Please enter a valid height and weight.
What Is BMI?
Body mass index (BMI) is a measure that uses your height and weight to estimate whether you are a healthy weight. It is the standard screening tool used across the NHS and in NICE guidance because it is quick, free and reasonably accurate for most adults at a population level.
BMI does not measure body fat directly — it is a screening tool, not a diagnosis. A high BMI is a prompt for a fuller assessment of your health, which may include waist circumference, blood pressure, blood tests and a discussion of your medical history. You can read more about how weight affects health in our overweight and obesity guide.
How Is BMI Calculated?
The formula is simple: BMI = weight (kg) ÷ height (m)².
1
Square your height in metres
If you are 1.75m tall: 1.75 × 1.75 = 3.06.
2
Divide your weight by that number
If you weigh 80kg: 80 ÷ 3.06 = 26.1. That is your BMI.
3
Check your category
Compare your result against the UK categories below — using the adjusted thresholds if they apply to your background.
Using imperial units, the formula is weight in pounds multiplied by 703, divided by height in inches squared. The calculator above handles both unit systems for you.
UK BMI Categories
These are the standard NICE and NHS categories for adults:
| BMI (kg/m²) | Category | Adjusted threshold* |
|---|---|---|
| Below 18.5 | Underweight | Below 18.5 |
| 18.5–24.9 | Healthy weight | 18.5–22.9 |
| 25.0–29.9 | Overweight | 23.0–27.4 |
| 30.0–34.9 | Obesity class I | 27.5 and above |
| 35.0–39.9 | Obesity class II | — |
| 40.0 and above | Obesity class III | — |
*For adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background (NICE NG246).
Why Thresholds Differ by Ethnicity
People of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean family backgrounds tend to carry more body fat — and more of it around the abdomen — at the same BMI as people of European background. Health risks such as type 2 diabetes and cardiovascular disease therefore begin at lower BMIs.
NICE recommends using 23 rather than 25 as the overweight threshold, and 27.5 rather than 30 as the obesity threshold, for these groups. This also affects eligibility assessments for weight management support and treatment.
Limitations of BMI
BMI is a useful starting point, but it has well-recognised blind spots:
- Muscle vs fat — BMI cannot tell the difference, so muscular people may be misclassified as overweight or obese.
- Fat distribution — abdominal fat carries more health risk than fat on the hips and thighs, but BMI ignores where fat is stored.
- Age — older adults naturally lose muscle, so a “healthy” BMI can hide low muscle and higher fat.
- Pregnancy — BMI is not a valid measure during pregnancy; weight is assessed differently in antenatal care.
- Children and teenagers — under-18s are assessed with age- and sex-specific BMI centile charts, never adult thresholds.
Waist Measurements: A Useful Second Check
Because abdominal fat carries particular risk, NICE recommends measuring waist circumference alongside BMI when BMI is below 35. Measure at the midpoint between the bottom of your ribs and the top of your hips, breathing out normally.
| Group | Raised risk | High risk |
|---|---|---|
| Women | Over 80cm (31.5in) | Over 88cm (34.6in) |
| Men | Over 94cm (37in) | Over 102cm (40in) |
| South Asian men | — | Over 90cm (35.4in) |
An even simpler rule of thumb is the waist-to-height ratio: your waist should measure less than half your height. This works across all ethnic groups and body types.
What to Do About Your Result
If your BMI is below 18.5
Being underweight can indicate undernutrition or an underlying health problem. Speak to your GP, particularly if you have lost weight without trying.
If your BMI is 18.5–24.9
You are in the healthy range. Focus on maintaining it with a balanced diet, regular activity and good sleep — our guide to healthy weight management for busy adults has practical strategies.
If your BMI is 25 or above
A BMI in the overweight range is a prompt to look at trends: is your weight stable, rising, or affecting your health? Losing even 5–10% of body weight meaningfully improves blood pressure, cholesterol and blood sugar. Start with our overweight and obesity guide for the full picture of causes, risks and options.
If your BMI is 30 or above (or 27.5 with the adjusted thresholds)
This is the level at which NICE recommends considering structured support — from NHS weight management programmes to prescription treatment where clinically appropriate. Our guide to weight loss medication explains the options available in the UK, and What Is Mounjaro? covers the most effective licensed medicine in detail.
BMI is a screening tool, not a diagnosis. Whatever your result, treatment decisions should always follow a full clinical assessment of your health, history and circumstances — never a BMI number alone.
Concerned About Your BMI Result?
If your BMI is in the range where NICE recommends considering treatment, Access Doctor’s GPhC-registered pharmacist independent prescribers can assess your eligibility through a short online consultation.
View Weight Loss Treatments →Frequently Asked Questions
What is a healthy BMI in the UK?
For most adults in the UK, a healthy BMI is 18.5 to 24.9. A BMI of 25 to 29.9 is classed as overweight, and 30 or above as obesity. NICE recommends lower thresholds for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background: for these groups, 23 or above is classed as overweight and 27.5 or above as obesity, because weight-related health problems develop at lower BMIs.
How do I calculate my BMI manually?
BMI is your weight in kilograms divided by your height in metres squared. For example, if you weigh 80kg and are 1.75m tall: 1.75 × 1.75 = 3.06, and 80 ÷ 3.06 = 26.1. Using imperial units, multiply your weight in pounds by 703, then divide by your height in inches squared. Our calculator does this for you and also applies the NICE ethnic-adjusted thresholds where relevant.
Why are BMI thresholds lower for some ethnic groups?
People of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean family backgrounds tend to carry more body fat, and more abdominal fat, at the same BMI as people of European background. This means health risks such as type 2 diabetes and heart disease begin at lower BMIs. NICE therefore recommends thresholds of 23 for overweight and 27.5 for obesity in these groups, rather than 25 and 30.
Is BMI accurate if I am muscular?
BMI does not distinguish between muscle and fat, so very muscular people — such as athletes, rugby players or weight trainers — may have a BMI in the overweight or even obese range without carrying excess fat. If this applies to you, waist circumference and waist-to-height ratio are more useful measures: aim for a waist measurement less than half your height. A clinician can also assess body composition directly.
What BMI qualifies for weight loss treatment in the UK?
Under NICE guidance, prescription weight loss medication may be considered for adults with a BMI of 30 or above, or 27.5 or above with a weight-related health condition such as type 2 diabetes, high blood pressure or sleep apnoea. Lower thresholds apply for some ethnic backgrounds. Bariatric surgery is considered at a BMI of 40 or above, or 35–40 with significant comorbidities. Meeting a BMI threshold is only the starting point — a full clinical assessment is always required.
Does BMI apply to children and teenagers?
Not in the same way. Children and young people under 18 are assessed using BMI centile charts, which compare their BMI against others of the same age and sex, because healthy body composition changes throughout childhood. Adult thresholds should never be applied to children. If you are concerned about a child’s weight, speak to your GP, school nurse or health visitor. This calculator is designed for adults aged 18 and over.
References
- NICE NG246. Obesity: identification, assessment and management. 2022. nice.org.uk
- NHS. Body mass index (BMI). nhs.uk
- World Health Organization. Obesity and overweight fact sheet. who.int
- NICE. BMI: preventing ill health and premature death in black, Asian and other minority ethnic groups (PH46). nice.org.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.