Your BMI
—
—
15
18.5
25
30
40+
Underweight< 18.5
Healthy18.5 – 24.9
Overweight25 – 29.9
Obese30+
What Is BMI?
Body Mass Index (BMI) is a simple measurement that uses your height and weight to estimate whether you're a healthy weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is still used by the NHS, WHO, and doctors worldwide as a quick screening tool.
The Formula
BMI = weight (kg) ÷ height (m)²
For example, if you're 1.75m tall and weigh 70kg: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 (healthy weight).
NHS BMI Categories
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk of malnutrition, osteoporosis |
| 18.5 – 24.9 | Healthy weight | Lowest risk of weight-related health problems |
| 25 – 29.9 | Overweight | Increased risk of heart disease, type 2 diabetes |
| 30 – 34.9 | Obese (Class I) | High risk of weight-related health problems |
| 35 – 39.9 | Obese (Class II) | Very high risk |
| 40+ | Obese (Class III) | Highest risk — seek medical advice |
BMI Limitations — What It Doesn't Tell You
BMI is a useful starting point, but it has well-known limitations:
⚠️ BMI doesn't distinguish between muscle and fat. A muscular person (like a rugby player or bodybuilder) can have a BMI over 25 while being perfectly healthy. If you exercise regularly and have significant muscle mass, BMI may overestimate your body fat.
- Athletes and gym-goers: Muscle is denser than fat. A BMI of 27 on someone with 12% body fat is very different from 27 on someone with 30% body fat.
- Older adults: Muscle mass naturally decreases with age. An "healthy" BMI in an elderly person might mask higher body fat percentages.
- Ethnicity: The NHS notes that people of South Asian, Chinese, and Black African heritage may have higher health risks at lower BMI levels. Some guidelines suggest a healthy range of 18.5–23 for these groups.
- Children and teens: BMI categories are different for under-18s. Use the NHS child BMI calculator which accounts for age and sex.
- Pregnant women: BMI isn't applicable during pregnancy.
💡 Better measures of health: Consider combining BMI with waist circumference (NHS recommends under 94cm for men, under 80cm for women) and waist-to-hip ratio for a more complete picture.
Healthy Weight Range by Height
Quick reference table showing the healthy weight range (BMI 18.5–24.9) for common heights:
| Height | Healthy Weight Range | Midpoint |
|---|---|---|
| 5'0" (152cm) | 42.7 – 57.2 kg (6st 10lb – 9st 0lb) | 49.9 kg |
| 5'3" (160cm) | 47.4 – 63.5 kg (7st 7lb – 10st 0lb) | 55.4 kg |
| 5'5" (165cm) | 50.4 – 67.5 kg (7st 13lb – 10st 8lb) | 58.9 kg |
| 5'7" (170cm) | 53.5 – 71.7 kg (8st 6lb – 11st 4lb) | 62.6 kg |
| 5'9" (175cm) | 56.7 – 76.0 kg (8st 13lb – 11st 13lb) | 66.3 kg |
| 5'11" (180cm) | 59.9 – 80.3 kg (9st 6lb – 12st 8lb) | 70.1 kg |
| 6'0" (183cm) | 62.0 – 83.1 kg (9st 11lb – 13st 1lb) | 72.5 kg |
| 6'2" (188cm) | 65.4 – 87.7 kg (10st 4lb – 13st 11lb) | 76.6 kg |
| 6'4" (193cm) | 69.0 – 92.4 kg (10st 12lb – 14st 7lb) | 80.7 kg |
How to Reach a Healthy BMI
If You're Overweight (BMI 25–29.9)
- The NHS recommends losing weight gradually — 0.5 to 1kg per week
- A calorie deficit of 500–600 calories per day typically achieves this
- Focus on whole foods, reduce ultra-processed food, and increase daily movement
- The NHS Better Health programme offers free 12-week plans: nhs.uk/better-health
If You're Underweight (BMI < 18.5)
- Gradually increase calorie intake with nutrient-dense foods
- Eat more frequently — 5-6 smaller meals rather than 3 large ones
- Strength training helps build healthy muscle mass
- See your GP if you're losing weight unintentionally — it may indicate an underlying condition
When to See Your GP
- BMI over 30 (obese range)
- BMI over 25 with other risk factors (high blood pressure, diabetes in family)
- Unexplained weight loss or gain
- BMI under 18.5 with symptoms like fatigue, dizziness, or hair loss
⚠️ Medical disclaimer: This calculator is for informational purposes only. It is not a substitute for professional medical advice. Always consult your GP or a qualified healthcare professional for personalised health guidance.
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