BMI Calculator UK Guide: Understanding Your Body Mass Index

Body Mass Index (BMI) is the most widely used method for assessing whether adults are at a healthy weight. Used by the NHS and healthcare professionals worldwide, understanding your BMI can be an important first step in managing your health.

What is BMI?

BMI (Body Mass Index) is a simple calculation that uses your height and weight to work out if your weight is healthy. The concept was developed by Belgian statistician Adolphe Quetelet in the 1830s and remains the standard screening tool for weight classification today.

How to Calculate BMI

Imperial Formula (Pounds and Inches)

If you prefer imperial measurements:

NHS BMI Categories

The NHS uses the following BMI ranges to classify adult weight:

BMI Range	Classification	Health Risk
Below 18.5	Underweight	Increased risk of nutritional deficiency
18.5 - 24.9	Healthy Weight	Lowest health risk
25 - 29.9	Overweight	Increased risk of health problems
30 - 34.9	Obese Class I	High risk of health problems
35 - 39.9	Obese Class II	Very high risk
40+	Obese Class III	Extremely high risk

BMI Reference Chart

Here are BMI values for common height and weight combinations:

Height	Healthy Weight Range	BMI 25 Weight
5'0" (152 cm)	43.5 - 58.5 kg	58.5 kg
5'4" (163 cm)	49.4 - 66.7 kg	66.7 kg
5'6" (168 cm)	52.2 - 70.3 kg	70.3 kg
5'8" (173 cm)	55.3 - 74.4 kg	74.4 kg
5'10" (178 cm)	58.5 - 78.9 kg	78.9 kg
6'0" (183 cm)	61.7 - 83.5 kg	83.5 kg
6'2" (188 cm)	65.3 - 88.0 kg	88.0 kg

Limitations of BMI

While BMI is a useful screening tool, it has several important limitations:

Doesn't Distinguish Muscle from Fat

Muscle weighs more than fat. Athletes and bodybuilders may have a high BMI (technically "overweight" or "obese") while having very low body fat. For example, many professional rugby players have BMIs over 30 but are extremely fit.

Doesn't Account for Fat Distribution

Where you carry fat matters for health. Abdominal fat (visceral fat) is more dangerous than fat on hips and thighs. Two people with identical BMIs may have very different health risks based on fat distribution.

Varies by Ethnicity

Research shows that health risks associated with BMI differ by ethnicity:

• South Asian, Chinese, African-Caribbean: May have higher health risks at lower BMIs. The NHS suggests using 23 as the overweight threshold and 27.5 for obesity.
• Black African backgrounds: May have lower health risks at higher BMIs compared to White Europeans.

Age and Gender Differences

BMI doesn't account for:

• Older adults typically have more body fat than younger adults at the same BMI
• Women generally have more body fat than men at the same BMI
• Muscle mass naturally decreases with age

BMI for Different Groups

Children and Teenagers

BMI is calculated the same way for children, but the results are interpreted differently using age and sex-specific percentile charts. The NHS doesn't use adult BMI categories for under-18s.

Pregnant Women

BMI is not appropriate during pregnancy. Pre-pregnancy BMI is used to assess healthy weight gain during pregnancy.

Elderly Adults

For adults over 65, a slightly higher BMI (up to 27) may actually be associated with lower mortality risk. Consult your GP for personalised advice.

Health Risks by BMI Category

Underweight (BMI Below 18.5)

Being underweight can indicate:

• Nutritional deficiencies
• Weakened immune system
• Osteoporosis risk
• Fertility problems
• Underlying health conditions

Overweight and Obese (BMI 25+)

Carrying excess weight increases risk of:

• Type 2 diabetes
• Heart disease and stroke
• High blood pressure
• Some cancers
• Sleep apnoea
• Osteoarthritis
• Fatty liver disease
• Mental health conditions

Better Measures Than BMI Alone

For a more complete picture of health, consider these additional measures:

Waist Circumference

Measure around your waist at the narrowest point, usually just above the belly button:

• Men: Below 94 cm (37") is low risk; 94-102 cm is high risk; above 102 cm (40") is very high risk
• Women: Below 80 cm (31.5") is low risk; 80-88 cm is high risk; above 88 cm (34.5") is very high risk

Waist-to-Height Ratio

Your waist should be less than half your height. This is often a better predictor of health risks than BMI alone.

Body Fat Percentage

Measured using specialised scales, DEXA scans, or skinfold callipers. Healthy ranges are approximately:

• Men: 10-20% body fat
• Women: 18-28% body fat

What to Do With Your BMI Result

If You're Underweight

Speak to your GP to rule out underlying health conditions. A registered dietitian can help create a healthy weight gain plan focusing on nutritious, calorie-dense foods.

If You're in the Healthy Range

Maintain your weight through balanced eating and regular physical activity. Continue monitoring your weight and waist circumference.

If You're Overweight or Obese

The NHS recommends:

• Aim to lose weight gradually (0.5-1 kg per week)
• Reduce calorie intake sensibly
• Increase physical activity (aim for 150 minutes moderate activity per week)
• Consider NHS weight management programmes
• Speak to your GP about support options

Using Our BMI Calculator

Our free BMI calculator makes it easy to check your BMI. Simply enter your height and weight in either metric or imperial units, and get instant results with your NHS weight classification.

Conclusion

BMI is a useful starting point for understanding whether you're at a healthy weight, but it's not the complete picture. It works well as a screening tool for the general population but has limitations for certain groups.

For the most accurate assessment of your health, combine BMI with waist measurements and consider consulting a healthcare professional who can evaluate your individual circumstances.

Remember: small, sustainable changes to diet and activity levels can make a significant difference to your health, regardless of your current BMI.