How to Calculate BMI (Body Mass Index Formula)

BMI is calculated using a simple mathematical formula. There are two versions depending on whether you use metric or imperial measurements.

BMI formula (metric)

For example, if you weigh 70 kilograms and your height is 1.75 metres: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 (normal weight).

BMI formula (imperial)

For example, if you weigh 154 pounds and your height is 5 feet 9 inches (69 inches): BMI = (154 × 703) ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7 (normal weight).

BMI Chart: Categories and Health Risk Ranges

The World Health Organisation (WHO) defines the following BMI categories for adults over 18 years old. These categories apply equally to men and women of all ethnicities, though adjusted thresholds exist for certain populations.

BMI Calculator for Women

The BMI formula is the same for women and men. However, women naturally carry a higher percentage of body fat than men at the same BMI level. A woman with a BMI of 24 typically has a body fat percentage of around 30-33%, while a man with the same BMI would have approximately 20-25% body fat.

During pregnancy, BMI should be calculated using pre-pregnancy weight. The NHS recommends that women with a pre-pregnancy BMI over 30 receive additional monitoring due to increased risks of gestational diabetes and pre-eclampsia. After menopause, women tend to gain weight around the abdomen. A waist measurement over 80cm (31.5 inches) indicates increased health risk for women regardless of BMI.

BMI Calculator for Men

Men who are physically active or carry significant muscle mass may find that BMI overestimates their body fat. Muscle tissue is denser than fat, meaning a muscular man can weigh more without being unhealthy. For men, a waist circumference under 94cm (37 inches) is considered low risk. Between 94-102cm is high risk, and over 102cm (40 inches) is very high risk for weight-related conditions regardless of BMI.

Research shows men are more likely to carry visceral fat (around organs) rather than subcutaneous fat (under skin). Visceral fat carries greater health risks, which is why waist measurement matters more for men than BMI alone suggests.

BMI by Age: Does BMI Change as You Get Older?

For adults aged 18 and over, the standard BMI categories remain the same regardless of age. However, research suggests optimal BMI may shift slightly with age:

• Adults 18-65: A BMI of 18.5-24.9 is considered healthy by WHO standards.
• Adults over 65: Some studies suggest a BMI of 23-27 is associated with the lowest mortality risk in older adults. Being slightly above the "normal" range may provide a protective buffer against illness and age-related muscle loss.
• Children and teenagers (2-17): BMI is interpreted using age-specific and sex-specific percentile charts. A child in the 85th-95th percentile is considered overweight, and above the 95th percentile is obese.

BMI for Different Ethnic Groups

BMI categories were developed primarily using data from white European populations. Health risks can begin at different BMI thresholds for people of different ethnic backgrounds:

• South Asian, Chinese, and Japanese heritage: Health risks may increase at a BMI of 23 rather than 25. The WHO recommends considering overweight at BMI 23+ and obese at BMI 27.5+ for these populations.
• Black African and Caribbean heritage: Some studies suggest health risks may not increase until a slightly higher BMI than in white European populations.
• Middle Eastern heritage: Similar risk profile to South Asian populations, with increased risk at lower BMI thresholds.

Limitations of BMI

BMI is a useful screening tool but has important limitations:

• Does not measure body fat directly — uses weight as a proxy, cannot distinguish between muscle, bone, water, and fat.
• Overestimates fat in muscular individuals — athletes and bodybuilders may be classified as overweight despite being healthy.
• Underestimates fat in older adults — people who have lost muscle with age may have normal BMI but excess body fat.
• Does not account for fat distribution — abdominal fat is more dangerous than fat on hips and thighs.
• Not suitable for pregnant women without additional clinical context.

How to Improve Your BMI

If your BMI is too high

A sustainable calorie deficit of 500-750 calories per day typically produces 0.5-1kg of weight loss per week. Focus on whole foods: vegetables, fruits, lean proteins, whole grains, healthy fats. Aim for 150 minutes of moderate aerobic activity weekly plus two strength training sessions. Prioritise 7-9 hours of sleep — poor sleep disrupts hunger hormones. Even a modest 5-10% weight loss significantly improves blood pressure, cholesterol, and blood sugar levels.

If your BMI is too low

Eat more frequently — 5-6 smaller meals per day. Choose nutrient-dense foods: nuts, avocados, olive oil, whole-milk dairy, dried fruits, lean meats. Include strength training to build muscle. Consult your GP to check for thyroid disorders, coeliac disease, or other underlying conditions.

If your BMI is normal

Maintain with a balanced diet, regular exercise (150 minutes/week moderate activity), stress management, adequate sleep, and regular health check-ups.

Other Ways to Measure Body Composition

• Waist circumference: Over 94cm (men) or 80cm (women) indicates increased risk.
• Waist-to-hip ratio: Above 0.90 (men) or 0.85 (women) indicates increased cardiovascular risk.
• Body fat percentage: Healthy ranges typically 10-20% (men) and 18-28% (women). Measured via callipers, bioelectrical impedance, or DEXA scans.
• DEXA scan: Gold standard for body composition, measuring fat, muscle, and bone separately.

Related Calculators

Frequently Asked Questions About BMI