How to Calculate Your BMI: Formula, Categories, and What It Means

What Is BMI?

Body Mass Index (BMI) is a numerical value calculated from your height and weight. It is used as a screening tool to identify whether a person is underweight, at a healthy weight, overweight, or obese.

BMI was developed in the 1830s by Belgian mathematician Adolphe Quetelet and was adopted by the World Health Organisation as a standard measurement in the 1980s. Despite being nearly 200 years old, it remains the most widely used body weight screening tool in medicine.

Use our BMI Calculator to calculate your BMI instantly.

The BMI Formula

BMI is calculated differently depending on whether you use metric or imperial measurements.

Metric formula: BMI = weight (kg) ÷ height (m)²

Example: A person weighing 75kg at 170cm: BMI = 75 ÷ (1.70)² = 75 ÷ 2.89 = 25.95

Imperial formula: BMI = (weight (lbs) × 703) ÷ height (inches)²

Example: A person weighing 165lbs at 5 feet 7 inches (67 inches): BMI = (165 × 703) ÷ (67)² = 116,000 ÷ 4,489 = 25.84

BMI Categories

The World Health Organisation defines the following BMI categories for adults:

• Under 18.5: Underweight
• 18.5–24.9: Normal weight
• 25.0–29.9: Overweight
• 30.0–34.9: Obese Class I
• 35.0–39.9: Obese Class II
• 40.0 and above: Obese Class III (severe)

These categories apply to adults aged 18 and over. BMI is calculated differently for children and teenagers using age and sex-specific percentile charts.

What Your BMI Number Means

Underweight (BMI below 18.5) Being underweight can indicate malnutrition, an eating disorder, or an underlying health condition. It is associated with weakened immune function, bone density loss, fertility problems, and increased risk of complications during illness or surgery. If you are underweight, speak to your doctor.

Normal weight (BMI 18.5–24.9) This range is associated with the lowest risk of weight-related health problems for most adults. However, being in this range does not guarantee good health — body composition, diet, fitness, and other factors matter enormously.

Overweight (BMI 25–29.9) Overweight is associated with increased risk of type 2 diabetes, cardiovascular disease, high blood pressure, and joint problems. However, the risk increase at the lower end of this range (25–27) is relatively modest for most people.

Obese (BMI 30 and above) Obesity is associated with significantly elevated risk of type 2 diabetes, heart disease, stroke, certain cancers, sleep apnoea, and joint conditions. Risk increases with BMI level — Class III obesity (BMI 40+) carries the highest risk.

Ideal Weight by Height

Based on the normal BMI range (18.5–24.9), here are the healthy weight ranges by height:

• 5'0" (152cm): 48–61 kg (105–135 lbs)
• 5'2" (157cm): 51–65 kg (113–143 lbs)
• 5'4" (163cm): 54–70 kg (119–154 lbs)
• 5'6" (168cm): 57–74 kg (126–163 lbs)
• 5'8" (173cm): 61–79 kg (134–173 lbs)
• 5'10" (178cm): 64–83 kg (141–183 lbs)
• 6'0" (183cm): 68–88 kg (149–194 lbs)
• 6'2" (188cm): 71–93 kg (157–204 lbs)

For a personalised ideal weight calculation, use our Ideal Weight Calculator which uses multiple formulas including Robinson, Miller, and Devine.

BMI for Different Populations

Asian populations Research shows that people of Asian descent face higher metabolic health risks at lower BMI values than the standard categories suggest. Several health organisations recommend lower thresholds for Asian adults:

• Overweight: BMI 23 (vs 25 in standard categories)
• Obese: BMI 27.5 (vs 30 in standard categories)

Older adults For adults over 65, some research suggests a slightly higher BMI (25–27) may actually be protective. Older adults with slightly higher BMI may have greater nutritional reserves and better outcomes during illness than those at the lower end of the normal range.

Athletes and muscular individuals Muscle is significantly denser than fat. A person with very high muscle mass can have a BMI in the overweight or obese range while having low body fat and excellent health. BMI is a poor tool for screening athletic populations.

Calculating BMI for Children

BMI is interpreted differently for children and teens aged 2–19. Rather than fixed categories, children are compared to others of the same age and sex using percentile charts:

• Below 5th percentile: Underweight
• 5th to 85th percentile: Healthy weight
• 85th to 95th percentile: Overweight
• Above 95th percentile: Obese

This percentile approach accounts for the fact that healthy body fat levels change throughout childhood and differ between boys and girls.

BMI Limitations You Should Know

Does not measure body fat directly BMI is a proxy for body fat, not a measurement of it. Two people with identical BMIs can have very different body compositions — one heavily muscled, one with high body fat.

Does not indicate fat distribution Abdominal fat (visceral fat around the organs) is far more metabolically dangerous than fat stored elsewhere. Two people with identical BMIs can have very different health risks depending on where they carry their weight.

Does not account for age-related muscle loss As people age they lose muscle mass (sarcopenia). An older person can have a normal BMI while having unhealthy levels of fat relative to lean tissue — a condition sometimes called sarcopenic obesity.

Same thresholds for men and women Women naturally carry more body fat than men. A woman and a man with identical BMIs have different body compositions — the woman will typically have a higher body fat percentage.

Better Measures to Use Alongside BMI

Waist circumference A waist measurement above 94cm (37 inches) for men or 80cm (31.5 inches) for women indicates excess abdominal fat and elevated metabolic risk — regardless of BMI.

Waist-to-height ratio Divide your waist measurement by your height. A ratio below 0.5 is associated with lower health risk. This simple measure outperforms BMI in predicting metabolic disease risk in many studies.

Body fat percentage Measures the actual proportion of fat in your body. Use our Body Fat Calculator to estimate yours.

Resting metabolic rate and fitness level Cardiovascular fitness is one of the strongest predictors of long-term health outcomes — stronger than BMI. A person with a BMI of 27 who is fit and active is at lower risk than a person with a BMI of 23 who is sedentary.

How to Use BMI Effectively

BMI is best used as one data point among several rather than a definitive health verdict. Here is how to use it well:

• Use it as a starting point for conversations with your doctor rather than a conclusion
• Combine it with waist circumference and body fat percentage for a fuller picture
• Track it over time — trends matter more than any single measurement
• Adjust interpretation based on your ethnicity, age, and muscle mass
• Focus on health behaviours (exercise, diet, sleep, stress) rather than hitting a specific BMI number

Frequently Asked Questions

• Is BMI accurate for everyone?

BMI is reasonably accurate for sedentary adults of average build at a population level. It is less accurate for athletes, older adults, pregnant women, people with very high or very low muscle mass, and people of Asian descent.

• What BMI is considered obese?

A BMI of 30 or above is classified as obese by the WHO. Class I obesity is 30–34.9, Class II is 35–39.9, and Class III (severe) is 40 and above.

• Can I have a high BMI and still be healthy?

Yes, particularly if high BMI is due to muscle mass rather than fat. However, if high BMI is due to excess body fat — particularly abdominal fat — health risks are elevated regardless of how you feel currently.

• What is a good BMI for my age?

The standard BMI categories (18.5–24.9 for normal weight) apply to adults of all ages, though some research suggests slightly higher BMIs (25–27) may be acceptable for older adults. For children, BMI is assessed using age-specific percentile charts.

• How often should I check my BMI?

For most adults, checking BMI every 3–6 months is sufficient if you are monitoring weight. More frequent tracking adds little value and can increase anxiety around minor fluctuations.