⚖️ BMI Calculator

What Is BMI?

Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has since become the most widely used screening tool for assessing whether an adult has a healthy body weight. BMI is not a direct measure of body fat, but it correlates well enough with body fat that health organisations worldwide — including the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) — use it as a population-level screening tool.

The value obtained tells you whether you fall into one of several broad categories: underweight, normal weight, overweight, or obese. These categories are associated with different levels of health risk, making BMI a useful first indicator that further assessment may be warranted.

How Is BMI Calculated?

BMI is calculated by dividing your weight in kilograms by the square of your height in metres. In the imperial system, a correction factor of 703 is applied.

For example, a person who is 175 cm tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which falls in the Normal Weight range.

BMI Classification (WHO)

The World Health Organization uses the following categories for adults aged 20 and older. These thresholds apply globally, though some Asian-Pacific health authorities use lower cutoffs (overweight at 23, obese at 27.5) due to differences in body composition across ethnicities.

BMI for Children and Teens

The standard BMI formula applies to adults 20 and over. For children and teenagers aged 2–19, BMI is calculated the same way but interpreted differently — using age- and sex-specific percentile charts rather than fixed cutoffs. This is because body composition changes significantly as children grow, and boys and girls develop differently.

Health Risks of Being Overweight

Carrying excess body weight increases the risk of a wide range of serious health conditions. The risk rises progressively with BMI above 25, and is substantially elevated at BMI 30 and above. According to the CDC, the conditions associated with excess weight include:

• High blood pressure (hypertension)
• High LDL cholesterol, low HDL cholesterol, and high triglycerides
• Type 2 diabetes
• Coronary heart disease and stroke
• Gallbladder disease
• Osteoarthritis — joint breakdown from excess load
• Sleep apnoea and breathing problems
• Certain cancers: endometrial, breast, colon, kidney, gallbladder, liver
• Mental health issues including depression and anxiety
• Reduced quality of life and physical function
• Higher all-cause mortality compared to those with a healthy BMI

Health Risks of Being Underweight

Being underweight (BMI below 18.5) carries its own serious risks, which are often underappreciated. These include:

• Malnutrition and vitamin deficiencies
• Anaemia (reduced ability to carry oxygen in the blood)
• Osteoporosis — weakened bones at increased fracture risk
• Weakened immune system and greater vulnerability to illness
• Fertility issues in women due to hormonal disruption
• Delayed growth and development in children and adolescents
• Higher risk of complications from surgery
• Elevated all-cause mortality, particularly in older adults

Limitations of BMI

BMI is a useful population-level screening tool, but it has well-documented limitations when applied to individuals. The American Medical Association (AMA) issued guidance in 2023 recommending that BMI not be used as the sole diagnostic criterion for obesity.

BMI cannot distinguish muscle from fat

A muscular athlete may have a BMI in the "Overweight" or "Obese" range despite having very low body fat. Conversely, an inactive person with a "Normal" BMI may carry dangerous levels of visceral fat — fat stored around internal organs — without it being detected by BMI.

BMI does not account for age, sex, or ethnicity

Older adults tend to have more body fat than younger people at the same BMI. Women naturally carry more fat than men at equivalent BMI values. People of Asian descent tend to have higher body fat percentages at the same BMI compared to people of European descent, which is why many Asian health authorities use lower BMI cutoffs.

BMI does not reflect fat distribution

Where fat is stored matters as much as how much fat you carry. Abdominal (visceral) fat is strongly associated with metabolic disease, even in people with a normal BMI. Waist circumference — ideally below 80 cm for women and 94 cm for men — is a useful complementary measure.

BMI Prime and Ponderal Index

BMI Prime

BMI Prime is the ratio of your BMI to the upper limit of normal (25 kg/m²). A BMI Prime below 1.0 means you are within the normal range; above 1.0 means overweight; above 1.2 means obese. It allows quick comparison across populations with different BMI thresholds.

Ponderal Index

The Ponderal Index (PI) is similar to BMI but uses the cube of height rather than the square, making it more reliable for very tall or very short individuals — who are disproportionately misclassified by BMI.

How to Improve Your BMI

If your BMI is outside the healthy range, small, consistent changes are more sustainable than dramatic ones. Evidence-based approaches include:

• Caloric deficit for weight loss: A deficit of 500 kcal/day typically produces roughly 0.5 kg of fat loss per week. Use our Calorie Calculator to find your baseline.
• Strength training: Builds muscle mass, which increases resting metabolic rate — meaning you burn more calories even at rest.
• Protein intake: Higher protein diets (1.6–2.2 g/kg body weight) preserve muscle during weight loss and reduce hunger.
• Sleep: Sleeping fewer than 7 hours per night consistently is associated with weight gain and impaired appetite regulation.
• Reduce ultra-processed foods: These are engineered to be hyper-palatable and are strongly associated with overconsumption.

Frequently Asked Questions