BMI Calculator

Calculate your Body Mass Index (BMI) using metric or imperial units. Instant results — no data stored.

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Reference

BMI Categories

Standard classification by the World Health Organization (WHO)

Underweight
Below 18.5

May indicate nutritional deficiency or other health concerns. Consider speaking with a healthcare professional.

Normal Weight
18.5 – 24.9

Associated with the lowest health risk. Maintain a balanced diet and regular physical activity.

Overweight
25 – 29.9

Slightly elevated risk for certain conditions. Lifestyle changes like diet and exercise can help.

Obese
30 and above

Higher risk for cardiovascular disease, diabetes, and other conditions. Medical guidance is recommended.

Disclaimer: BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, age, or sex differences. Always consult a qualified healthcare professional for medical advice.

FAQ

Common Questions

How is BMI calculated?
BMI is calculated by dividing your weight (in kilograms) by the square of your height (in metres): BMI = kg / m². In imperial units: BMI = (lbs / inches²) × 703. This calculator handles both formulas automatically based on your selected unit system.
Is BMI accurate for everyone?
BMI is a useful population-level screening tool but has limitations for individuals. It can overestimate body fat in athletes with high muscle mass and underestimate it in older adults who have lost muscle. It also doesn't account for fat distribution, which is a key risk factor.
Is my data saved or shared?
No. All calculations happen entirely in your browser using JavaScript. Your height and weight values are never sent to any server or stored anywhere.
What is a healthy BMI for women vs. men?
The WHO uses the same BMI ranges (18.5–24.9 for normal weight) for both men and women. However, women naturally carry more body fat than men at the same BMI — around 10% more on average. Some researchers argue for sex-specific cut-offs, but standard medical practice still applies the same ranges to both. For a fuller picture, combining BMI with waist circumference gives a better sense of health risk.
Can BMI be used for children?
Yes, but with important differences. For children and teenagers (ages 2–19), BMI is interpreted using age- and sex-specific growth charts rather than fixed thresholds. Paediatricians use "BMI-for-age percentiles" — for example, a child is considered overweight if their BMI falls between the 85th and 95th percentile for their age and sex. This calculator is designed for adults (18+); consult a paediatrician for children's weight assessment.
Why does BMI underestimate body fat in older adults?
As people age, they tend to lose muscle mass (sarcopenia) and bone density while gaining fat — a process that often leaves body weight unchanged. This means an older adult can have a "normal" BMI while carrying a higher-than-expected percentage of body fat. For this reason, clinicians treating older patients often use additional metrics such as waist circumference, lean mass measurements, or grip strength alongside BMI.
Does ethnicity affect BMI interpretation?
Yes. Research shows that people of South Asian, East Asian, and some other ethnic backgrounds tend to develop metabolic health risks (such as type 2 diabetes and cardiovascular disease) at lower BMI values than the standard cut-offs suggest. The WHO now recommends that Asian populations be assessed for risk at a BMI of 23 (overweight) and 27.5 (obese). If you are of Asian descent, discuss ethnicity-adjusted thresholds with your doctor.
What steps can I take to move into a healthy BMI range?
If your BMI is in the overweight or obese range, sustainable lifestyle changes are more effective than crash diets. Evidence-backed approaches include: reducing ultra-processed food intake, increasing fibre and protein at meals, accumulating at least 150 minutes of moderate aerobic activity per week (such as brisk walking), adding two sessions of resistance training to preserve muscle, and prioritising sleep (poor sleep raises hunger hormones). A registered dietitian or your GP can create a plan tailored to your health history.
Background

What does BMI actually measure?

Body Mass Index is a simple numerical index derived from a person's weight and height. It was developed in the 19th century by Belgian mathematician Adolphe Quetelet — who called it the "Quetelet Index" — as a way to describe population-level weight distributions, not to assess individual health. The term "Body Mass Index" was coined by physiologist Ancel Keys in a 1972 study, and the World Health Organization (WHO) adopted it as a global screening tool in the 1990s.

The formula is straightforward: BMI = weight (kg) ÷ height² (m²). For imperial measurements, the equivalent is BMI = (weight in lbs ÷ height in inches²) × 703. The result is a single number that places you in one of four WHO categories — Underweight, Normal weight, Overweight, or Obese — each linked to statistical health risks at the population level.

Because BMI uses only weight and height, it is a proxy for body fatness rather than a direct measurement. It is best understood as a starting-point screening tool: an elevated BMI signals that further assessment may be worthwhile, not that a person is definitely unhealthy.

Special Populations

How BMI applies differently across groups

Athletes & High Muscle Mass

Muscle is denser than fat, so athletes and strength-trained individuals often have a high BMI that wrongly categorises them as overweight or obese. A rugby player weighing 100 kg at 180 cm has a BMI of 30.9 (obese) yet may carry very little body fat. For this group, body fat percentage — measured by DEXA scan, underwater weighing, or skinfold callipers — is a far more relevant metric.

Older Adults (60+)

Ageing typically involves a loss of muscle and bone density, which is not reflected in body weight. An older adult can maintain a normal BMI while carrying a higher proportion of body fat — a condition called "normal weight obesity." Some research suggests a BMI of 22–27 may be optimal for adults over 65, with lower BMIs associated with frailty and higher mortality risk in this age group.

Children & Teenagers

Standard adult BMI cut-offs do not apply to people under 18. Instead, paediatricians use age- and sex-specific growth percentile charts. A child above the 95th BMI percentile for their age and sex is considered obese; between the 85th and 95th is overweight. This calculator is designed for adults only — always consult a paediatric healthcare professional for children.

Ethnic Background

Research shows that people of South and East Asian descent develop cardiometabolic health risks at lower BMI values than the standard WHO thresholds. The WHO now recommends lower action points for Asian populations: overweight at BMI ≥ 23, obese at ≥ 27.5. If you are of Asian heritage, discuss these adjusted thresholds with your doctor when interpreting your result.

Going Further

BMI vs. other health measurements

BMI is a useful first filter, but it works best alongside complementary measurements. Here is how the most common ones compare:

Waist Circumference

Measures abdominal fat directly. High waist circumference (above 94 cm / 37 in for men; above 80 cm / 31.5 in for women) is independently associated with increased risk of type 2 diabetes, heart disease, and stroke — even in people with a normal BMI. This is one of the most practical additional checks you can do at home.

Body Fat Percentage

Directly measures the proportion of your body weight that is fat. Healthy ranges are roughly 10–20% for men and 18–28% for women, though these vary by age. Methods include DEXA scanning (most accurate), bioelectrical impedance scales (widely available but variable accuracy), and hydrostatic weighing. Body fat percentage sidesteps BMI's muscle-mass blind spot entirely.

Waist-to-Hip Ratio (WHR)

Divides waist circumference by hip circumference to indicate fat distribution. A "apple-shaped" pattern (fat stored centrally around the abdomen) carries higher risk than a "pear-shaped" pattern (fat stored around the hips and thighs). The WHO considers a WHR above 0.90 for men and above 0.85 for women as high risk. Some studies show WHR predicts cardiovascular risk better than BMI alone.

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