BMI Calculator
Enter your height and weight to get your BMI with WHO categories and healthy range guidance. Supports metric and imperial inputs.
Calculate Your BMI
Enter your details to calculate your BMI
Check your BMI in ten seconds β no sign-up, no data stored
Your doctor mentioned BMI at your last check-up but you forgot what the number means. A friend posted their BMI on social media and you're curious where you land. Your life insurance form asks for your BMI category and you don't have it handy. Enter your weight and height β either metric or imperial β and this calculator gives you your BMI number and its WHO category (underweight, normal, overweight, or obese) in a colour-coded label. The whole thing takes about ten seconds.
Toggle between metric (kilograms and centimetres) and imperial (pounds, feet, and inches) using the tabs at the top. Enter your numbers and click Calculate BMI. The result appears immediately: your BMI value rounded to one decimal place, and a colour-coded badge showing your category. Blue for underweight, green for healthy weight, amber for overweight, red for obese.
Who uses this calculator
Life insurance applications, workplace health assessments, and occupational health questionnaires frequently ask for BMI. If you don't have a recent measurement from a clinic, this calculator gives you the number in seconds. A 75kg person who is 170cm tall has a BMI of 25.95 β just into the overweight category β and knowing that before a form avoids guessing.
If your doctor has asked you to track your weight between visits, calculating BMI at home lets you monitor the trend. A 5kg weight loss for someone at 175cm shifts BMI from 27.1 (overweight) to 25.5 (still overweight, but moving in the right direction). The colour-coded category makes it easy to see progress without interpreting raw numbers.
Many parents calculate their own BMI while researching their child's health. This calculator is designed for adults aged 20+, and knowing your own BMI first helps you understand the context before looking at age-specific percentile charts for children.
If you're building muscle through strength training, your BMI may increase even as your body composition improves. Tracking BMI alongside waist circumference and body fat percentage gives a fuller picture. A regular check every 4-6 weeks during a training block reveals whether the trend is heading in the right direction.
How BMI is calculated β and where the number comes from
The two formulas behind your result
What happens when you enter your numbers
When you type in your weight and height and hit Calculate, the calculator converts your measurements into the standard units the formula expects. If you're using metric, your height in centimetres is converted to metres by dividing by 100, then squared, then your weight is divided by that squared value. If you're using imperial, your feet are converted to inches (multiplied by 12), added to any extra inches, then your weight in pounds is divided by the square of total inches and multiplied by 703 β a constant that makes imperial inputs equivalent to metric results.
The number that comes out is rounded to one decimal place and matched against four WHO thresholds: under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obese. Each category gets its own colour so you can see at a glance where you land. No health data leaves your device β the calculation runs locally in your browser and nothing is stored or sent anywhere.
Worked example: 82 kg, 175 cm
A 175cm person weighing 82kg: height in metres is 1.75, squared is 3.0625, weight divided by that gives 26.8 β just into the overweight category. That person would need to weigh under 76.2kg (a loss of about 5.8kg) to reach the healthy weight range. At 76kg, their BMI would be 24.8 β green.
Worked example: 11 stone 4 lbs, 5 ft 7 in
Converting: 11 stone 4 lbs equals 158 lbs, and 5 ft 7 in equals 67 inches. Squaring 67 gives 4,489. Dividing 158 by 4,489 and multiplying by 703 gives 24.7 β healthy weight, just under the 25 threshold. If that person gained half a stone (7 lbs) to 165 lbs, the BMI would rise to 25.8, crossing into overweight.
Why BMI matters β and when it doesn't
BMI is a quick screening tool, not a diagnosis. It correlates with chronic disease risk at a population level β people in the overweight and obese categories statistically face higher rates of type 2 diabetes, heart disease, and certain cancers. But it has real blind spots: a muscular 85kg rugby player and a sedentary 85kg office worker at the same height produce the same BMI, despite having very different body compositions. BMI also doesn't account for where fat is distributed β abdominal fat is more metabolically harmful than fat on the hips or thighs. For a complete picture, pair BMI with waist circumference and, if available, body fat percentage.
What the calculator does with edge cases
Entering zero or leaving fields blank produces an error message instead of an invalid result. Negative numbers are rejected. Extremely large values β someone weighing 400kg or standing 220cm tall β are accepted without capping, because legitimate measurements do exist at those extremes. The result is computed to full decimal accuracy before rounding to one decimal place for display.
Frequently Asked Questions
What is a healthy BMI?
The WHO classifies a BMI between 18.5 and 24.9 as a healthy weight range for most adults. Under 18.5 is underweight, 25β29.9 is overweight, and 30 or above is obese. These thresholds are based on population-level studies linking BMI to chronic disease risk. They apply to adults aged 20 and over β children and adolescents use different, age-specific percentile charts.
Is BMI accurate for athletes?
BMI can overestimate body fat in athletes and muscular people because it doesn't distinguish between muscle and fat. Muscle tissue is roughly 18% denser than fat, so a fit person with significant lean mass may register as overweight despite having a low body fat percentage. For athletes, BMI works best when paired with other measurements like waist circumference, skinfold thickness, or DEXA scans.
Can I use this calculator for children?
No β this calculator is designed for adults aged 20 and older. For children and adolescents aged 2β19, BMI is interpreted using age- and sex-specific percentile charts published by the CDC. A child's BMI percentile, not the raw number, determines their weight category. A 12-year-old boy with a BMI of 21 might be in the 75th percentile (normal), while an adult with the same BMI would also be normal β but the interpretation process is entirely different. Consult your paediatrician or use the CDC's BMI-for-age growth charts.
How often should I check my BMI?
For most adults, checking every few months or whenever you notice significant weight changes is sufficient. BMI is a single snapshot β it doesn't tell you about body composition, fat distribution, or metabolic health. If you're actively managing your weight, tracking BMI every 4β6 weeks alongside waist circumference gives you a useful trend without obsessing over daily fluctuations.
Why does the imperial formula multiply by 703?
The factor 703 converts pounds-and-inches measurements into the same BMI scale used by the metric formula. It's derived from the mathematical relationship between kilograms and pounds (1 kg β 2.2046 lbs) and centimetres and metres (100 cm = 1 m). Without this conversion, the same person would get different BMI values depending on which units they used, which would make the WHO thresholds meaningless.
What is the difference between BMI and body fat percentage?
BMI is a ratio of weight to height β it doesn't measure fat directly. Body fat percentage tells you what proportion of your total weight is fat. Two people with the same BMI can have very different body compositions: one might carry excess fat while the other has high muscle mass. BMI is faster and cheaper to calculate, but body fat percentage is a more precise indicator of metabolic health.
How does BMI compare to waist circumference as a health indicator?
Waist circumference measures abdominal fat, which is more metabolically dangerous than fat stored on the hips or thighs. A person with a normal BMI but a large waist (over 94cm for men, 80cm for women) may still face elevated health risks β a condition sometimes called 'normal weight obesity.' The NHS recommends checking both BMI and waist circumference together for a more complete picture. A tape measure around your navel gives you the waist number in seconds.
Does BMI change as I get older?
BMI thresholds don't change with age for adults, but body composition does. After about age 30, most people lose muscle mass gradually and gain fat, even if their weight stays the same. This means two 70kg people aged 25 and 65 might have the same BMI, but the older person likely has a higher body fat percentage. For people over 65, BMI is less reliable as a standalone measure β pairing it with waist circumference and functional strength assessments gives better insight.