Both unit systems
Metric and imperial with a proper feet-and-inches input rather than making you convert to decimal feet first.
BMI Calculator
Calculate body mass index from height and weight, metric or imperial.
- Input
- No upload needed — instant
- Privacy
- Nothing is uploaded
- Cost
- Free · no sign-up · no watermark
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Overview
About the BMI Calculator
Free BMI calculator with metric and imperial units, WHO category ranges, a visual scale, a healthy weight range for your height, and honest limits explained.
Body mass index is a population screening tool that has been widely repurposed as an individual health measure. It is worth understanding both what it does well and where it stops being informative.
The formula
BMI = weight (kg) ÷ height (m)²The imperial equivalent multiplies by 703:
BMI = (weight (lb) ÷ height (inches)²) × 703Squaring the height is an empirical choice made by Adolphe Quetelet in the 1830s because it happened to fit the weight-to-height relationship he observed across a population. It was never derived from physiology, and it was not intended as a measure of individual health — Quetelet called it the "average man" statistic.
The categories
Where BMI works
At population scale, BMI correlates reasonably well with body fat and, more importantly, with health outcomes. It requires no equipment beyond a scale and a tape measure, it is cheap, it is comparable across studies, and it flags risk groups effectively. For epidemiology and for a quick clinical screen, it remains genuinely useful.
Where BMI fails
It cannot distinguish muscle from fat. A strength athlete and a sedentary person of identical height and weight have the same BMI and completely different health profiles.
It ignores fat distribution. Visceral fat around the abdomen carries substantially more metabolic risk than subcutaneous fat elsewhere. Two people at BMI 27 can have very different risk, and waist circumference or waist-to-height ratio captures the difference while BMI cannot.
The height scaling is imperfect. Squaring rather than cubing systematically over-penalises tall people and under-penalises short ones. Some researchers advocate a 2.5 exponent for exactly this reason.
Cut-offs are population-specific. The international thresholds were largely derived from European-ancestry populations. South Asian, Chinese and Japanese populations show elevated cardiometabolic risk at lower BMI, and several national bodies use 23 as the overweight threshold.
It says nothing about fitness, diet, sleep, blood pressure, lipids or glucose. Those matter more, individually and collectively, than the index does.
A more useful framing
Treat BMI as one weak signal among several. Pair it with waist circumference (aim for less than half your height), waist-to-height ratio, resting heart rate, blood pressure and bloodwork. If those are good, a BMI outside the "normal" band is unlikely to be telling you anything important. If they are not, BMI is not where the useful information is either.
This is general information, not medical advice. For any decision about your health, talk to a clinician who can look at the whole picture rather than one ratio.
Step by step
How to use the BMI Calculator
Choose metric (kg and cm) or imperial (lb and ft/in).
Enter your height and weight.
Read your BMI, the WHO category and where you sit on the scale.
Check the healthy weight range for your height.
Adjust the inputs to see how the category boundary falls.
Why use it
Benefits and common use cases
What this tool is good for, and what it deliberately does not try to do.
Standard WHO categories
Uses the World Health Organization classifications, so the result matches what a clinician or an insurer would quote.
Healthy weight range, not just a number
Shows the weight range that would place you in the healthy band for your height, which is more actionable than the index alone.
States its own limitations
BMI cannot distinguish muscle from fat. The tool says so plainly rather than presenting a single number as a health verdict.
Questions
Frequently asked questions
Short, honest answers about quality, limits and privacy.
How is BMI calculated?
Weight in kilograms divided by height in metres squared: BMI = kg ÷ m². In imperial units, (pounds ÷ inches²) × 703 gives the same figure — the 703 is the unit conversion factor.
What are the BMI categories?
WHO classifications for adults: under 18.5 is underweight, 18.5 to 24.9 is healthy weight, 25 to 29.9 is overweight, and 30 and above is obesity, subdivided into class I (30–34.9), class II (35–39.9) and class III (40+).
Is BMI accurate for athletes?
No, and this is its best-known weakness. BMI measures excess weight, not excess fat. A muscular athlete at 6'2" and 230 lb registers around 29.6 — technically overweight — with a very low body fat percentage. Waist circumference, body fat measurement or a DEXA scan are far more informative.
Does BMI apply to children and older adults?
Not directly. Children and teens are assessed against age-and-sex percentile charts rather than fixed thresholds. In older adults, the healthy range is often considered slightly higher — around 23 to 30 — because some reserve is protective against illness and falls.
Are the cut-off points the same for every population?
No. Several populations, particularly South and East Asian, face elevated metabolic risk at lower BMI values. Some national guidelines use an overweight threshold of 23 rather than 25 for these groups. The WHO international cut-offs used here are a general standard, not a universal one.