BMI Calculator

What is BMI Calculator?

Body mass index (BMI) is a screening number calculated as weight in kilograms divided by height in metres squared.

How to use

1. Choose Metric or Imperial units.
2. Enter height and weight.
3. Read the BMI value and screening category.

About BMI Calculator

Body mass index, or BMI, is a screening measure based on height and weight. This calculator supports metric centimetres and kilograms as well as imperial feet, inches, and pounds. It converts imperial values using exact unit factors, calculates weight in kilograms divided by height in metres squared, and displays the result to one decimal place. The category is based on that displayed value so boundary results are easy to understand.

For adults, the screening ranges used here are under 18.5 for Underweight, 18.5 through 24.9 for Normal, 25.0 through 29.9 for Overweight, and 30.0 or above for Obese. These cutoffs match the WHO's adult definitions: overweight is a BMI of 25 or more, and obesity is a BMI of 30 or more. These categories are commonly used adult ranges, but BMI cannot distinguish muscle, bone, fat distribution, age, pregnancy, or many health conditions. It is not a diagnosis. Children and teenagers need age- and sex-specific charts, so this page is intended for adults aged 18 and over.

Switching between Metric and Imperial clears the input fields so a value from one unit system is never silently interpreted as another. Height is checked against a practical range of 50 to 250 centimetres after conversion, and weight is checked from 10 to 500 kilograms. Empty fields show a placeholder instead of an error. Values outside the range receive an inline message that explains which field needs attention.

Use the result as a prompt for a broader conversation with a qualified health professional, especially if you have a medical condition or a rapid change in weight. The calculator makes no diagnosis and stores nothing. It performs the conversion locally in your browser and does not send height or weight to a server. A clear, labelled form and keyboard-friendly unit control make it usable on mobile screens as well as desktop displays.

For consistent results, measure height without shoes and use the same unit system for the entire entry. Rounding is applied to the displayed BMI before the category is selected, which makes the boundary behavior visible. A value of 24.95, for example, is displayed to one decimal place and classified from that displayed number. This is a deliberate presentation rule, not a claim about clinical precision. If you need a health assessment, share the underlying measurements and your context with a qualified professional rather than relying on a single screening number.

BMI can be useful for a population-level screening conversation, but a single number does not describe fitness, nutrition, or health on its own. Record the units beside any value you share, because a pound and a kilogram are not interchangeable. The page keeps the conversion factors visible in its implementation and gives an error rather than producing a misleading result for an impossible height or weight.

Two quick examples show the arithmetic. A person weighing 70 kg at 1.75 m has a BMI of 70 divided by (1.75 times 1.75), which is 22.9 — inside the normal range. A person weighing 85 kg at 1.70 m has a BMI of 85 divided by (1.70 times 1.70), which is 29.4 — in the overweight range, near the obesity cutoff. Small changes move the number more than people expect: at 1.70 m, each kilogram of weight shifts BMI by about 0.35. Try the calculator with your own numbers rather than trusting a remembered figure.

BMI's biggest blind spot is composition. A strength athlete with heavy muscle can score above 25 while carrying little body fat; an older adult can score normal while having lost significant muscle. The formula also ignores where fat sits — abdominal fat is more closely linked to health risk than fat elsewhere — and it says nothing about fitness, diet quality, sleep, or blood markers. Two people with the same BMI of 27 can have very different health profiles. Treat the number as one input among many, not a verdict.

Cutoffs are debated for Asian populations. The WHO has noted that health risks in Asian populations tend to rise at lower BMI values than in European populations, and some countries publish lower action thresholds. There is no single agreed alternative scale, so this page keeps the standard WHO adult cutoffs — 25 for overweight, 30 for obesity — for consistency and labels the result a screening number. If you are of Asian descent, mention it when discussing the result with a health professional rather than reinterpreting the category yourself.

Two cheap companions make BMI more informative. Waist circumference reflects the abdominal fat that BMI misses; a tape measure around the waist at navel level takes seconds. Body-fat percentage, estimated by calipers, bioimpedance scales, or a DEXA scan, separates fat from lean mass directly. None of these replaces clinical judgement either. The practical move is to track the same measurements the same way over time — trends beat single readings.

A result outside the normal range is a prompt, not a prescription. General, low-risk steps include re-measuring in a week to rule out error, reviewing sleep, activity, and eating patterns honestly, and booking a check-up if the number stays high or weight changed quickly. Avoid crash diets and unregulated supplements; rapid weight loss can cost muscle and rebound. This page gives general information only and is not medical advice — a qualified professional who knows your history should interpret the number.

For children and teenagers, adult cutoffs do not apply because healthy BMI changes with growth. Paediatricians use BMI-for-age percentiles plotted on sex-specific growth charts: a percentile, not the raw number, decides the category. Online adult calculators will misclassify a growing child, so do not use this page for anyone under 18. If you are checking a child's growth, ask the paediatrician which chart they use and bring the raw height and weight, not just a BMI value.