BMI Calculator
Calculate body mass index from height and weight.
BMI categories follow the World Health Organization (WHO) classification: under 18.5 underweight · 18.5–24.9 normal · 25–29.9 overweight · 30+ obese. BMI is a general guide and doesn't account for muscle mass, age, or body composition. Source: WHO ↗
Use on any page (bookmarklet)
Want BMI Calculator without leaving the page you're on? Drag the button below to your bookmarks bar, then click it on any website to open BMI Calculator right there — it runs entirely in your browser.
Use responsibly: a bookmarklet runs on whatever page you click it on. Avoid sensitive sites such as online banking, payment, or healthcare pages — you run it at your own risk. Everything is processed locally and no data is sent anywhere. See our Terms.
- Show your bookmarks bar if it's hidden — Ctrl+Shift+B (⌘+Shift+B on Mac).
- Drag the button above onto the bookmarks bar.
- Open any website and click the bookmark — the BMI Calculator panel appears in the top-right corner. Use ✛ to move it between corners, or ›/‹ to tuck it against the edge and pull it back out.
- Click the bookmark again (or the ✕) to close it.
Note: a few sites with strict security policies may block bookmarklets.
Examples
70 kg, 175 cm
Input
Metric
Output
BMI 22.9 · Normal weight
154 lb, 5 ft 9 in
Input
Imperial
Output
BMI 22.7 · Normal weight
About this tool
This free online BMI calculator works out your Body Mass Index from your height and weight, in metric or imperial units, and shows which WHO category it falls into. The result updates instantly as you type, and everything runs in your browser with no upload.
How to use
- Choose metric (kg, cm) or imperial (lb, ft/in).
- Enter your weight and height.
- Read your BMI and category below.
Good to know
BMI is a quick screening number, not a diagnosis. It doesn't measure body fat directly or account for muscle, bone, age, or sex — so use it as a starting point and talk to a healthcare professional for personal advice.
What BMI was actually designed for
The formula was devised in the 1830s by Adolphe Quetelet, a mathematician studying the statistical distribution of body types across populations. It was never intended to assess an individual, and Quetelet said so. Its survival into modern clinical use comes down to practicality: it needs two measurements anyone can take, requires no equipment, and correlates well enough with body fat across large groups to be useful for public health statistics.
That history explains its central weakness. A measure built to describe a population tells you where you sit relative to others — not what your body is made of.
Where BMI misleads
- Muscle weighs more than fat. Athletes and regular strength trainers routinely score “overweight” or “obese” on BMI while carrying very little body fat, since the formula cannot tell muscle from anything else.
- It ignores fat distribution. Two people with identical BMI can carry very different health risk depending on whether fat sits around the abdomen or the hips. Waist circumference, or waist-to-height ratio, predicts metabolic risk better — a common rule of thumb is keeping your waist under half your height.
- The thresholds are not universal. Risk begins at lower BMI values in South and East Asian populations, and several health authorities apply a lower overweight cut-off accordingly. The standard categories reflect data gathered largely from European populations.
- It is not designed for everyone. Children and teenagers need age- and sex-specific percentile charts rather than adult categories; pregnancy, older age with muscle loss, and limb differences all distort the result.
Using the number sensibly
BMI is most useful as a trend rather than a verdict. Tracking your own figure over months tells you something real about a change in direction; a single reading compared against a category boundary tells you very little, especially near a threshold where a kilogram moves you across a line that was drawn for statistical convenience.
If the result concerns you, the useful next step is a conversation with a clinician who can look at waist measurement, blood pressure, blood work, activity, and family history together. None of those are things a height-and-weight formula can see.
Your height and weight are processed entirely in your browser and never uploaded — this tool has no server to send them to. For other everyday health estimates, the water intake calculator works out a daily target from body weight and activity, and the sleep calculator works backwards from your wake time through 90-minute sleep cycles. Both are guides rather than medical advice, for the same reasons set out above.
Frequently asked questions
How is BMI calculated?
BMI is your weight in kilograms divided by your height in metres squared (kg/m²). In imperial units it's 703 × pounds ÷ inches². This calculator does both for you.
What are the BMI categories?
Using WHO ranges: below 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30 or above is obese.
Where do these BMI ranges come from?
The category cut-offs come from the World Health Organization (WHO) classification of body mass index for adults. A source link to the WHO is shown beneath the result. Note that some countries and clinicians use slightly different thresholds, especially for certain populations.
Can I use pounds and feet/inches?
Yes. Switch to Imperial to enter weight in pounds and height in feet and inches.
Is BMI accurate for everyone?
BMI is a general screening tool. It doesn't distinguish muscle from fat or account for age, sex, and body composition, so athletes and some groups may get misleading numbers. Treat it as a rough guide.
Is my health data uploaded anywhere?
No. The calculation runs entirely in your browser with no upload, so your measurements stay private.
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