BMI Calculator

Calculate your BMI online — free and private

Body Mass Index is the quickest way to see where your weight sits relative to your height. Enter your numbers and this calculator returns your BMI and its category instantly — no sign-up, no app, and nothing uploaded. It works in both metric (centimetres and kilograms) and imperial (feet, inches, and pounds) units, so you can use whichever you think in.

Because the whole calculation runs in your browser, the height and weight you type never leave your device. For something as personal as body measurements, that matters: you get the number without handing your health data to a server.

What is BMI?

Body Mass Index is a single number that relates your weight to your height. It was designed as a fast population-level screening tool, and it's still the most common way to flag whether someone might be underweight, at a healthy weight, overweight, or in the obese range. It doesn't measure body fat directly — it's a ratio — but for most adults it gives a reasonable first indication worth discussing with a doctor.

How BMI is calculated

The formula is your weight in kilograms divided by the square of your height in metres: BMI = kg / m². Someone who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9.

If you prefer imperial units, the arithmetic is the same once the numbers are converted — this tool turns your feet, inches, and pounds into metres and kilograms behind the scenes, so you get an identical result either way. The equivalent imperial formula multiplies weight in pounds by 703 and divides by height in inches squared.

BMI categories

For adults, the World Health Organization uses these standard ranges:

• Below 18.5 — Underweight
• 18.5 to 24.9 — Normal weight
• 25.0 to 29.9 — Overweight
• 30.0 and above — Obese

The calculator highlights your category and marks where you fall on a scale, so you can see how close you are to the next band.

The limits of BMI

BMI is useful, but it's blunt. It only knows your height and weight, so it can't tell muscle from fat. A muscular athlete can read as "overweight" despite low body fat, while someone with little muscle can read as "normal" while carrying excess fat. It also doesn't account for age, sex, ethnicity, or where fat is distributed on the body — all of which affect health.

It isn't designed for children, pregnant people, or the very elderly, who need different measures. Treat BMI as one starting data point, not a diagnosis. Waist circumference, body-fat percentage, blood work, and a conversation with a healthcare professional give a much fuller picture.

Works on your phone

The calculator runs in your phone's browser with nothing to install. Switch between metric and imperial with a tap, type your numbers, and the result updates live. Since everything happens locally, it responds instantly and keeps your measurements on your device.

Privacy: your numbers never leave your device

Your height and weight are used only to compute the BMI in your browser. Nothing is uploaded, nothing is stored on a server, and there's no account to create. When you close the page, the numbers are gone — because they were never sent anywhere in the first place.

Frequently asked questions

What is BMI?

A ratio of your weight to your height that sorts you into a broad category — underweight, normal, overweight, or obese. It's a quick screening number, not a full health assessment.

How is BMI calculated?

Weight in kilograms divided by height in metres squared (kg/m²). Imperial inputs are converted automatically before the same formula is applied.

What is a healthy BMI?

For adults, 18.5 to 24.9 is the normal range. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese — general guidelines that don't account for muscle, age, or sex.

Is my data uploaded to a server?

No. The calculation runs entirely in your browser — your height and weight never leave your device, and nothing is stored or sent anywhere.