BMI Calculator

What is a BMI calculator?

A BMI calculator works out your Body Mass Index — a single number that relates your weight to your height. It is the most widely used screening figure for whether an adult sits in a healthy weight range, and it appears everywhere from doctors' offices to fitness apps. The appeal is its simplicity: with just two measurements you get a number and a category, no special equipment required.

The tool above accepts both metric (centimetres and kilograms) and imperial (feet, inches and pounds) measurements. Whatever you enter, it converts to a common base, applies the standard formula, and shows your BMI to one decimal place alongside a colour-coded category and a scale that marks where you fall.

BMI was never meant to diagnose anything on its own. It is a fast, repeatable estimate that flags when a closer look might be worthwhile — which is exactly what makes it useful as a quick check rather than a final answer.

How to use it

1. Pick your units with the toggle at the top. Metric asks for height in centimetres and weight in kilograms; imperial asks for feet plus inches and weight in pounds.
2. Enter your height. In imperial mode you can fill in feet and inches separately, and either box on its own still works.
3. Enter your weight. The result updates live as you type, so you never need to press a button.
4. Read your BMI and category. The big number is your index; the coloured badge names your band, and the scale below shows your position from underweight through to obese.
5. Copy the result with one click if you want to note it down or share it with a clinician.

Everything happens on your device, so it stays instant even as you adjust the numbers, and it keeps working offline once the page has loaded.

The BMI formula in plain words

The metric formula is:

BMI = weight (kg) ÷ height (m)²

So someone 1.75 m tall weighing 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9, which lands in the normal range.

For imperial measurements the calculator converts first. One inch is 0.0254 metres, so 5 ft 9 in is 69 inches = 1.7526 m. One pound is about 0.4536 kg, so 154 lb is about 69.85 kg. Plug those into the same formula and you get the same BMI as the metric example. Converting before calculating — rather than using a separate imperial formula — keeps the result consistent no matter which units you prefer.

What the categories mean

The calculator uses the standard adult bands:

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

These ranges come from large population studies linking BMI to health outcomes. They describe groups, not individuals, so a single number near a boundary is not meaningful on its own. Someone at 24.8 and someone at 25.1 are, in practice, in the same place — the line is a convention, not a cliff.

Why BMI is only a rough guide

BMI uses just height and weight, which means it cannot tell muscle from fat or see how weight is distributed. A few well-known limitations are worth keeping in mind:

• Muscle reads as weight. Athletes and very muscular people often score "overweight" or "obese" despite low body fat, because muscle is dense.
• It misses fat distribution. Where fat sits — around the waist versus the hips — affects health risk, and BMI says nothing about it. A waist measurement adds useful context.
• It is not for everyone. BMI is built for adults. Children and teenagers use age- and sex-specific percentile charts, and it is not a reliable measure during pregnancy.
• Age and build matter. Older adults and different body frames can carry the same BMI with very different amounts of muscle and fat.

For these reasons, treat your BMI as one data point. A waist-to-height ratio, body-fat estimate, a sense of your daily calorie needs from a TDEE Calculator, blood markers and a conversation with a healthcare professional give a far fuller picture.

Tips for getting a useful reading

• Measure consistently. Weigh yourself at the same time of day, ideally in the morning, for comparable numbers over time.
• Watch the trend, not the digit. A single BMI matters less than whether it is drifting up or down over weeks and months.
• Round honestly. Small input errors barely move the result, but guessing your height by several centimetres can shift a category — measure rather than estimate when you can.
• Pair it with a waist check. A waist under half your height is a common rule of thumb that complements BMI nicely.

Private, instant and always free

Because this BMI calculator runs entirely in your browser, there are no accounts, no limits and nothing leaves your device. Keep it open whenever you want a quick health check-in — it turns two measurements into a clear number and category in seconds, while your figures stay yours.