BMI Calculator

About this tool

Body Mass Index is a single number worked out from your height and weight. It's used everywhere as a quick screening measure — a starting point for a conversation, not a diagnosis. This calculator takes metric or imperial input and shows both the BMI value and the population category it falls into, updating as you type.

How it's calculated. In metric units, BMI is your weight in kilograms divided by the square of your height in meters. In imperial units it's your weight in pounds times 703, divided by the square of your height in inches — the 703 factor just converts the result back onto the same scale as the metric formula.

Worked example. Someone 1.75 m tall weighing 70 kg: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = a BMI of about 22.9, which sits in the typical range. The same person entered as 5′9″ and 154 lb comes out at 22.7 — the tiny difference is rounding in the height and weight, not a different calculation.

The categories. Below 18.5 is classed as underweight, 18.5–24.9 as a typical range, 25–29.9 as overweight, and 30 or above as obese (sometimes split into classes I, II and III at 35 and 40). These thresholds are population screening bands — average health outcomes across large groups shift around those points — and the exact boundary is not meaningful for one person. A BMI of 24.9 and one of 25.1 are not different in any real sense.

Where BMI is unreliable. It cannot tell muscle from fat, so a muscular, lean athlete can register as overweight. It says nothing about where fat is carried, and abdominal fat carries more risk than fat on the hips and thighs. It's less informative for older adults, who tend to lose muscle while keeping fat, and the adult bands do not apply to children and teenagers at all — those are read against age- and sex-specific percentile charts. Some health bodies also use a lower overweight threshold of 23 for people of Asian descent.

Treat the number as one cheap data point. For a fuller picture, look at waist measurement, the body fat percentage estimator, and how you actually feel and function. The guide to what your BMI means goes through all of this in more detail.

Frequently asked questions

How is BMI calculated?

Metric: weight in kilograms divided by height in meters, squared. Imperial: weight in pounds multiplied by 703, divided by height in inches, squared. Both formulas land on the same scale.

What do the categories mean?

Under 18.5 is underweight, 18.5–24.9 is the typical range, 25–29.9 is overweight, and 30+ is obese. These are general population screening bands, not individual medical advice, and the boundaries are approximate.

Is BMI accurate for athletes?

Often not. Muscle is denser than fat, and BMI can't distinguish the two, so someone with a lot of muscle and low body fat can land in the overweight or obese band. Body fat percentage or a waist measurement is more useful for them.

Does BMI work for children?

Not with the adult categories above. Children's and teenagers' BMI is interpreted using age- and sex-specific percentile charts, because healthy body composition changes throughout growth.

Why do the metric and imperial results differ slightly?

The underlying formula is the same. Small differences come from rounding when you convert your height and weight between unit systems — enter your real measurements in either system and the result is accurate to the input.

Should I use a different threshold for my background?

Some health organizations apply a lower overweight cut-off (around 23) for people of South Asian, Chinese and other Asian descent, because cardiometabolic risk tends to rise at a lower BMI in those populations.

My BMI is "normal" but I'm worried about my weight.

A normal BMI can still sit alongside a high body fat percentage or a large waist measurement, particularly with age. If something feels off, that's a reason to look at the other measures and talk to a healthcare provider rather than to rely on this one number.