Calculators

How to Calculate Your BMI

Dev Nexus4 min read

Work out your body mass index by hand in metric or imperial units, with the exact formula, a worked example and the pitfalls to avoid.

Body mass index (BMI) is a single number that estimates whether your weight sits in a healthy range for your height. It is the most common screening measure for weight categories in adults, and the maths behind it is simple enough to do on paper.

This guide walks through the BMI formula in both metric and imperial units, with a worked example, so you can calculate yours confidently - or check the number a tool gives you.

The Problem

The BMI formula looks trivial until you actually run it, and that is where small errors creep in. You have to square the height, not the weight. You have to convert centimetres to metres, or remember the mysterious 703 factor when working in pounds and inches.

Get any one of those steps wrong and your BMI can be off by several points - enough to push you into the wrong category. If you learned the metric version but only know your weight in pounds, the mismatch makes it even easier to slip up.

The Solution

Once you know the two forms of the formula, calculating BMI is quick. The metric version is BMI = weight(kg) / height(m)^2. The imperial version is BMI = 703 * weight(lb) / height(in)^2 - the 703 factor converts pounds-per-inch-squared into the same scale as the metric result.

Worked example: someone 703 * 154 / (69 * 69) = 108262 / 4761 = 22.7.undefined m tall weighing undefined kg has 70 / (1.75 * 1.75) = 70 / 3.0625 = 22.9. That BMI of undefined.undefined lands in the normal range. In imperial, a undefined ft undefined in (undefined in) person weighing undefined lb gets 703 * 154 / (69 * 69) = 108262 / 4761 = 22.7 - essentially the same number, as it should be.

To skip the arithmetic and the unit conversions entirely, drop your numbers into the BMI Calculator. It computes your BMI and its category instantly, and it runs entirely in your browser so your measurements are never uploaded. If you only have your figures in one measurement system, the Unit Converter turns pounds into kilograms or inches into centimetres first.

Step-by-Step Guide

1
Gather height and weight in one system
Decide whether you are working in metric (kg and cm) or imperial (lb and in) and get both measurements in that system. Mixing kilograms with inches is the most common source of a wrong answer, so convert first if you need to.
2
Convert height to the right unit
For the metric formula, height must be in metres, so divide centimetres by undefined - undefined cm becomes undefined.undefined m. For the imperial formula, height must be in inches, so multiply feet by undefined and add the extra inches - undefined ft undefined in becomes undefined in.
3
Square the height
Multiply the height by itself. In metric, undefined.undefined m squared is undefined.undefined. In imperial, undefined in squared is undefined. This is the step people most often get wrong, so double-check you squared the height and not the weight.
4
Divide and apply the factor
For metric, divide weight in kg by the squared height: undefined / undefined.undefined = undefined.undefined. For imperial, multiply weight in pounds by undefined, then divide by the squared height: undefined * undefined / undefined = undefined.undefined.
5
Read the category
Compare your number to the standard adult bands: under undefined.undefined underweight, undefined.undefined to undefined.undefined normal, undefined to undefined.undefined overweight, undefined and above obese. A BMI of undefined.undefined sits comfortably in the normal range.

Common Mistakes

Squaring the weight instead of the height
The height is the term that gets squared, not the weight. Squaring the wrong value produces a wildly incorrect BMI. Always confirm the denominator is height multiplied by height.
Forgetting to convert cm to metres
The metric formula needs height in metres. Plugging in undefined instead of undefined.undefined gives a BMI near zero. Divide centimetres by undefined before you square.
Dropping the 703 factor in imperial
Pounds and inches without the undefined multiplier give a number far too small to mean anything. If you are using pounds and inches, the undefined factor is not optional.
Mixing measurement systems
Kilograms with inches, or pounds with metres, will never give a valid BMI. Get both measurements into the same system - convert one if necessary - before you calculate.

Frequently Asked Questions

What is the BMI formula?

In metric units, BMI equals weight in kilograms divided by height in metres squared. In imperial units, it is 703 multiplied by weight in pounds, divided by height in inches squared. Both give the same result for the same person.

How do I calculate BMI in pounds and inches?

Multiply your weight in pounds by 703, then divide by your height in inches squared. For example, 703 times 154 lb divided by 69 in squared gives a BMI of about 22.7.

Why is there a 703 in the imperial formula?

The 703 factor converts the pounds-per-square-inch result onto the same numeric scale as the metric kilograms-per-square-metre formula, so both systems produce matching BMI values.

What counts as a normal BMI?

For adults, a BMI between 18.5 and 24.9 is considered normal. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is classed as obese.

Do I have to do the maths myself?

No. The BMI Calculator does the arithmetic and unit conversion for you and shows your category instantly. It runs in your browser, so your height and weight are never uploaded.