Body mass index, or BMI, is a simple ratio of weight to height. It is not a diagnosis, but it is a quick screening number used by clinics and fitness programmes worldwide. You can work it out by hand in under a minute.
How it works
In metric units, BMI = weight in kilograms divided by height in metres squared. In imperial units, BMI = weight in pounds divided by height in inches squared, times 703. The standard adult categories are: below 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, and 30 and above obese. These cut-offs apply to adults; children use age and sex specific charts instead.
Step by step
- Measure your height and weight with the same care you would for any health record.
- For metric: square your height in metres, then divide your weight in kilograms by that number.
- For imperial: square your height in inches, divide your weight in pounds by that number, then multiply by 703.
- Round to one decimal place.
- Compare with the standard categories above.
- Repeat every few months with the same method, rather than switching formulas.
Worked example
Metric: 78 kg, 1.75 m. Height squared = 1.75 x 1.75 = 3.0625. BMI = 78 / 3.0625 = 25.5, in the overweight category. Imperial: 172 lb, 69 inches. 172 / (69 x 69) x 703 = 172 / 4761 x 703 = 25.4, the same answer allowing for rounding. A second example: 62 kg, 1.68 m gives 62 / 2.8224 = 22.0, in the healthy range.
Common mistakes
- Using height in centimetres without converting to metres, which gives a nonsense answer.
- Forgetting to square the height, which is the most common hand-calculation error.
- Applying adult categories to children or teenagers.
- Treating BMI as a body-fat measurement. Muscular people can score high with low body fat.
Try it with the free tool
You can follow every step above by hand, but the free tool below does the same calculation instantly. Try your own numbers in it, then check one result by hand so you trust the method as well as the answer. Open the full tool here: open the tool.
Checklist before you finish
- Write down every input number first, with its unit, before you calculate anything.
- Work through the steps in order, and do not skip the unit conversions.
- Check the answer against the worked example above: is it the right size, and does the direction make sense?
- Round only at the end, and keep the unrounded figure for any follow-up calculation.
Practice with your own numbers
Pick three cases of your own and work each one by hand. First, try a simple round-number case you can check mentally. Second, try a realistic case with awkward numbers, the kind you will actually meet. Third, try an edge case, such as a very small or very large input, and confirm the answer still behaves sensibly. If any case gives a surprising result, re-check the units and the formula step by step before you trust it.
If your answer looks wrong
Most errors come from a small set of causes. Check that you used the same units throughout, that you divided where the formula divides and multiplied where it multiplies, and that you applied any percentage as a decimal. Check the order of operations: powers and brackets come before multiplication and division, which come before addition and subtraction. Finally, test the method on the worked example in this guide. If your method reproduces the example answer, the method is right and the inputs deserve a second look.
Quick tips
Measure height in the morning and weight at the same time of day for consistent tracking. If your BMI is near a category boundary, a small measurement error can tip the category, so re-measure before drawing conclusions.