BMI Calculator

What BMI measures, and what it cannot see

Body mass index is one division: your weight divided by the square of your height. In metric that is kilograms over meters squared; in imperial it is pounds over inches squared, multiplied by 703. The result is a single number that describes how heavy you are for your height, and nothing else. It was designed in the nineteenth century to describe populations, and it is still used that way, because measuring the height and weight of ten thousand people is cheap and measuring the body composition of ten thousand people is not.

That cheapness is also the catch. The formula has no way of knowing what the weight is made of. Muscle is denser than fat, so a trained athlete and a sedentary person of the same height and weight get the same BMI and are in completely different states of health. It does not know where the weight sits either, although fat around the abdomen carries more risk than the same amount on the hips. It was derived from European populations, so it reads differently for groups that carry more body fat at a given weight. It does not apply to children, whose proportions change as they grow, or to anyone pregnant, whose weight includes a second person. And because people tend to lose muscle and gain fat as they age, a BMI that has not moved in decades can hide a real change in what the weight is made of.

None of that makes BMI useless: it is a fast, free screening figure that tells you whether anything is worth looking at more closely.

Which measurement answers which question

• BMI: a quick screen, and a way to watch your own trend over months. Best read as a direction of travel rather than a category you belong to.
• The healthy weight range for your height: more useful than the BMI figure itself when you are setting a target, because it is expressed in the units you actually weigh yourself in.
• Waist circumference: where BMI is silent. Fat stored around the abdomen is the part most closely tied to metabolic risk, and a tape measure costs almost nothing.
• Waist to height ratio: the same measurement made comparable between tall and short people. The common rule of thumb is to keep your waist under half your height.
• Body fat percentage: what BMI is standing in for. Worth estimating if you train, if your BMI and your appearance disagree, or if you are tracking a body recomposition.
• Percentile charts: the only correct tool for anyone under about 20. Adult cutoffs do not transfer to a growing body.

How to find the healthy weight range for your height

1. Pick your units at the top of the calculator. Imperial gives you feet, inches and pounds; metric gives you centimeters and kilograms. Switching converts what you have already typed, so you can check the same body in both.
2. Enter your height and weight. The result updates as you type, so there is no button to press.
3. Read the line marked Healthy weight range under your BMI. That is the span of weights that would put you between BMI 18.5 and 25 at the height you entered.
4. Compare it with where you are now. If you are outside the band, the gap in pounds or kilograms is a more workable target than a BMI figure, because it is the number your scale shows.
5. Check it against the list headed WHO BMI categories, which shows every band and its cutoffs, with your own band highlighted.

How to weigh and measure so the number means something

1. Weigh yourself in the morning, after using the bathroom and before eating, wearing roughly the same thing each time. Day to day swings of two to four pounds (one to two kilograms) are mostly water and food in transit.
2. Measure your height without shoes, standing straight with your heels against a wall and your eyes level. Most people are a little shorter than they think, and half an inch of error moves an adult BMI by roughly 0.3.
3. Use the same scale every time. Two bathroom scales can differ by a couple of pounds, which is enough to move you between bands for no real reason.
4. Press Log this measurement to keep the entry. The tool holds your last eight in this browser, which is what makes the trend visible.
5. Weigh weekly rather than daily if the daily number bothers you. A month of weekly readings tells you far more than any single BMI.

Keeping the result and taking it to an appointment

The measurement log lives in your browser, not in an account, so there is nothing to sign into and nothing to sync. That also means it is tied to one browser on one device: clearing site data or switching to private browsing loses it, and it will not follow you from your phone to your laptop.

• To put it in a health app or a form: type the height and weight in, not the BMI. Apps and clinics recalculate the index themselves, and entering a rounded BMI by hand only adds error.
• To take it to a doctor: bring the dates and the weights rather than the categories. A clinician wants to see the direction and the speed of a change, which is exactly what a run of logged entries shows.

Where the standards differ

• WHO, international: the bands this page uses. Underweight below 18.5, healthy 18.5 to 25, overweight 25 to 30, and obesity from 30, divided into class I from 30, class II from 35 and class III from 40.
• Asian populations: a WHO expert consultation published in 2004 concluded that risk of type 2 diabetes and cardiovascular disease rises at a lower BMI in many Asian populations, and proposed additional public health action points near 23 and 27.5 rather than new categories. The NHS in England, for example, counts a BMI of 23 to 27.4 as overweight and 27.5 or more as obese for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background.
• United States: the CDC adult calculator uses the same four WHO bands and the same three obesity classes, so a US result and an international result agree.
• Taiwan: the health ministry classifies from 18.5, 24 and 27, which puts the overweight line a point lower than the WHO figure.
• Older adults: the WHO bands do not change with age, but MedlinePlus, from the US National Library of Medicine, notes that a slightly higher BMI at 65 or older may help protect against thinning of the bones. If you enter an age of 65 or more, the calculator adds a note under your result and still shows the WHO adult bands.
• Athletes and heavily trained people: no standard adjusts for muscle mass, because no formula based on height and weight can. Sports medicine uses body composition testing instead.
• Children and adolescents: BMI is read against age and sex percentile charts, not adult cutoffs. Which reference chart is used differs between the WHO growth reference and national ones, so a percentile from one is not directly comparable with a percentile from another.

Limits worth knowing before you read too much into it

This calculator is an information tool, not medical advice, and it cannot diagnose anything. Talk to a qualified health professional about your own health, and do not start or stop anything on the strength of a number on a web page.

A category is a band, not a cliff: a BMI of 24.9 and a BMI of 25.1 describe essentially the same body, so a reading that sits close to a cutoff says very little. A single figure says less than a trend, and most of what you learn from this tool comes from measuring the same way for a few months. And if the number bothers you, it is reasonable to stop looking at it: weight is only one of many inputs into health, alongside blood pressure, blood tests, sleep, activity and how you actually feel.