BMI Calculator

What BMI actually is

So you put in your height and weight, hit calculate, and now there is a number on the screen with a word next to it, maybe "Normal" or "Overweight". The fair question is what that number is really telling you, and how much weight to give it.

This page is the straight version. We will go through what BMI actually measures, the exact formula this tool runs, a worked example you can follow line by line, and the places where the number quietly falls short. That last part is not filler. This is a health number, and being upfront about what it cannot do is the honest thing to do.

BMI stands for Body Mass Index, and all it really does is weigh your weight against your height. Weight on its own does not tell you much.

Here is a way to picture it. Take two people who both weigh 80 kilograms. One is six foot three, so that weight is spread down a long frame. The other is five foot two, so the exact same 80 kilograms is packed into a much smaller one. Same reading on the scale, completely different picture. BMI is the tool that tries to tell those two apart using nothing more than height and weight. It hands you back a single number, and that number drops you into one of four categories.

One honest note before we go on. BMI was never built to judge a single person. A Belgian named Adolphe Quetelet worked it out back in the 1830s to describe whole populations, not to tell any one individual whether they were healthy. It got borrowed for that job much later. It does that job well enough as a quick flag, but keep the origin in mind: you are pointing a population tool at one specific person, and that person is you.

The formula this calculator uses

Plenty of BMI pages never show you the maths. We would rather put it in front of you, because there is nothing to hide. The formula is short:

BMI = weight (kg) ÷ height (m) ÷ height (m)

Said the normal way, that is your weight in kilograms divided by your height in metres, squared. No age adjustment, no gender adjustment, no correction for anything else going on in the background. It is the same formula the World Health Organization and the CDC use, and it has been the standard for decades.

You do not have to convert anything yourself. Put your weight in as kilograms or pounds, put your height in as feet and inches, and the tool quietly converts everything to metric before it runs the sum. It then rounds the answer to one decimal place, so you get 22.9 rather than 22.94381.

One thing worth flagging honestly: this version takes height as feet and inches, not centimetres. If you naturally think in centimetres, convert first. One inch is 2.54 centimetres, so divide your height in centimetres by 2.54 to get your total inches, then split that into feet and inches. For example, 170 centimetres works out to roughly 5 feet 7 inches. It is a small extra step, and smoothing it out is on our list.

A worked example

A number makes more sense once you have watched one go all the way through. Take someone 5 feet 7 inches tall who weighs 70 kilograms. Here is exactly what happens behind the button.

First the tool turns the height into metres. 5 feet 7 inches is 67 inches, and 67 × 0.0254 gives about 1.70 metres. Then it runs the formula:

BMI = 70 ÷ (1.70 × 1.70)
BMI = 70 ÷ 2.89
BMI ≈ 24.2

So the BMI is 24.2, which sits in the Normal range, just under the upper line. Now picture the same height but 78 kilograms. That works out to 78 ÷ 2.89 ≈ 27.0, which tips over into Overweight. Same height, eight kilograms, a different category. That is the whole mechanism in one example.

What the four categories mean

Once the tool has your number, it sorts you into one of four buckets. These are the standard adult cut-offs used by the WHO and the CDC:

Two things worth noticing. The lines are identical whether you are a man or a woman, which is why the calculator never asks your sex. And they are meant for adults, roughly age 20 and up. If you are checking this for a child or a teenager, these fixed numbers simply do not apply, because a growing body is read against age and sex growth charts instead. There is more on that below, and our Child BMI Calculator is built for exactly that.

Where BMI gets it wrong

This is the part a health tool has no business skipping, and with BMI there is a fair bit to say. The number is useful, but it is a blunt instrument, and it is only fair to be clear about where it slips.

• It cannot tell muscle from fat. The number only sees your total weight, not what that weight is made of. A lean, heavily muscled athlete can land in "Overweight", or even "Obese", while carrying very little fat. That is plainly nonsense, and it is a good reminder not to hand this number more authority than it has earned. Muscle is dense and heavy, and BMI has no way of knowing the difference.
• It can miss fat the other way too. An older person who has lost muscle over the years can show a perfectly "Normal" BMI while actually carrying more body fat than the number lets on.
• It says nothing about where the fat sits. Fat around your middle carries more health risk than fat on your hips and thighs, but BMI counts every kilogram the same, wherever it happens to be.
• It uses one set of lines for every body. It does not adjust for age, sex or ethnicity. This one matters if you are of South or East Asian descent: a WHO expert group found that the health risks that usually appear around a BMI of 25 tend to show up earlier, which is why lower cut-offs are often recommended for these groups. So a "Normal" reading on this chart is not always the full story.
• It is built for adults, not children and not pregnancy. For children and teenagers, BMI is read against growth charts for their age and sex rather than these fixed numbers, so use our Child BMI Calculator for that. And it is not a measure meant for use during pregnancy.

None of this makes BMI pointless. It makes it a decent first glance rather than a verdict. It is a fine way to check whether your weight is worth a second thought. It was never meant to be the last word on your health, and it cannot see the things that actually decide that, like your waist, your blood work, how active you are, or your family history. If your number surprises or worries you, that is a conversation for a doctor, who can look at the whole picture instead of one ratio.

Questions people ask

Is BMI different for men and women?

No. The formula and the four categories are exactly the same for both, which is why the tool never asks your sex. Men and women do carry weight differently, but that difference is not built into the BMI number itself.

My BMI says overweight, but I am pretty fit. What is going on?

Almost always, muscle. Muscle is dense, so if you carry a lot of it, BMI can push you a category higher than your actual body fat would suggest. This is a known blind spot in the number, not a problem with you. A body fat measurement will tell you far more here than BMI can, so the Body Fat Calculator is a better next stop.

What counts as a healthy BMI?

For adults, 18.5 to 24.9 is the range usually called healthy. Just keep in mind that "healthy" is a much bigger idea than a single number, and plenty of healthy people sit a little outside that band.

Can I use this for my child?

Not this one. A child's BMI has to be read against growth charts for their age and sex, so use the Child BMI Calculator instead.

Why does it ask for height in feet and inches?

That is simply how this version takes it. If you think in centimetres, convert first: 170 centimetres is roughly 5 feet 7 inches.

How often should I check my BMI?

It barely moves from one day to the next, so there is little point checking it daily. If you are tracking a change over time, once a month is plenty.

References

A quick word on where these numbers come from. The four categories and the weight-over-height-squared formula this tool uses are the standard adult definitions published by the CDC, which in turn follow the World Health Organization. The point that BMI is a poor guide to actual body fat is drawn from Nuttall's critical review in Nutrition Today. And the note about lower cut-offs for people of Asian descent comes from the WHO expert consultation published in The Lancet.

1. Centers for Disease Control and Prevention. About Body Mass Index (BMI). https://www.cdc.gov/bmi/about/
2. Nuttall FQ. Body Mass Index: Obesity, BMI, and Health: A Critical Review. Nutrition Today. 2015;50(3):117-128. https://pmc.ncbi.nlm.nih.gov/articles/PMC4890841/
3. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet. 2004;363(9403):157-163. https://doi.org/10.1016/S0140-6736(03)15268-3