Body mass index compares your weight to your height. It is a screening number, not a diagnosis: it takes two measurements almost everyone already knows and places you on a scale that public health bodies use to talk about weight across whole populations.
Enter your height and weight above and the result updates as you type. Switch between metric and imperial with the toggle — the calculator converts for you, so there is no need to work out your height in centimetres first.
Key terms
- BMI — your weight in kilograms divided by the square of your height in metres. The units are kg/m², which is why the number itself carries no unit label.
- Healthy weight range — the span of weights that would put you between 18.5 and 24.9 at your current height. It is shown alongside your result so the number has something concrete attached to it.
- Category — the band your BMI falls into. The bands used here are the CDC’s adult categories, listed in full below.
How it works
The calculation is a ratio. Weight rises roughly with the cube of height in a growing child but only with the square across adults of different builds, and squaring height turned out to track adult body fat better than any simpler comparison. That is the whole idea: divide mass by height squared and you get a number that is broadly comparable between a tall person and a short one.
Formula
BMI = weight (kg) ÷ height (m)²
Imperial input is converted first: 1 lb = 0.45359237 kg and 1 in = 0.0254 m exactly. Many charts use a shortcut of 703 × lb ÷ in² instead, which agrees with the exact conversion to two decimal places.
The CDC groups adult results into these bands:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class 1 |
| 35.0 – 39.9 | Obesity class 2 |
| 40.0 and above | Obesity class 3 |
How to use this calculator
- Pick Metric or Imperial. The fields change to match.
- Enter your height. In imperial, feet and inches are separate fields; leave inches blank if you are an exact number of feet.
- Enter your weight. The result appears immediately — there is no calculate button.
- Read the healthy weight range underneath. It is often more useful than the index itself, because it is expressed in the units you weigh yourself in.
A worked example: someone 5 ft 5 in tall weighing 150 lb has a BMI of 24.96 — inside the healthy band, but only just. At that height the healthy range tops out at about 150 lb, which shows how narrow the upper edge of the band can be.
What BMI does not tell you
BMI knows your weight, not what that weight is made of. A muscular athlete and a sedentary person of the same height and weight get identical results, because the index cannot see the difference between muscle and fat. It also says nothing about where weight sits on the body, and abdominal fat carries risks the index cannot detect.
The bands were derived largely from populations of European descent, and several bodies use lower thresholds for people of South Asian and East Asian descent, where health risks appear at a lower index. Pregnancy, older age and childhood each need different reference points — the categories above apply to adults aged 20 and over who are not pregnant.
Treat the number as a conversation starter with a clinician rather than a verdict. Waist measurement, blood pressure, blood glucose and activity level all describe your health more directly than a ratio of two numbers.
Frequently asked questions
Is a BMI of 25 unhealthy?
25.0 is where the CDC’s overweight band begins, but a threshold is a line drawn through a gradient, not a cliff edge. Someone at 25.1 is not meaningfully different from someone at 24.9. What matters more is the direction of travel over years, and the other measures a clinician can take.
Why do metric and imperial sometimes give different answers?
Because most imperial charts use the rounded shortcut of 703 × pounds ÷ inches², while the exact conversion factor is closer to 703.07. This calculator converts imperial input to kilograms and metres exactly, then applies the standard formula, so both modes agree.
Does BMI work for children?
Not with these bands. Children and teenagers are assessed against age-and-sex percentile charts rather than fixed numbers, because healthy body composition changes considerably as they grow. Ask a paediatrician for the right chart.
Does muscle really change the result?
It changes the interpretation, not the arithmetic. A heavily trained athlete can land in the overweight or obesity bands while carrying very little fat. If your weight comes largely from muscle, the index is the wrong tool and body composition measurement is the right one.
What is a healthy weight for my height?
The calculator shows it directly: the weight range that would put you between 18.5 and 24.9 at the height you entered. Because the index depends on height squared, that range widens noticeably for taller people.
