01What BMI means
Body Mass Index (BMI) is a single number, worked out from your height and weight, that estimates whether you are at a healthy weight. It is used worldwide by doctors, insurers, and health bodies such as the WHO and CDC because it is fast, free, and needs nothing more than a set of scales and a tape measure.
BMI is best understood as a screening tool: it flags whether your weight may be worth a closer look, rather than giving a complete picture of your health on its own. The Overview tab covers what the calculator does, the weight categories, and practical guidance; the Formula tab shows the maths and a worked example.
02What this calculator does
This BMI calculator takes two simple inputs — your height and your weight — in either metric or imperial units. From those it works out your Body Mass Index, matches it to a weight category, and shows you where you fall on a colour-coded scale alongside the healthy weight range for your height.
The result is rounded to one decimal place and the category follows the standard WHO and CDC bands: underweight, healthy weight, overweight, or obesity. The calculator also rejects impossible entries — a blank field, a zero, a negative number, or an extreme value — so you always get a meaningful result rather than a misleading one.
03BMI categories & ranges
For adults aged 20 and over, the same BMI thresholds apply regardless of age or sex. The four standard categories are shown below; a healthy BMI sits between 18.5 and 24.9.
The obesity band is often split further into Class 1 (30.0–34.9), Class 2 (35.0–39.9), and Class 3 or severe obesity (40.0 and above). These sub-classes help clinicians judge risk and, in some cases, eligibility for treatments such as bariatric surgery, which is generally considered from a BMI of 40 — or 35 with a weight-related condition.
BMI is a useful first check, but it does not measure body fat directly, and it can mislead in several situations:
- Athletes and muscular builds. Muscle is denser than fat, so a fit, muscular person can score in the overweight or obese range while carrying very little fat.
- Older adults. Muscle mass tends to fall with age, so an older person can have a “healthy” BMI yet still carry a high proportion of body fat.
- Sex and ethnicity. At the same BMI, women on average carry more body fat than men, and some populations face raised risks at lower BMIs.
- Pregnancy. Standard adult categories are not designed for pregnancy, when weight gain is expected and healthy.
- Children and teens. Under-20s use age- and sex-specific BMI percentiles, not the fixed adult cut-offs.
- Zero, negative, or blank inputs are rejected. Height and weight must be positive numbers within sensible ranges; the calculator flags anything outside them instead of returning a misleading BMI.
- Pick Metric (cm, kg) or Imperial (ft/in, lb) at the top of the calculator.
- Enter your height, then your weight.
- Tap Calculate to see your BMI, your category, where you sit on the colour-coded scale, and the healthy weight range for your height.
Tips for reading your result
- Treat BMI as a starting point, not a verdict — pair it with a waist measurement, which captures fat stored around the middle that BMI can miss.
- Track the trend over months rather than reacting to a single reading, since day-to-day weight naturally fluctuates.
- If your BMI sits outside the healthy range, focus on small, steady changes rather than crash diets.
01The BMI formula
BMI is based on your weight divided by your height squared. The same idea is written two ways depending on whether you think in metric or imperial units — both give the same result for the same person:
Where:
- weight = your body weight, in kilograms (metric) or pounds (imperial)
- height = your standing height, in metres (metric) or inches (imperial)
- [height]² = your height multiplied by itself (height squared)
- 703 = a conversion factor that makes the pound-and-inch version line up with the metric result
Both formulas agree because the 703 factor simply rescales imperial units so the answer matches the metric one. The result is a plain number with no units, which is then read against the category bands.
02Worked example
Take an adult who weighs 70 kg and stands 1.75 m tall. Here weight = 70 and height = 1.75. Work through the metric formula one line at a time:
If the same person used pounds and inches instead (about 154 lb and 68.9 in), the imperial formula — 703 × 154 ÷ 68.9² — lands on essentially the same result, confirming the two formulas agree. For their height, a healthy weight runs from about 57 kg (BMI 18.5) to 76 kg (BMI 24.9).