Learn how to calculate Body Mass Index (BMI) using the correct formula, interpret your result accurately, and understand what BMI can and cannot tell you about your health.
Body Mass Index, or BMI, is one of the most widely used health metrics in the world. It appears in doctor's offices, insurance questionnaires, fitness apps, and government health guidelines. Despite its ubiquity, a surprising number of people calculate it incorrectly — or never calculate it at all because the formula looks intimidating.
This guide walks through the BMI formula step by step, shows you two worked examples using metric and imperial units, explains what the result actually means, and points out the situations where BMI is misleading. By the end you will be able to calculate your BMI on paper, in a spreadsheet, or with a single tap in a browser-based calculator.
BMI is defined as weight divided by the square of height. The important part is that the units have to match, so there are two versions of the formula — one metric, one imperial.
BMI = weight (kg) ÷ height (m)²
Note that height must be in meters, not centimeters. A person who is 175 cm tall needs to use 1.75 in the formula.
BMI = [ weight (lb) ÷ height (in)² ] × 703
The 703 factor converts the result so it lines up with the metric version. Without it the numbers would be off by a large margin.
A person weighs 70 kg and is 1.75 m tall.
A BMI of 22.9 falls into the "normal weight" category according to the World Health Organization classification.
A person weighs 170 lb and is 5'9" (69 inches) tall.
A BMI of 25.1 falls just into the "overweight" category.
The World Health Organization publishes the standard adult BMI classification:
These ranges were established using data from large populations. Individual interpretation always benefits from context — age, muscle mass, body composition, and medical history all matter.
The number itself is only useful if you know what it implies. Here is a practical read of each range — not medical advice, but the kind of context a clinician might actually say in a 10-minute appointment.
Being underweight is often ignored because the public-health conversation focuses on the other end, but it carries its own risks: weaker immune response, thinner bones, menstrual irregularities, and slower recovery from illness. The first question to ask is whether this is your natural build or a recent change. A BMI of 17 that has been stable for years in a small-framed adult is very different from a BMI of 17 that was 21 a year ago.
What to do next: track your weight weekly for a month, note energy levels, and if the number keeps drifting down or you feel off, book a check-up. Blood tests for thyroid function and iron are the usual starting point.
You are squarely inside the range most clinicians consider low-risk on this single metric. That is not the same thing as "healthy" — two people with identical BMIs can have wildly different blood pressure, cholesterol, and fitness — but BMI is not the thing to focus on.
What to do next: nothing BMI-related. If you want a better health picture, get a blood panel, measure your resting heart rate for a week, and notice whether you can walk briskly for 30 minutes without getting winded. Those tell you more than any body-weight number.
This is where most adults in many Western countries sit, which is partly why the boundary is so fuzzy in practice. A BMI of 27 in a sedentary office worker usually reflects excess body fat and is worth addressing. The same number in a regular gym-goer with visible muscle often reflects lean mass, not fat — and the risk profile is very different.
What to do next: measure your waist (at the belly button, without sucking in). For men, over 94 cm (37 in) is a mild warning sign and over 102 cm (40 in) a stronger one. For women, the thresholds are 80 cm (31.5 in) and 88 cm (34.5 in). If your waist is inside those numbers, your BMI of 27 is probably not clinically urgent. If it is over them, focus there — waist size responds faster than the scale to the kind of changes that actually help.
At this range, the statistical risk of type 2 diabetes, high blood pressure, and cardiovascular issues rises noticeably. That does not mean it is urgent today — plenty of people live at this BMI for decades — but it is worth treating as an early signal rather than a background number.
What to do next: rather than a crash diet, aim for a 5–10% weight reduction over six months. Research consistently shows that even this modest drop substantially lowers cardiovascular risk. A conversation with your GP about blood glucose and blood pressure is worth having at this stage, even if you feel fine.
At these levels the risk curve gets steeper, and most public-health guidelines recommend clinical support rather than purely self-directed change. This is not a moral judgement — the research is simply clear that sustained weight change above this threshold usually benefits from professional guidance, whether that is a dietitian, a behavioural programme, or in some cases medication.
What to do next: if you have not recently had a blood panel and blood pressure check, that is the right starting point. A BMI this high combined with normal bloods is a much more reassuring picture than the same BMI with already-elevated glucose or cholesterol.
This is by far the most common error. Plugging 175 (cm) into the metric formula instead of 1.75 (m) gives a BMI value so small it is meaningless. Always convert height to meters first: divide centimeters by 100.
When using pounds and inches, skipping the multiplication by 703 produces a number that is roughly 700 times too small. The factor is not optional.
Squaring the height and then rounding before the division introduces meaningful error. Do all the arithmetic first and round only the final answer.
BMI is a population-level statistic that works reasonably well for sedentary adults of average build. It becomes unreliable in several specific cases:
A better health picture comes from combining BMI with waist circumference, body composition measurements, blood pressure, and a conversation with a clinician.
For anyone under 20, the raw BMI number is converted into a percentile based on age and sex. A 12-year-old with a BMI of 20 might be at the 85th percentile (overweight) while a 17-year-old with the same BMI is at the 50th percentile (healthy). Pediatricians use growth charts to interpret these values, and parents should avoid comparing a teen's BMI directly to adult categories.
Treat BMI as one data point among several. It is a quick, free, and non-invasive screening tool, which is why it remains popular in primary care and public health research. It is not a diagnostic test, and a single reading does not define a person's health.
Consistency matters more than precision. Tracking your BMI every few months using the same scale and the same height measurement tells you whether your body composition is trending in the direction you want. One-off numbers taken after a large meal or at different times of day add noise without insight.
BMI is easy to calculate once you know which formula to use and remember to keep the units consistent. The categories give a rough sense of where a person sits relative to a healthy-weight range, but the number is meaningless without context. Use it as a starting point for a conversation about health, not as a verdict.
For a fast check with no math at all, the free BMI calculator on the Toolific Hub handles both metric and imperial inputs, shows your category, and includes a healthy-weight range for your height.