BMI is your weight in kilograms divided by the square of your height in metres (kg/m²), or, in imperial units, 703 times your weight in pounds divided by your height in inches squared. The number is sometimes called the Quetelet index, after the Belgian mathematician Adolphe Quetelet who devised the weight-divided-by-height-squared ratio in the 1830s while studying average physical characteristics of human populations. Today it is the most widely used population-level screening measure for adult weight status, recommended by the World Health Organization and adopted by clinicians and public-health bodies worldwide. Calculating BMI from your own height and weight takes only two numbers and a calculator — but it pays to know which unit system you are using, because the imperial formula carries a conversion constant of 703 that does not appear in the metric version. Once you have a BMI value, it is compared against fixed WHO cut-offs to land in one of four adult weight categories: underweight, normal weight, overweight, or obese. This article walks through both formulas, the WHO category table, and a tool — the BMI Calculator — that does the arithmetic for you and updates the result as you type.
The BMI formula in plain English
BMI is a single number that summarises how heavy you are relative to how tall you are. The math is deliberately simple, which is exactly why it has survived as a screening tool for nearly two hundred years.
The core idea is this: taller people are expected to weigh more, so a height-squared scaling keeps the score from drifting upward simply because someone is tall. Squaring the height compresses the effect: a person who is 10% taller is not 10% heavier in BMI terms, but roughly 21% heavier, because 1.10² ≈ 1.21.
You will see the formula written two ways depending on the unit system:
- Metric: BMI = weight (kg) ÷ height² (m²)
- Imperial: BMI = 703 × weight (lb) ÷ height² (in²)
Both expressions evaluate to the same units — kilograms per square-metre. The factor 703 is the conversion constant that rescales pounds-per-square-inch into kilograms-per-square-metre. It is not arbitrary: it comes from the exact relationship 1 lb ≈ 0.4536 kg and 1 in ≈ 0.0254 m, so 1 lb/in² ≈ 0.4536 ÷ 0.0254² ≈ 703.07, which is rounded down to 703 for everyday use.
Calculating BMI in metric units
If your scale reads in kilograms and you know your height in centimetres or metres, use the metric formula. It is the cleanest of the two because there is no conversion constant.
Worked example using 70 kg and 1.75 m:
1. Square the height: 1.75 × 1.75 = 3.0625 m².
2. Divide the weight by that value: 70 ÷ 3.0625 ≈ 22.86 kg/m².
A reading of about 22.9 falls inside the WHO normal-weight band of 18.5 to 24.9, so this is a textbook result. If you have your height in centimetres instead of metres, divide by 100 first — for instance, 175 cm becomes 1.75 m — and then plug into the same formula. Using centimetres directly will give you a BMI that is 10,000 times too small, which is the most common slip people make with the metric version.
For readers who only know imperial numbers, the step-by-step pounds-and-inches BMI guide walks through the same idea in US units.
Calculating BMI in imperial units
US scales and medical forms often record weight in pounds and height in inches (or feet and inches). The imperial formula adds the factor 703 so that a BMI of 25 in pounds and inches means exactly the same thing as a BMI of 25 in kilograms and metres.
The process in plain steps:
1. Convert your full height into inches only — multiply feet by 12 and add the remaining inches.
2. Square the inches value (multiply it by itself).
3. Multiply your weight in pounds by 703.
4. Divide the result of step 3 by the result of step 2. The quotient is your BMI in the same kg/m² units used everywhere else.
Heavier weights and shorter heights push the BMI up; lighter weights and taller heights push it down. For the exact number for your own height and weight, use the BMI Calculator rather than estimating by hand.
WHO weight categories by BMI range
After you compute a BMI value, the next step is to place it in one of the WHO adult weight categories. The cut-offs were chosen as population-level screening thresholds, not as individual targets, and they are the same numbers used by clinicians and public-health bodies worldwide.
| BMI (kg/m²) | WHO weight category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight (healthy range) |
| 25.0 to 29.9 | Overweight |
| 30.0 or above | Obese |
The ranges are continuous, applied at the boundaries as <18.5, <25, <30, and ≥30 respectively, so a BMI of exactly 25.0 sits in the overweight band and a BMI of exactly 18.5 sits in the normal band. According to the World Health Organization's fact sheet on obesity and overweight, these categories are used as a quick, low-cost triage step in public-health screening, not as a final verdict on any one person's health.
How to calculate BMI step by step using the calculator tool
If you would rather skip the arithmetic, the BMI Calculator applies both formulas in your browser and updates the result the moment you change a value or switch unit systems. Everything runs locally — nothing is uploaded.
- Pick a unit system. Click the Metric or Imperial toggle at the top of the tool. Metric expects kilograms and metres; imperial expects pounds and inches.
- Type in your weight. In metric, enter kilograms. In imperial, enter pounds. Use a recent reading from a calibrated scale.
- Type in your height. In metric, enter metres — for example, 1.75 rather than 175. In imperial, enter inches (convert feet and inches to total inches first).
- Read your BMI value. The tool displays the number as soon as both fields are filled, so you can experiment with different heights or weights and see the result update live.
- Check your WHO category. Below the BMI number, the tool labels your reading as underweight, normal weight, overweight, or obese using the WHO adult cut-offs.
For most adults, a single BMI reading is enough for a rough screen. If your number sits near a boundary, type your values again with a slightly different weight or height to see how stable the category is, or switch between metric and imperial to confirm both formulas give the same answer — they should match to within rounding.
What BMI can and cannot tell you
BMI is popular because it requires only two measurements and almost no equipment, which makes it ideal for population studies and quick screenings. It has clear strengths:
- It is reproducible: anyone with a scale and a tape measure can produce a BMI.
- It is comparable across countries because it uses fixed unit definitions.
- It correlates well with several health risks at the population level, which is why the WHO still uses it as a triage tool.
It also has a well-known and important limitation. Because the formula uses only weight and height, it cannot distinguish muscle from fat. A muscular athlete and a sedentary person of the same height and weight receive the same BMI, even though their body composition and health risks differ. BMI also does not account for fat distribution, bone density, age, sex, or ethnicity, all of which affect what a given number means for a given individual. According to the Wikipedia overview of body mass index, these limits are widely recognised in the medical literature.
For these reasons, treat a BMI result as a screening signal rather than a diagnosis. The calculator is an educational estimate and a starting point, not a substitute for professional advice.
Going beyond BMI: when to talk to a professional
If your BMI falls outside the normal-weight band, or sits near one of the cut-offs, the next sensible step is a conversation with a qualified healthcare professional rather than further self-calculations. A clinician can layer in measurements that BMI cannot capture — waist circumference, body-fat percentage, blood markers, family history — and interpret the number in the context of your age, sex, ethnicity, and fitness level.
BMI is also a poor fit for certain groups. It was designed for non-pregnant adults, and the cut-offs were derived from mostly Western populations, so adolescents, pregnant people, very elderly adults, and adults from some Asian populations may need adjusted thresholds or different metrics entirely. If you fall into any of these groups, ask a clinician which weight-status measure applies to you.
The BMI Calculator is best used as a quick, private way to turn two numbers into a single screening value, then to take that value — together with the WHO category — into a real conversation if anything about it concerns you.