BMI Calculator: Free Online Body Mass Index Calculator
Calculate your Body Mass Index using the standard WHO ranges. Private and free — everything runs in your browser.
What BMI Measures — and Its Limits
Body Mass Index (BMI) is a screening measure calculated from height and weight: BMI = weight (kg) ÷ height (m)². Enter your numbers in either metric or imperial units above and the calculator converts everything internally before running that formula, so you never have to do the unit math yourself. The result flags whether your weight sits in a range associated with elevated health risk at a population level, and it uses the same four categories the World Health Organization defined for adults worldwide.
How the calculation works
The calculator converts feet and inches to meters, or pounds to kilograms, depending on which units you pick, then divides your weight in kilograms by the square of your height in meters. Squaring height rather than using it directly is what makes BMI scale reasonably across a wide range of heights: without that square term, taller people with an otherwise proportional build would score higher purely because they weigh more in absolute terms, not because they carry more body fat. Once the result lands in one of four bands, below 18.5, 18.5 to 24.9, 25 to 29.9, or 30 and above, the calculator labels it underweight, normal weight, overweight, or obese and shows the exact range your number fell into.
Limitations
- 💪BMI doesn't distinguish muscle from fat — athletes and heavily muscled people are often classified "overweight" despite low body fat.
- 📏It doesn't account for fat distribution — visceral (abdominal) fat carries more health risk than the same fat elsewhere.
- 🧓It isn't calibrated for children, pregnant people, or the elderly, who need different reference standards.
These limitations exist because BMI was designed as a population-level screening tool for epidemiological research, not an individual diagnostic measure. A single number derived from just height and weight cannot see the difference between a bodybuilder carrying 8% body fat and a sedentary person carrying 30%, if both happen to weigh the same amount at the same height. Treat a high or low BMI as a prompt to look closer, through body composition testing, waist circumference, or a conversation with a healthcare provider, rather than a verdict on your health by itself.
Reading your category
A BMI under 18.5 falls in the underweight range, which can point to inadequate calorie intake, an underlying health condition, or a naturally lean build depending on the rest of the picture. 18.5 to 24.9 sits in the normal weight range most public health guidance targets for the general adult population. 25 to 29.9 is classified overweight, a range associated with modestly elevated risk for conditions like type 2 diabetes and cardiovascular disease at the population level. 30 and above is classified obese, where that associated risk rises further according to WHO data, though individual risk still depends heavily on factors BMI alone cannot capture.
Metric and imperial units
Switch between centimeters and feet-and-inches for height, or kilograms and pounds for weight, using the toggle above each field, and the calculator handles the conversion behind the scenes so the formula always runs on metric values regardless of what you typed in. This matters if you are used to seeing your height quoted in feet and your weight in pounds, common in the US, or in centimeters and kilograms, common almost everywhere else, since you never need to convert anything by hand or look up a conversion factor before getting a result.
Getting an accurate reading
Measure your height without shoes and your weight at a consistent time of day, ideally in the morning before eating, since body weight fluctuates by a pound or two over the course of a normal day from food and water intake alone. Round to the nearest reasonable unit rather than guessing, since a few centimeters or a couple of pounds of error can shift a borderline result into a different category. If you switch between metric and imperial units, double check the converted value looks right before reading the result, since a typo in feet or inches produces a height conversion that throws the whole calculation off.
Common use cases
People track BMI over time as a rough proxy for progress alongside diet or exercise changes, watching the trend rather than fixating on any single reading. Healthcare providers use it as a first-pass screening number before ordering more detailed assessments. Insurance and fitness programs sometimes reference BMI bands as a baseline metric, even though more precise measures like waist-to-hip ratio or DEXA body composition scans capture health risk more accurately for any given individual. Used the way it was designed, as one data point among several rather than a complete health assessment, BMI still serves as a fast, free way to see roughly where you stand.
Related: Percentage Calculator for other quick math, or Unit Converter for converting height and weight between metric and imperial.
Frequently Asked Questions
Is the BMI Calculator free?
Yes — completely free. No account and no data sent to a server; the calculation happens entirely in your browser.
What BMI ranges does it use?
The standard WHO adult ranges: below 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30 or above is obese.
Is BMI accurate for everyone?
No. BMI is a population-level screening tool, not a diagnosis. It doesn't account for muscle mass, bone density, or fat distribution, so athletes and very muscular people can score "overweight" despite low body fat. Talk to a healthcare provider for an individual assessment.