BMI Calculator
Calculate your Body Mass Index from height and weight, in metric or imperial units, with your category.
Content updated August 10, 2026
BMI is a screening measure, not a diagnosis and not a measure of body fat. It does not distinguish muscle from fat or account for where fat is stored. These adult categories do not apply to children, and results should be interpreted with a healthcare professional. Not medical advice.
Body Mass Index (BMI) is a quick screening number that relates your weight to your height. It's widely used by health services as a rough indicator of whether someone is in a healthy weight range for their height. This calculator works in both metric and imperial units and shows your BMI category.
BMI is a general guide, not a diagnosis. It doesn't distinguish muscle from fat, so very muscular people can read as 'overweight' while the number can understate risk for others. Treat it as one data point among many, and talk to a healthcare professional about your individual health.
The BMI formula
BMI = weight (kg) ÷ height (m)² · US customary shortcut: BMI ≈ 703 × weight (lb) ÷ height (in)²weight = body mass · height = standing height. This calculator converts US customary input with the exact factors 1 lb = 0.45359237 kg and 1 in = 2.54 cm and then applies the metric formula; the 703 constant is a rounded shortcut for mental arithmetic.
Worked example
Someone weighing 70 kg at 1.75 m has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9, inside the healthy band. At that height the healthy band spans 18.5 × 3.0625 = 56.7 kg up to but not including 25 × 3.0625 = 76.6 kg.
Assumptions, rounding, and limitations
Assumptions
- The person is an adult. Adult categories do not apply to children or teenagers, who are assessed on age- and sex-specific percentile charts.
- Height is standing height without shoes and weight is current body weight.
- US customary input is converted with the exact definitions 1 lb = 0.45359237 kg and 1 in = 2.54 cm before the metric formula is applied, rather than using the rounded 703 constant.
- Category boundaries are half-open, matching the CDC: healthy weight is 18.5 up to but not including 25, overweight is 25 up to but not including 30, and obesity is 30 or greater.
Rounding: BMI and the healthy weight range are computed at full floating-point precision. BMI is displayed to one decimal place and the weight range to one decimal place; the category is derived from the unrounded value, so a displayed 25.0 is already in the overweight band.
Limitations
- BMI is a screening measure, not a diagnosis and not a measurement of body fat. The WHO describes it as a surrogate marker for fatness.
- It cannot distinguish muscle from fat or identify where fat is stored, so it misclassifies muscular people and can understate risk in others. Waist circumference and other measures add information BMI does not carry.
- Risk thresholds differ across some populations, and pregnancy, older age and athletic training all change how the number should be read.
- This is general health information for education, not medical advice, diagnosis or treatment.
Sources
- Adult BMI Categories — Centers for Disease Control and Prevention
- About Body Mass Index (BMI) — Centers for Disease Control and Prevention
- Obesity and overweight — World Health Organization
Adult BMI categories
The CDC's adult categories are: below 18.5 underweight; 18.5 to less than 25 healthy weight; 25 to less than 30 overweight; and 30 or greater obesity. Obesity is further divided into class 1 (30 to less than 35), class 2 (35 to less than 40) and class 3, also called severe obesity (40 or greater). The World Health Organization uses the same adult thresholds, defining overweight as a BMI at or above 25 and obesity at or above 30.
Note that the intervals are half-open. You will often see the healthy range written as 18.5–24.9, which is a rounding convenience rather than the boundary: a BMI of 24.95 is not in the healthy band. This calculator uses the exact boundaries, so 25.0 reads as overweight.
These ranges apply to adults. They are not appropriate for children or teenagers, who are assessed on age- and sex-specific percentile charts, and clinicians interpret them alongside other measures for pregnant people, athletes and older adults.
How BMI is calculated here
The formula is weight in kilograms divided by height in metres squared: BMI = kg ÷ m².
You will often see the US customary shortcut 703 × pounds ÷ inches². The 703 is a rounded constant. This calculator instead converts your input using the exact definitions — one pound is 0.45359237 kilogram and one inch is 2.54 centimetres — and then evaluates the metric formula, so metric and US customary entries of the same body size return the same number.
What BMI does not tell you
BMI is a surrogate marker. The WHO notes that additional measurements such as waist circumference help in diagnosing obesity, because BMI alone does not describe how much of your weight is fat or where that fat sits.
That limitation runs in both directions. A very muscular person can read as overweight with low body fat, and someone with little muscle can sit inside the healthy band while carrying elevated risk. Treat the number as one screening input, not a verdict.
Frequently asked questions
▶Is BMI accurate?
BMI is a useful population-level screening tool but a blunt instrument for individuals. It can't tell muscle from fat or where fat is stored, so use it as a rough guide alongside other measures and professional advice.
▶What is a healthy BMI?
For adults the CDC defines the healthy range as 18.5 to less than 25. It is usually written as 18.5–24.9, but the real boundary is 25, so a BMI of exactly 25.0 falls into the overweight category. This is a general screening guideline, not a personal medical assessment.
▶How do I calculate BMI manually?
Divide your weight in kilograms by your height in metres squared. For 70 kg at 1.75 m: 70 ÷ (1.75 × 1.75) = 22.9. In imperial units, multiply pounds ÷ inches² by 703.
▶Does BMI work for athletes and very muscular people?
Not well. Muscle weighs more than fat, so muscular people can register as 'overweight' despite low body fat. For them, a body fat measurement is a more useful guide than BMI.
▶Is BMI calculated differently for men and women?
No — the formula and adult categories are the same for both. Body composition does differ between sexes, which is one reason BMI is only a rough screening tool for individuals.
▶Can I use this BMI calculator for children?
No. Children and teenagers are assessed with age- and sex-specific BMI percentile charts, not the adult categories here. Speak to a paediatrician or use a child-specific tool.
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Open calculator →Disclaimer: BMI Calculator results are estimates for general information and education only, and are not financial, tax, legal or medical advice. Verify important decisions with a qualified professional.