BMI Calculator

For most adults a healthy BMI is 18.5–24.9; below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese, per WHO and CDC classifications.

What your BMI does and doesn't tell you

Body Mass Index (BMI) is a quick screening number that compares your weight to your height. It is calculated as weight in kilograms divided by height in metres squared. Because it takes seconds to work out and needs no special equipment, health services use it to flag whether an adult may be underweight, a healthy weight, overweight, or living with obesity.

The value is useful at a population level, but it has real limits for individuals. BMI does not distinguish muscle from fat, so very muscular people can read as "overweight" even with low body fat. It also says nothing about where fat is stored — carrying weight around the waist carries more health risk than the same weight on the hips and thighs. For these reasons, treat BMI as one signal among several rather than a verdict on your health.

How to read your BMI category

• Under 18.5 — underweight. May signal undernutrition or an underlying condition worth discussing with a clinician.
• 18.5 to 24.9 — healthy weight. Associated with the lowest risk for most adults.
• 25 to 29.9 — overweight. A cue to look at habits; a small, steady change often moves the number.
• 30 and above — obese. Linked to higher risk of type 2 diabetes, heart disease, and joint problems.

If your number sits near a boundary, don't over-read a single decimal point — day-to-day weight shifts with hydration and food. Track the trend over weeks instead. To go deeper, combine BMI with a waist measurement, a body-fat percentage estimate, and how you feel day to day.

BMI categories for adults

Adult BMI cut-offs per WHO and CDC. Some Asian populations use a lower overweight threshold (BMI 23+).

A few hundred grams decide your category, and twenty kilograms do not

BMI has an odd property that the tidy table above hides: it is brutally precise at the boundaries and completely indifferent everywhere else. Both facts fall straight out of the arithmetic, and you can check them against the range this calculator prints for you.

Take someone 175 cm tall. The healthy band runs from 56.7 kg to 76.3 kg — 19.6 kg wide. Anywhere inside that span BMI says the same thing about them: normal weight. Two people at the same height, one at 57 kg and one at 76 kg, get an identical verdict. But at the top edge, moving from BMI 24.9 to 25.0 — normal weight to overweight — takes 306 grams. A large glass of water, drunk before you stand on the scale, is enough to change the category.

Band is 18.5 to 24.9 × height in metres squared, which is the same arithmetic this calculator runs to print your own healthy range. The last column is the weight change that moves BMI by 0.1 — one displayed decimal, and at 24.9 the width of a category boundary.

The practical consequence is that a single reading near a cut-off tells you almost nothing. Body weight moves through the day with food, fluid and salt, and at 175 cm every kilogram of that movement is worth a third of a BMI point — several times the distance to the line. If your number lands within about half a point of 18.5, 25 or 30, treat the category as undecided rather than as a result, and weigh yourself the same way on several mornings before believing either side of it.

Read the other way round, the width of the band is the more useful half. Nineteen kilograms of latitude at one height is BMI admitting it cannot tell a heavy-boned, muscular build from a light one — it only ever knew your height and your weight. If you want to know which end of your own band you are at and why, that is a body-composition question rather than a BMI one: the body fat calculator and the waist-to-height ratio both separate the things BMI cannot.

Categories follow WHO and U.S. CDC adult BMI classifications. See CDC, "About Adult BMI," and NHS, "BMI healthy weight calculator."