BMI Calculator
Calculate BMI from metric or imperial weight and height, then labels the index with a standard category.
How to use
- Set Unit to Metric (kg, cm) or Imperial (lb, in). The field labels switch to Weight (kg) and Height (cm), or to Weight (lb) and Height (in).
- Enter weight and height. Both have to be numbers greater than zero.
- Click Calculate BMI. The result shows the index to one decimal and a category. The detail line repeats the cut points.
When it helps
Metric BMI is weight in kilograms divided by height in meters squared. Imperial BMI is weight divided by height squared, then multiplied by 703. The labels on the result are Underweight below 18.5, Normal weight from 18.5 to 24.9, Overweight from 25 to 29.9, and Obesity at 30 or above.
If you want resting calories from sex, age, weight, and height, use the BMR calculator. The calorie calculator multiplies that resting figure by an activity factor. This page stops at the index and the category.
Limits
The number and category are a screening estimate, not a diagnosis of illness or fitness. The formula does not know muscle versus fat, so two people with different builds can share a category.
The form does not ask age, sex, waist, or pregnancy. Blank or non-positive values show Enter valid values. Changing Unit changes the labels, but it does not convert the numbers already typed. You still enter kilograms with metric, or pounds with imperial.
Questions
Where do the categories change?
Underweight is below 18.5, Normal weight is 18.5 through 24.9, Overweight is 25 through 29.9, and Obesity starts at 30. The detail line prints those same bounds.
Why can the same BMI look different on two people?
Only weight and height go into the formula. It cannot separate muscle from fat.
Does imperial use different cut points?
No. Both units use the same category lines. Only the arithmetic that produces the index changes, including the 703 factor for inches and pounds.
BMI is only one number. For related calculators on calories, metabolic rate and a broader health context, see the free tools on Longevity Health.