Your body mass index (BMI) is easy to calculate. Simply multiply your weight by 703 then divide by your height in inches twice.

Example:
Your weight is 140 lbs
* 703
= 98420
You are 5’6″ tall, which is 66 inches
98420 / 66 = 1491.2121
1491.2121 / 66 = 22.59 BMI

For an adult, a BMI of 25-29 is considered overweight and a BMI of 30+ is considered obese. Below 18.5 is considered underweight for an adult.

cdc.gov provides an easy to use Body Mass Index Calculator and a Body Mass Index for Adults Table as well.