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Body Mass Index (BMI): Chart, Calculation, & Healthy BMI Ranges ' Expert Bulletin
BMI Calculator

Assuming you are in the USA and use these units:

W is your weight in pounds
H is your height in inches

BMI = W / H^2 x 703.

So if your height is 5' 10" (70") and weight is 180 lbs

BMI = 180 / (70 x70) x 703 = 25.82

So <18.5 is thin
18.6 to 24.9 is healthy
25 to 29.9 is overweight
>30 is obese