20-25 is supposed to be a 'normal' bmi...
Less than 20 is supposed to be underweight...
So according to your body mass index... you aren't underweight...
but considering you've posted in the fitness forum... you may get some people who are built like tanks replying that would be morbidly obese according to bmi... so one of the main problems with this measure is that it doesn't differenciate between muscle and fat... so obviously it isn't a perfect measure but it is gives you an idea...