How to make this calculation in asp.net

Hi,

I have a textbox(txtamt) to enter some amount(decimal value)

In the next textbox(txtpercentage) i am asking user to enter some percentage and in another textbox(txtvalue) i have to calculate the percentage value for the amount entered .. Like suppose if user enters 100 as amount and 1% so in the 3rd text box i will be showing 1 which is the calculated value for 1% on 100 amount. Similarly when user enters some other percentage for the same amount how do i calculate it.. I mean at what event should i write the calculation.. ?