Overtime Pay per hour? Time and a half Taxed more so than regular pay - system of doing so?

Can anyone explain to me how we are taxed differently for income after overtime (40 hours) while getting time and a half?

I like to work on my weekly budget before i get paid but this is the first time I have to calculate Time and a half - and I have no idea how to work out thetaxes for it because I know its not the same as regualr pay.=

paycheckcity.com has a pretty good paycheck calculator.