loan amortization

[Jordan Lagrassa]

I recently built a house and with the loan it was for 300K for 30 years . I have heard of you make one extra payment per year to principal it would cut down your loan to about 18 years. My question is does anyone know of a tool I can use to figure out how much I would have to pay every year towards principle to cut the loan down to 7 years . If there is no tool I'm open to any ideas anyone has to help me figure this out. Thanks