Short term loan calculator

[Stu Bennett]

New investor and member here so forgive the dumb question! I found a property that needs to be a cash deal.  I’d like to pay half with my equity line from another property, and then ask the owner to finance the other half for a two year period then i'll refinance and pay him off. If i offer a set interest rate (assuming something slightly over market rate)  i'm struggling with what the payment structure looks like. Do I base the payback on a 30 year fixed? Is there a calculator for such deals?