It's important to know how much your monthly payments will be. This calculator will give you a rough estimate. Your lender will of course give you more accurate details. It's important to know how much your monthly payments will be. A mortgage calculator is a springboard to helping you estimate your monthly mortgage payment, broken down by principal and interest. You’ll also be able to take it a step further by adding your amortization period. Your lender will of course give you more accurate details.