2 This calculator was created in response to an article I read about what to do with extra money at the end of the month. Apply it to the Mortgage or invest it? I wrote an article explaining the exact numbers. This calculator can be used to figure it out for yourself. The extra payment you enter in this calculator will be applied at the start of the plan (not the start of the mortgage, unless those coincide). The calculator assumes that after you’ve paid off the mortgage, the entire mortgage payment along with the extra payment is redirected to investing. […]