Debt Investment Calculator - One-time Investment This calculator will compute how much interest you will save by adding an additional principal payment to the next regularly scheduled payment on any given debt. For example, if you find yourself with an extra $20 this month, this calculator will tell you how much interest you will save (guaranteed, tax-free return on your investment) if you add that $20 to the next payment of one of your debts (preferably the one with the highest interest rate). Enter the principal balance owed: Enter the annual interest rate: Enter the regular monthly payment amount (principal and interest only): Enter the amount to add to the next payment: Current Payoff Term (Months): Revised Payoff Term (Months): Time Saved (Months): Current Interest Cost: Revised Interest Cost: Total interest savings (Return on investment): Guaranteed Annual Rate of Return: