When you receive some extra money it may be difficult to determine whether you should invest the funds or use them to retire debt. Financial theory recommends that if your after-tax return on investments is greater than your after-tax cost of debt then you should invest. However, remember to consider the inherent riskiness of the investment you select (i.e. you may lose the money you invest yet still have obligations to pay back the liability). Use this calculator to help analyze your situation.