Interest Only Calculator FAQ

An interest-only loan keeps the monthly payment lower during the IO period because you only pay interest on the outstanding balance without reducing principal. This improves short-term cash flow but means the balance stays the same until the IO period ends, at which point the payment increases to amortize the remaining term.

The calculator switches to a standard amortizing payment based on the remaining original term. Because the principal has not been reduced during the IO period, the post-IO payment is typically higher than both the IO payment and what a fully amortizing payment would have been from the start. The results show this jump clearly so you can plan for it.

A HELOC or line of credit usually tracks the drawn balance rather than the full credit limit. The calculator lets you enter the credit limit separately from the drawn amount so you can model scenarios where you borrow less than your maximum. This distinction matters because you only pay interest on what you actually draw.

Enter 2 to 5 draw rows, each with a month index and a dollar amount. The calculator adds each new draw to the running balance before charging monthly interest for that period. This means the payment starts low and increases as draws are funded, mirroring how real construction loans disburse money at project milestones rather than all at once.

The result cards show the starting IO payment, the payment after IO ends, and the cost delta versus a fully amortizing path when a total term is present. The delta compares total interest paid under each scenario so you can weigh the cash flow benefit of the IO period against the higher total cost over the full loan term.

An IO loan can make sense when you expect higher income in the future, plan to sell or refinance before the IO period ends, or want to maximize cash flow for other investments. However, it costs more in total interest over a full term and carries the risk of payment shock when the IO period ends. The calculator helps you compare these tradeoffs quantitatively.

Use the rate you expect to pay during the IO period. For fixed-rate IO loans, this is straightforward. For adjustable-rate products, use your best estimate of the average rate over the IO term. The calculator assumes a constant rate throughout, so testing multiple rate scenarios is a good way to understand sensitivity.