An interest only period can end with a larger required payment when principal repayment begins. This calculator estimates that payment from the balance, interest rate and repayment time remaining at the change. It shows the monthly increase and the effect on cash flow under your assumptions.
What you enter
Enter the balance expected when interest only ends, the current interest only rate, the total original term and the length of the interest only period. Enter the rate you want to model after the change. If you know the remaining repayment term directly, use that instead of reconstructing it.
Use the loan documents to confirm whether the rate also changes and whether the balance is amortized or becomes due. This calculator models monthly amortization after the interest only period. A balloon payoff or a different payment schedule needs its own analysis.
Keep taxes, insurance and HOA dues outside the principal and interest calculation, then add them to compare full housing costs. If those bills are expected to change, use separate before and after amounts. A flat tax assumption can otherwise hide another payment increase.
Example
Assume a $300,000 balance and a hypothetical example rate of 6.00% as of October 16, 2026. During interest only, monthly interest is $1,500 under this example's monthly rate convention.
Assume a 30 year total term with the first ten years interest only. After that period, 20 years remain to repay principal. If the balance and example rate are unchanged, a fully amortizing payment is about $2,149 a month.
The increase is about $649, or 43.29%. With rent and other costs unchanged, monthly cash flow falls by the same dollar amount. These are made up loan terms for a calculation, not an offered program or rate.
What happens to the payment when interest only ends?
Under this model, principal repayment begins over the remaining term. The same principal must be repaid over that remaining time rather than over a new full original term.
See Interest Only vs. Amortizing DSCR Loans for the broader choice. The calculator handles the future payment arithmetic rather than choosing the loan structure for you.
How much higher is the new payment?
It is the difference between the modeled amortizing payment and the interest only payment. The percentage increase divides that difference by the original payment.
Change the reset rate separately to see its effect. Do not assume an adjustable rate will stay at today's rate just because a calculator defaults to it.
What happens to my cash flow?
With unchanged revenue and other costs, the payment increase reduces cash flow by that amount. Add changed housing charges or operating expenses to build a fuller future budget.
An estimated positive result does not establish that future rent will support the payment. It simply tests the figures you entered.
What is my balance at the reset?
If you have paid only interest and made no principal reductions or other balance changes, the principal stays the same in this model. Enter the expected actual balance if you plan principal payments.
This calculator gives an estimate only. It is not a loan offer or approval.
Sources
- Fannie Mae Mortgage Calculator. Disclaimer; How to use this calculator. Checked October 16, 2026.