- Interest-only mortgage
- A mortgage whose payments cover only interest for an initial period, so the balance does not go down. After that period, the payment is recalculated to repay the full balance over the remaining years of the term. This calculator shows both payments and the increase between them.
- Interest-only period
- The years at the start of the loan when each payment covers only that month’s interest and no principal is repaid. The monthly payment is the loan amount times the rate divided by 12 — $300,000 at 6.5% is $1,625. At the end of the period you still owe the full original amount.
- Amortizing period
- The part of the term after the interest-only period, when payments include principal. Because the full balance must be repaid in fewer years than the full term, the payment is higher than on a standard loan of the same size and rate. With a 30-year term and 10 interest-only years, the balance amortizes over 20.
- Payment increase
- The difference between the amortizing payment and the interest-only payment, sometimes called payment shock. Even with the same rate, as this calculator assumes, the increase can be large because principal repayment starts all at once. Borrowers often plan to sell or refinance before this point, which depends on future values and rates.
- Principal
- The amount borrowed and still owed, not counting interest. Payments reduce principal only when they exceed the interest due. On an interest-only mortgage principal stays at the original loan amount throughout the interest-only period, so no equity is built through payments — only through a down payment or rising home value.
- Interest rate
- The annual percentage the lender charges on the outstanding balance, before fees. This calculator divides it by 12 for the monthly rate and applies the same rate in both the interest-only and amortizing periods. It is not the APR, which also reflects certain loan fees.
- Loan term
- The total number of years over which the loan is scheduled to be repaid, including both the interest-only and amortizing periods. A longer interest-only period within the same term leaves fewer years to repay the balance, which raises the later payment and total interest.
- Amortization
- Paying off a loan through equal scheduled payments that each cover that month’s interest plus part of the principal, so the balance reaches zero by the last payment. Early payments are mostly interest; the principal share grows over time. Here amortization begins only once the interest-only period ends.
- Total interest
- The sum of all interest paid over the life of the loan if every payment is made on schedule. Interest-only loans generally carry more total interest than fully amortizing loans at the same rate, because no principal is repaid early. Total cost here is the loan amount plus this figure.
