Interest-Only Formula Explained
The Interest-Only formula explained in plain English: the equation behind the Interest-Only Calculator, its variables, and the assumptions it makes.
By Onias Derilus, Mortgage Capital · NMLS# 1859012 · Last Updated: June 2026
It estimates payments on an interest-only mortgage during the interest-only period. It also shows the higher fully-amortizing payment that begins once principal repayment kicks in.
The Interest-Only Formula, Explained
IO payment = balance × rate / 12; later payment amortizes over remaining term
Interest-only payments are lower because they do not reduce the balance. The catch is that when the interest-only period ends, the same balance must be repaid over fewer remaining years, which raises the payment sharply.
These loans suit borrowers with irregular income or a clear plan to sell, refinance, or pay a lump sum before the payment jumps. The calculator shows both payment levels so the step-up is visible.
The formula is only the starting point. Open the interest-only calculator to plug in your own numbers and see the result instantly. For a rate tied to your actual file, talk to a licensed broker before you decide.
Turn Your Interest-Only Estimate Into a Real Pre-Approval
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Rates are illustrative only. APR and payments vary by credit score, loan amount, and market conditions. Subject to credit approval. Not a commitment to lend. NMLS# 1859012. Equal Housing Lender.