📉 Interest-Only Mortgage Calculator

Estimate your monthly payment during an interest-only period, the higher principal-and-interest payment once the interest-only period ends, and how much extra interest an interest-only loan costs compared to a standard fully-amortizing mortgage.

Interest-Only Monthly Payment

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Payment After IO Period — Total Interest (IO Loan) — Standard Loan Payment — Standard Loan Total Interest — Extra Interest Cost of IO —

Interest-Only vs. Standard Loan Comparison

Metric	Interest-Only Loan	Standard Loan

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What Is an Interest-Only Mortgage?

An interest-only mortgage lets a borrower pay only the interest charge on the loan for an initial period — commonly 5 to 10 years — without reducing the principal balance at all. After this interest-only period ends, the loan converts to a standard amortizing loan, where the remaining principal must be fully repaid over the remaining term, typically causing a significant jump in the monthly payment. Interest-only loans are used by borrowers who expect rising income, plan to sell or refinance before the payment increases, or want to free up cash flow for investing elsewhere. Because none of the principal is paid down during the interest-only period, the borrower builds no equity through payments alone during that time — only through market appreciation.

How Interest-Only Payments Are Calculated

During the interest-only period, the monthly payment is simply the loan balance multiplied by the monthly interest rate: Payment = Loan Amount × (Annual Rate ÷ 12). Because the balance never decreases, this payment stays flat throughout the interest-only period. Once that period ends, the lender re-amortizes the full original principal over the remaining loan term using the standard amortization formula, M = P × r × (1 + r)^n / ((1 + r)^n − 1), where n is now the number of months remaining. Because the same principal must now be repaid over fewer months, the post-interest-only payment is substantially higher than a standard mortgage payment would have been from day one.

The "Payment Shock" After the Interest-Only Period

The most important risk of an interest-only mortgage is payment shock — the sudden, often dramatic increase in the required monthly payment once the interest-only period expires and full amortization begins. Because the entire original principal must be paid off in the remaining, shorter time frame, the new payment can be meaningfully higher than what a fully-amortizing loan of the same size and rate would have required from the start. Borrowers should calculate this future payment well in advance and confirm they can afford it, rather than assuming they will refinance, sell, or have higher income by the time the interest-only period ends.

Interest-Only vs. Standard Fully-Amortizing Mortgages

A standard mortgage begins reducing principal from the very first payment, so the loan balance steadily declines and is fully repaid by the end of the term. An interest-only mortgage delays all principal reduction, which lowers payments early on but results in more total interest paid over the life of the loan, because a larger balance accrues interest for longer. This calculator directly compares the two structures side-by-side — total interest, monthly payments, and total cost — so you can see exactly how much more an interest-only structure costs. For a standard amortizing option, see our mortgage calculator or full amortization calculator.

Who Uses Interest-Only Loans and Why

Interest-only loans are popular with real estate investors who plan to sell a property before the interest-only period ends, borrowers with irregular or rising income (such as commission-based earners or business owners), and high-net-worth borrowers who prefer to invest the cash they would otherwise put toward principal. They can also appeal to buyers in high-cost markets seeking lower initial payments to qualify for a larger loan. However, lenders scrutinize these loans carefully since they carry higher long-term risk, and interest-only terms are less common in conventional owner-occupied mortgages since the 2008 financial crisis prompted stricter underwriting standards.

Building Equity Without Principal Paydown

Because interest-only payments do not reduce the loan balance, the only ways to build equity during the interest-only period are property value appreciation or making optional extra principal payments (allowed by most interest-only loan agreements). Borrowers relying solely on market appreciation take on additional risk: if property values stagnate or decline, they may owe as much as — or more than — the property is worth when the interest-only period ends, complicating any plan to sell or refinance. Making voluntary extra principal payments during the interest-only period, even small ones, directly reduces both future interest costs and the size of the post-IO payment.

Deciding If an Interest-Only Mortgage Is Right for You

An interest-only mortgage can make sense for disciplined borrowers with a clear plan to sell, refinance, or substantially increase income before the interest-only period ends, and who understand and can absorb the higher payment that follows. It generally costs more in total interest than a standard mortgage and carries more risk if plans change. Before committing, run the numbers with this calculator, compare against a standard loan calculator or mortgage calculator, and consider consulting a licensed mortgage professional to stress-test your ability to handle the post-interest-only payment under less favorable future conditions.

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Does my payment ever go down after the interest-only period?

No. Once the interest-only period ends, the loan is re-amortized so the full remaining principal is repaid over the remaining term, which produces a higher monthly payment than the interest-only payment, not a lower one.

Can I pay extra principal during the interest-only period?

Most interest-only loans allow optional extra principal payments during the interest-only period. Doing so reduces the balance that must be repaid later, lowering both future interest and the post-IO payment.

Why is the post-interest-only payment so much higher than a standard mortgage payment?

Because none of the principal was paid down during the interest-only years, the full original loan amount must be repaid over a shorter remaining term, which requires a larger monthly payment than if amortization had started on day one.

Do interest-only loans always cost more overall?

Generally yes — because the balance does not decrease during the interest-only period, more total interest accrues over the life of the loan compared with a standard fully-amortizing loan of the same amount, rate, and term.

Are interest-only mortgages common today?

They are less common for owner-occupied conventional mortgages since underwriting tightened after 2008, but they remain available through certain lenders and are more commonly used in investment property and jumbo loan financing.

Is this calculator financial advice?

No. This tool provides estimates for educational purposes only and is not financial, legal, or tax advice. Consult a licensed mortgage or financial professional before choosing a loan structure.