LOANS & DEBT
Interest-Only Mortgage Calculator
By Worldtickers ·
Use our free interest-only mortgage calculator to find your monthly payment during the interest-only period and the higher, fully-amortizing payment that follows once principal payments begin. Includes the formula, worked examples, and the risks to weigh before choosing this loan structure.
This interest tool focuses on use our free interest-only mortgage calculator to find your monthly payment during the interest-only period and the higher, fully-amortizing payment that follows once principal payments begin. Includes the formula, worked examples, and the risks to weigh before choosing this loan structure. Use it to compare borrowing costs, monthly payments, interest charges, payoff timelines, and refinance or repayment choices by changing the rate, term, balance, and payment assumptions.
Interest-Only Mortgage Calculator
Interest-Only Mortgage Calculator
Enter your loan amount, rate, interest-only period, and total loan term to see your payment during the interest-only years and what it jumps to once principal payments begin.
What Is an Interest-Only Mortgage?
An interest-only mortgage is a home loan structured so that, for an initial stretch of years, your monthly payment covers only the interest accruing on the balance — none of it reduces the principal you borrowed. That initial stretch is called the interest-only period, commonly 5, 7, or 10 years, after which the loan switches to a standard amortizing schedule for whatever term remains.
The appeal is straightforward: because you aren't paying down principal, the required monthly payment during the IO period is meaningfully lower than it would be on a fully amortizing loan of the same size and rate. Some borrowers use that lower payment to free up cash flow for other goals, while others use it as a bridge — planning to sell, refinance, or make a large lump-sum principal payment before the IO period runs out.
The tradeoff is equally straightforward: because the balance never shrinks during the IO period, the eventual amortizing payment has to repay the entire original loan amount in fewer years than a standard mortgage would have had. That produces a payment jump — sometimes a large one — the moment the IO period ends. Our ARM calculator is useful alongside this one if your interest-only loan also carries an adjustable rate, since both the amortization restart and a rate reset can hit at the same time.
How to Use This Calculator
Enter four numbers and the calculator does the rest.
Loan Amount
The total amount being borrowed (or the current balance, if you already have the loan). This is the figure both the interest-only payment and the eventual amortizing payment are calculated from, since no principal is paid down in between.
Interest Rate
The annual interest rate on the loan, entered as a percentage. This calculator assumes the rate stays constant through both phases; if your loan's rate can adjust after the IO period, pair this result with our ARM calculator to see the combined effect.
Interest-Only Period
How many years the interest-only phase lasts — typically stated directly in your loan terms, such as "10 years interest-only."
Total Loan Term
The full length of the loan, including the interest-only period — for example, a 30-year loan with a 10-year IO period amortizes over the remaining 20 years once the IO period ends. This must be longer than the interest-only period, and the calculator will flag it if it isn't.
The Formula Explained
During the interest-only period, the formula is just: Interest-Only Payment = Loan Amount × Monthly Interest Rate, where the monthly interest rate is the annual rate divided by 12 (as a decimal). Nothing more complicated is happening — the balance never moves, so the interest charge, and therefore the payment, never moves either.
After the interest-only period ends, the loan switches to the standard amortization formula, applied to the original loan amount over the months remaining: M = P × r × (1 + r)^n ÷ [(1 + r)^n − 1], where P is the loan amount, r is the monthly interest rate, and n is the number of monthly payments left (Total Loan Term − Interest-Only Period, converted to months). Because P is unchanged from day one — no principal was ever paid down — the entire original balance now has to be repaid in a shorter window than the loan's full term, which is exactly why the new payment is higher.
You can think of it this way: a standard 30-year loan amortizes $400,000 over 360 months from the start. An interest-only loan with a 10-year IO period instead amortizes that same $400,000 over just 240 months (30 years − 10 years), but only once those 240 months begin — which is why the post-IO payment on an interest-only loan is always higher than the payment on an equivalent standard loan of the same size, rate, and total term.
Worked Examples
Example 1: $400,000 Loan, 6% Rate, 10-Year IO Period, 30-Year Term
Monthly rate = 6% ÷ 12 = 0.5% = 0.005. Interest-only payment = $400,000 × 0.005 = $2,000.00 per month, and it stays exactly $2,000.00 for all 120 months of the IO period, since the balance never drops below $400,000.
Once the IO period ends, 20 years (240 months) remain, and the full $400,000 must be amortized over those 240 months at 0.5% monthly: M = 400,000 × 0.005 × (1.005)^240 ÷ [(1.005)^240 − 1] ≈ $2,865.72 per month — a jump of $865.72, or about 43% higher than the interest-only payment. Someone budgeting around the initial $2,000 payment needs to be ready to absorb that increase in year 11.
Example 2: $250,000 Loan, 5% Rate, 7-Year IO Period, 30-Year Term
Monthly rate = 5% ÷ 12 ≈ 0.4167% = 0.0041667. Interest-only payment = $250,000 × 0.0041667 ≈ $1,041.67 per month for the first 7 years (84 months).
After the IO period, 23 years (276 months) remain to amortize the same $250,000: M = 250,000 × 0.0041667 × (1.0041667)^276 ÷ [(1.0041667)^276 − 1] ≈ $1,526.01 per month — an increase of about $484.34, or roughly 46%. Notice the payment increase here is proportionally similar to Example 1 even though the loan is smaller, because a shorter IO period relative to the total term produces a bigger percentage jump; a 7-year IO period out of 30 years leaves less time to spread the balance than a 10-year IO period would on a 40-year loan, for instance.
Tips and Risks
Plan for the Payment Jump in Advance
Don't treat the low interest-only payment as your long-term budget number. Calculate the post-IO payment on day one (this calculator does it instantly) and confirm you could afford it if your plans to sell, refinance, or pay down the balance don't pan out on schedule.
Extra Principal Payments Are Your Safety Valve
Because most interest-only loans allow optional extra principal payments, even modest additional payments during the IO period lower both your current interest charge and the balance that eventually gets amortized — directly shrinking the size of the future payment jump.
Don't Count on Appreciation for Equity
Since your payments build no equity during the IO period, your equity position depends entirely on your down payment and home price movement. If prices are flat or fall, refinancing options can be limited right when you need them most — near the end of the IO period.
Check Whether the Post-IO Rate Can Adjust
Some interest-only loans are fixed-rate throughout; others convert to an adjustable rate once the IO period ends. If yours is the latter, run our ARM calculator using the remaining balance and term this calculator produces, to stress-test a worst-case rate on top of the amortization restart.
Frequently Asked Questions
What is an interest-only mortgage?
An interest-only mortgage lets you pay only the interest owed on the loan for a set number of years — the interest-only (IO) period — with no portion of your payment going toward the principal balance. Because none of the balance shrinks during that stretch, your payment is lower than a standard amortizing loan of the same size and rate. Once the IO period ends, the loan converts to a regular amortizing schedule over whatever term remains, and the payment increases because it must now also cover principal on the full original balance.
How is the interest-only payment calculated?
During the interest-only period, the math is simple: monthly payment equals the loan balance multiplied by the monthly interest rate (the annual rate divided by 12, expressed as a decimal). There is no amortization formula involved because nothing is being paid down — the same calculation repeats every month for as long as the IO period lasts, since the balance never changes. This is why the payment stays perfectly flat until the day the interest-only period ends.
What happens to my payment after the interest-only period ends?
The loan re-amortizes: the lender takes the original loan balance (since none of it was paid down) and spreads it over the remaining term using the standard amortization formula, at whatever rate applies at that point. Because the same principal now has to be repaid in fewer years, and interest is added on top, the new payment is noticeably higher than the interest-only payment — often by 30% to 50% or more, depending on how much term is left and the interest rate.
Do interest-only mortgages build any equity?
Not from your payments. Since none of your monthly payment reduces the principal during the IO period, your only source of equity growth is home price appreciation (or a down payment you made at closing) — not paydown from your own payments. If home values are flat or fall during your IO period, you could still owe exactly what you borrowed years later, which is an important risk to weigh against the lower initial payment.
Who are interest-only mortgages best suited for?
They tend to suit borrowers with irregular or back-loaded income — commissioned salespeople, business owners, or people expecting a near-term bonus or liquidity event — who want lower payments now and plan to pay down principal in a lump sum, refinance, or sell before the IO period ends. They are generally a poor fit for buyers who need the lowest possible long-term payment or who might struggle to absorb the payment jump when the IO period expires.
Can I pay extra principal during the interest-only period?
In most cases yes — interest-only mortgages typically allow optional extra principal payments even though they aren't required. Paying extra during the IO period directly reduces your balance, which lowers your interest-only payment going forward (since it is balance × rate) and also shrinks the post-IO amortizing payment, because that payment is calculated on whatever balance remains when the IO period ends rather than on the original loan amount. Check your specific loan documents, since terms vary by lender.
What are the main risks of an interest-only mortgage?
The biggest risk is payment shock: the jump from the interest-only payment to the fully amortizing payment can be large and sudden, and it happens automatically whether or not your finances have changed by then. A second risk is having built no equity through payments, which limits your options — refinancing, selling, or borrowing against the home — if property values haven't risen. Rate risk also applies if the loan has an adjustable rate for its post-IO phase, since payments could rise for two reasons at once: amortization starting and the rate resetting.
Is an interest-only mortgage the same as an ARM?
No, though the two are often combined. "Interest-only" describes how principal is (or isn't) paid down during an initial period; "ARM" (adjustable-rate mortgage) describes how the interest rate behaves over time. A loan can be interest-only with a fixed rate the whole time, or interest-only with a rate that adjusts after the IO period, or fully amortizing from day one with an adjustable rate. If your loan is both, use our ARM calculator alongside this one to see how a rate change layers on top of the IO-to-amortizing payment jump.