Interest Only Loan Calculator
By Talcart · Last updated July 10, 2026
Interest Only Calculator Guide
Understanding Interest Only Loans
Payment Structure
Initial period: Interest payments only
After IO period: Principal + Interest
Balloon payment at end
Calculation Method
Monthly Interest = Principal × (Annual Rate / 12)
Example: $200,000 loan at 4% = $666.67 monthly interest
Interest Only Loan Calculator
An interest-only calculator shows what a loan costs while only interest is due: $300,000 at 6% is $1,500 a month interest-only, versus $1,798.65 fully amortizing over 30 years. Just as important, it shows the payment shock afterwards - once a 10-year IO period ends, that same loan re-amortizes over 20 years at $2,149.29 a month, a 43% jump.
Key facts
- $300,000 at 6% costs $1,500 a month interest-only versus $1,798.65 amortizing over 30 years - 16.6% less, but the balance never falls.
- With a 10-year IO period on a 30-year term, the payment then rises to $2,149.29 (+43.3%), and lifetime interest totals $395,830 versus $347,514 on a standard 30-year loan.
- IO payment = principal x annual rate / 12; every 1% of interest rate adds exactly $250 a month per $300,000 borrowed.
What is the Interest Only Calculator?
An interest-only loan is a loan on which the borrower pays only the interest charge for an initial period - typically 5 to 10 years - so the principal balance stays unchanged until amortization begins. During the IO phase the payment is at its minimum and no equity is built from payments. When the period expires, the full balance must amortize over the remaining term, be refinanced, or be repaid from a sale. Under US qualified-mortgage rules an interest-only feature disqualifies a loan from QM status, so lenders underwrite these as non-QM products.
How does the Interest Only Calculator work?
During the IO period the payment is pure interest: payment = principal x annual rate / 12, so every 1% of rate costs $250 a month per $300,000 borrowed. The calculator then re-amortizes the untouched balance over the remaining months with M = P x r x (1 + r)^n / ((1 + r)^n - 1). For $300,000 at 6% on a 30-year note with a 10-year IO period, n falls to 240 and the payment rises from $1,500.00 to $2,149.29.
What is the Interest Only Calculator formula?
- P – outstanding principal
- r – monthly rate
$300,000 loan, 30-year term with a 10-year interest-only period
| Rate | IO payment | 30-yr amortizing payment | Payment after IO ends | Jump vs IO |
|---|---|---|---|---|
| 5% | $1,250.00 | $1,610.46 | $1,979.87 | +58.4% |
| 5.5% | $1,375.00 | $1,703.37 | $2,063.66 | +50.1% |
| 6% | $1,500.00 | $1,798.65 | $2,149.29 | +43.3% |
| 6.5% | $1,625.00 | $1,896.20 | $2,236.72 | +37.6% |
| 7% | $1,750.00 | $1,995.91 | $2,325.90 | +32.9% |
| 8% | $2,000.00 | $2,201.29 | $2,509.32 | +25.5% |
How do you use the Interest Only Calculator?
- Enter principal, rate, IO period and remaining term.
- See both phases of payment.
Worked example
| Scenario | $500,000 at 6%, 10-year IO, 30-year total |
| Calculation | IO: 500,000 × 0.005 = $2,500; Year 11+: amortize $500,000 over 240 months |
| Result | IO $2,500 → $3,581 after IO ends. |
Common use cases
Tips & best practices
IO loans can leave you owing the full principal — make sure you have an exit plan.
Frequently asked questions
Only for borrowers with a clear exit plan and disciplined cash flow - for example, investors intending to sell before the reset, or people with irregular income who will prepay principal voluntarily. The payment is lower now (16.6% lower at 6% on a 30-year comparison) but the debt never shrinks, total interest is higher, and the post-IO payment jump is substantial.
On a 30-year loan with a 10-year IO period, the jump ranges from about 25% to 58% depending on rate. At 6% on $300,000 the payment climbs from $1,500.00 to $2,149.29 (+43.3%); at 8% it goes from $2,000.00 to $2,509.32 (+25.5%). Lower rates produce bigger percentage jumps because principal dominates the new payment.
No. The scheduled IO payment covers only the interest accrued, so the balance you owe is identical at the end of the period to the day you borrowed. Any equity during the IO phase comes solely from your original down payment and from price appreciation - and it can turn negative if the property's value falls.
Yes, if you keep it to term. On $300,000 at 6% over 30 years, the IO structure (10 years at $1,500, then 20 years at $2,149.29) accrues $395,830 in lifetime interest versus $347,514 on a standard amortizing loan - about $48,300 more, because the full balance keeps accruing interest for the first decade.
Generally borrowers with strong credit, larger down payments, and documented assets or income, because these loans sit outside the US qualified-mortgage framework. Lenders typically underwrite the borrower against the higher post-IO amortizing payment, not the teaser IO payment, to make sure the reset is affordable.
Usually yes - most IO loans accept voluntary principal payments, and each one immediately lowers the next month's interest charge. Paying $50,000 down on a $300,000, 6% IO loan cuts the monthly interest from $1,500 to $1,250 and shrinks the eventual amortizing payment, since the post-IO payment is computed on whatever balance remains.
One of four things: the loan converts to an amortizing schedule over the remaining term (the standard structure), you refinance into a new loan, you sell the asset and repay the balance, or - on balloon variants - the entire principal falls due at once. Knowing which applies before signing is essential, because the full original balance is still outstanding.