See how an interest-only mortgage works during the IO period and what happens when payments reset to fully amortizing. Compare monthly payments, total interest, and remaining balance against a traditional fixed-rate mortgage.
An interest-only (IO) mortgage lets you pay just the interest on the loan for an initial period — typically 5 to 10 years — without paying down any principal. After the IO period ends, the loan converts to a fully amortizing payment that pays off the entire remaining principal over the shorter remaining term. The mechanism produces low monthly payments early in the loan but a large payment increase ("payment shock") when the amortization phase begins.
Interest-only mortgages largely disappeared from mainstream U.S. residential lending after the 2008 financial crisis — they were heavily implicated in subprime defaults because borrowers couldn't afford the post-IO payments. They've gradually returned in specialty lending: jumbo loans for high-income borrowers, investment property loans, and some portfolio products. They're also common in commercial real estate.
This calculator shows both phases of an interest-only loan: the low IO payment during years 1 through the IO term, and the higher amortizing payment from the end of IO through loan maturity. It also compares the lifetime cost against a standard 30-year amortizing loan at the same rate. Critical for borrowers: the post-IO payment is typically 30–80% higher than the IO payment. Affordability of the post-IO payment — not just the initial low payment — is the right test for whether to take an IO loan.
$600,000 loan at 6.75%, 7-year IO period, 30-year total term. IO payment: $600,000 × 0.0675 / 12 = $3,375/month (years 1–7) Post-IO payment: $600,000 × monthly payment factor over 23 years at 6.75% ≈ $4,547/month (years 8–30) Payment shock: +$1,172/month (35% increase) A self-employed borrower with high but variable income may use the IO period to keep payments low during business cycles, then transition to the higher amortizing payment when income stabilizes. Critical: must be able to afford the $4,547 payment when IO ends.
$500,000 investment property loan at 7.5%, 10-year IO period, 30-year total term. IO payment: $500,000 × 0.075 / 12 = $3,125/month (years 1–10) The low IO payment maximizes monthly cash flow from the rental property — useful for cash-on-cash return calculations during the early years of ownership. Many investors plan to sell or refinance before the IO period ends. Post-IO payment at $500K over 20 years at 7.5%: $4,029/month Payment shock: +$904/month Risk: if the property doesn't appreciate, refinancing or selling becomes harder when IO ends. The lower payments mask the underlying capital structure cost.
$400,000 loan at 6.5%, 10-year IO, 30-year total. Borrower pays IO ($2,167) plus voluntary $500/month principal. By year 10: principal paid voluntarily ≈ $60,000. Remaining balance: $340,000. Post-IO payment on $340,000 over 20 years at 6.5%: $2,536/month (vs. $2,983 without voluntary payments). Payment shock with voluntary payments: +$369 (vs +$816 without). This strategy uses the IO loan's flexibility — pay extra in good months, just IO in lean months — without the rigid commitment of a standard mortgage. Disciplined borrowers can achieve substantial benefit; undisciplined borrowers end up with the full post-IO shock.
Use this calculator when considering an interest-only mortgage for a primary residence, evaluating IO financing for investment property, or modeling the cash-flow impact of various IO structures for commercial real estate.
IO mortgages can make sense when: (1) the borrower has reliably high income to handle the post-IO payment, (2) the borrower expects to sell or refinance before the IO period ends, (3) the property is an investment with expected appreciation that justifies the deferred principal payments, or (4) the borrower wants payment flexibility (low minimum during IO, with voluntary principal payments when cash allows).
IO mortgages are inappropriate when: the borrower can't comfortably afford the post-IO payment, the borrower might not have a viable exit (sale or refinance) before IO ends, the property might not appreciate enough to support refinancing at post-IO date, or the borrower is using IO simply to qualify for more house than they can afford.
Pair this with the standard mortgage-payment calculator (for the amortizing alternative), the ARM calculator (for adjustable-rate alternatives that share some IO features), the mortgage-refinance calculator (the planned exit for many IO borrowers), and the rental-property calculator (when using IO for investment property).
A historical note: the worst residential lending failures of 2006–2008 involved IO loans combined with no-documentation underwriting, optional ARM features, and minimal down payments — buyers using IO to "qualify for" houses they couldn't actually afford. Modern Qualified Mortgage rules require lenders to qualify borrowers at the fully amortizing payment, which has dramatically reduced the risk of IO-related defaults but also reduced the appeal of IO for "stretch" borrowers. Today's IO loans are typically taken by well-qualified borrowers who could afford the standard amortizing payment but prefer the flexibility.
For most U.S. residential buyers in 2026, standard 30-year fixed-rate mortgages are simpler, safer, and economically similar after accounting for lifetime cost. IO loans are a specialty product for specific use cases, not a mainstream choice.
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Optional extra toward principal during IO period
IO Monthly Payment
$2,167
Post-IO Payment
$2,982
Payment Increase
37.6%
Total Interest
$575,750
| Year | Phase | Total Paid | Principal | Interest | Balance |
|---|---|---|---|---|---|
| 1 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 2 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 3 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 4 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 5 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 6 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 7 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 8 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 9 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 10 | Interest Only | $26,000.00 | $0.00 | $26,000.00 | $400,000.00 |
| 11 | Amortizing | $35,787.51 | $10,084.43 | $25,703.08 | $389,915.57 |
| 12 | Amortizing | $35,787.51 | $10,759.80 | $25,027.71 | $379,155.77 |