What is an interest-only mortgage?

An interest-only mortgage lets you pay only the interest on the loan for an initial period (typically 5–10 years). During the IO period, no principal is paid down. After the IO period ends, payments reset to a fully amortizing schedule that pays off the entire principal over the remaining loan term.

Why would someone choose interest-only?

IO mortgages provide lower initial payments, useful for: investors maximizing rental property cash flow, self-employed borrowers with variable income who want flexibility, those who plan to sell or refinance before the IO period ends, and high earners who prefer to invest the principal-payment difference rather than build home equity.

What is payment shock?

Payment shock is the sudden increase in monthly payment when the IO period ends and the loan converts to fully amortizing. Because the same principal now has to amortize over a shorter remaining term, the new payment is typically 30–80% higher than the IO payment. Borrowers who don't plan for this shock can face affordability problems.

Are interest-only mortgages risky?

They carry meaningful risks: payment shock at IO end, no principal pays down during IO (so total interest paid is higher), and exit strategies (sale or refinance) depend on home value and credit qualification holding up. The 2008 financial crisis was partly fueled by IO loans gone bad. Modern IO loans are subject to stricter underwriting that reduces but doesn't eliminate the risks.

Can I pay extra principal during the IO period?

Yes, with most modern IO mortgages. Voluntary principal payments during the IO period reduce both the eventual post-IO payment and lifetime interest. This is one way to use IO's flexibility productively — pay extra in good months, just IO in lean months.

Interest-Only Mortgage Calculator

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.

Inputs

Loan Amount

Interest Rate

Total Loan Term (years)

Interest-Only Period (years)

Extra Monthly Payment

Optional extra toward principal during IO period

Results

IO Monthly Payment

$2,167

Post-IO Payment

$2,982

Payment Increase

37.6%

Total Interest

$575,750

Monthly Payment Comparison

Loan Balance Over Time

Year-by-Year Breakdown

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

Last updated: May 26, 2026Reviewed by the CalcMountain editorial team

Formula

Interest-only payment (during IO period): IO_Payment = Loan × (Annual Rate / 12) Constant for the IO period. Does not reduce principal. After IO period ends, payment converts to fully amortizing on remaining principal over remaining term: Remaining term in months: n_remain = (Total Term − IO Period) × 12 Fully amortizing payment after IO: M_amort = Loan × [ r × (1 + r)^n_remain ] / [ (1 + r)^n_remain − 1 ] Where r = monthly interest rate. Payment shock at IO end: Shock = M_amort − IO_Payment Total cost over loan life: IO phase: IO_Payment × (IO Period × 12) Amortization phase: M_amort × n_remain Total = IO Total + Amort Total Example: $400,000 loan at 6.5%, 30-year total term, 10-year IO period. IO Payment: $400,000 × 0.065 / 12 = $2,167/month (years 1–10) Remaining term after IO: 20 years (240 months) Amortizing payment: $400,000 × [0.005417 × 1.005417^240] / [1.005417^240 − 1] = $2,983/month (years 11–30) Payment shock at year 11: +$816/month (38% increase) Total cost over 30 years: IO phase: $2,167 × 120 = $260,040 Amort phase: $2,983 × 240 = $715,920 Total: $975,960 For comparison, standard 30-year amortizing $400,000 at 6.5%: Monthly: $2,528 Total: $910,080 The IO loan costs $65,880 more lifetime — because no principal was paid down during the IO period, so more interest accrued.

How to use this calculator

Enter the loan amount.
Enter the interest rate. IO loans often have rates similar to standard mortgages, though specialty IO products may be slightly higher.
Set the total loan term (typically 30 years).
Set the interest-only period (typically 5 or 10 years).
Enter any optional extra monthly payment toward principal during the IO period. Voluntary principal payments during IO reduce both the post-IO payment AND lifetime interest.
Review the IO monthly payment (low) and the post-IO monthly payment (much higher). The payment shock at the transition is the critical number.
For affordability assessment: can you handle the POST-IO payment, not just the IO payment? If not, the IO loan is wrong for your situation.
Compare lifetime cost to a standard mortgage of the same loan amount and rate. IO loans are typically more expensive in lifetime interest because no principal pays down during the IO years.
Consider paying voluntary principal during IO. The IO loan structure doesn't require principal payments, but voluntary payments reduce both the eventual amortizing payment and total interest.

Worked examples

High-earner with variable income

$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.

Investor financing for rental property

$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.

IO with voluntary principal payments

$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.

When to use this calculator

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.

Common mistakes to avoid

Qualifying based on the IO payment alone. Modern QM rules require qualifying at the fully amortizing post-IO payment, but borrowers should self-test the same way — can you actually afford the post-IO payment?
Treating IO as principal forgiveness. The full principal must be paid eventually. IO just defers when payment begins, not whether you owe.
Assuming the property will appreciate enough to support refinancing. If home values stagnate or fall, refinancing out of IO at the end of the period may not be possible — leaving you with the full post-IO payment.
Forgetting to plan for the payment shock. Setting up a budget around the low IO payment without planning for the eventual increase creates a financial cliff at the IO transition.
Not making voluntary principal payments during IO. The structure lets you pay extra principal when cash allows. Without voluntary payments, you arrive at year 11 with the same balance you started with — and the now-shorter amortization period produces a much higher payment.
Confusing IO with balloon. IO loans become amortizing after the IO period — they don't have a balloon payment. Balloon loans require the full remaining principal at the end. Different structures with different risks.

Frequently Asked Questions

Sources & further reading

Interest-Only Mortgages — explainer — U.S. Consumer Financial Protection Bureau
Qualified Mortgage Rules — U.S. Consumer Financial Protection Bureau
Loan Estimate explainer — U.S. Consumer Financial Protection Bureau

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