An interest-only mortgage payment equals your loan balance multiplied by your annual interest rate, divided by 12. On a $400,000 loan at 7.5%, that is $2,500 per month — compared to roughly $2,797 for a standard 30-year fixed payment. The difference sounds appealing, but the payment jumps sharply when the interest-only period ends.
For a full comparison of loan structures, see the Mortgage Loan Types hub.
How to Calculate an Interest-Only Mortgage Payment
The formula is simple:
Monthly IO payment = (Loan Amount × Annual Interest Rate) ÷ 12
| Loan Amount | Interest Rate | IO Monthly Payment | 30-yr Fixed Payment | Difference |
|---|---|---|---|---|
| $250,000 | 7.00% | $1,458 | $1,663 | −$205 |
| $350,000 | 7.25% | $2,115 | $2,389 | −$274 |
| $400,000 | 7.50% | $2,500 | $2,797 | −$297 |
| $500,000 | 7.50% | $3,125 | $3,496 | −$371 |
| $700,000 | 7.75% | $4,521 | $5,007 | −$486 |
| $1,000,000 | 8.00% | $6,667 | $7,338 | −$671 |
Rates are illustrative. Your actual rate depends on credit score, loan-to-value, and lender.
During the interest-only period — typically 5 or 10 years — your loan balance does not decrease. Every payment covers only the interest charge. The moment the IO period ends, both the payment structure and the payment amount change dramatically.
Worked Example: The Real Cost of an IO Loan
Scenario: You take a $500,000 IO mortgage at 7.5% with a 10-year IO period followed by 20 years of full repayment.
- IO monthly payment (years 1–10): $500,000 × 0.075 ÷ 12 = $3,125
- Balance after year 10: Still $500,000 (no principal paid)
- Fully amortizing payment (years 11–30): $500,000 repaid over 240 months at 7.5% = $4,023/month
- Payment jump at reset: +$898/month — a 29% increase
Over 30 years you pay approximately $337,500 in interest during the IO phase plus roughly $465,520 during the amortizing phase — total interest: $803,020 on a $500,000 loan.
A 30-year fixed mortgage at the same rate produces roughly $752,000 in total interest — but with predictable, consistent payments and equity building from day one.
What Is an Interest-Only Mortgage?
An interest-only (IO) mortgage lets you pay only the interest portion for an initial fixed period, usually 5 or 10 years. After that, the loan converts to a fully amortizing payment where each monthly payment covers both interest and principal.
IO mortgages were popular before the 2008 financial crisis, when many borrowers used them to stretch into homes they could not otherwise afford. Regulators tightened qualifying rules significantly after the crisis. Today they are primarily offered as:
- Jumbo IO loans — for high-balance mortgages above the conforming loan limit ($806,500 in 2026 for most areas)
- Portfolio loans — kept on the lender’s own books rather than sold to Fannie Mae or Freddie Mac
- Hybrid ARM-IO loans — adjustable rate plus interest-only, combining two sources of payment risk
What Happens After the IO Period Ends?
At the end of the IO period, your payment recalculates over the remaining loan term. This is the critical risk:
If you have a 30-year loan with a 10-year IO period, you still have only 20 years to repay the full principal — not 30. The shorter repayment window pushes the principal-plus-interest payment significantly higher than a standard 30-year fixed.
If you also have an adjustable rate: the interest rate may reset at the same time, compounding the payment increase.
When an IO Mortgage Makes Sense
IO loans are legitimate tools for specific situations:
- High-income earners with variable income — physicians in residency, commission salespeople, or business owners with cyclical cash flow may want low payments during lean periods
- Real estate investors — investors focused on short-term appreciation or rental cash flow may prefer lower carrying costs
- Short-term ownership plans — if you plan to sell or refinance before the IO period ends, you avoid the reset entirely
- Bridging a liquidity gap — some borrowers buy a new home before selling their current one, using an IO loan to reduce short-term payments
Who Should Avoid Interest-Only Mortgages
IO loans are a poor fit if you:
- Depend on home equity as a retirement strategy
- Cannot comfortably afford the fully amortizing payment at reset
- Have a variable income that may decline over time
- Are stretching your budget to afford the purchase price
If the only way you can afford a home is with an IO payment, the safer path is usually a smaller purchase price or a longer saving period.
Interest-Only vs. Other Low-Payment Mortgage Options
| Option | Lower Payment? | Builds Equity? | Risk Level |
|---|---|---|---|
| Interest-only loan | Yes | No (appreciation only) | High |
| 30-year fixed | No | Yes (slowly) | Low |
| 5/1 ARM | Moderate | Yes | Moderate |
| 15-year fixed | No (higher) | Yes (quickly) | Low |
| FHA 30-year | Slight (low down) | Yes | Low–Moderate |
See our Mortgage Types guide for a full breakdown of loan structures, and the mortgage payment calculator to compare total costs across loan types.
How to Qualify for an IO Loan in 2026
Requirements are stricter than for conventional mortgages:
- Credit score: 720 or higher at most lenders
- Down payment: Typically 20–30%; LTV above 80% is uncommon
- Debt-to-income ratio: Usually limited to 43%, though portfolio lenders may allow up to 49%
- Reserves: 12–24 months of mortgage payments in liquid assets
- Full documentation: IO loans almost always require full income verification
Key Takeaways
- Your IO payment = (Loan Amount × Rate) ÷ 12 — the formula is simple, but the long-term costs are not
- Loan balance stays flat during the IO period; equity only grows if the property appreciates
- Payments jump significantly at the IO period end — budget for the fully amortizing amount from day one
- Best suited for high-income earners with variable cash flow, short-term ownership plans, or real estate investors
- Use the amortization calculator to model your exact payment schedule and see how much interest you would pay under each scenario
Before choosing an IO loan, compare it against a 30-year fixed using the mortgage refinance section to understand all your options.
The content on Wealthvieu is for informational purposes only and should not be considered financial, tax, or investment advice. Consult a qualified professional before making financial decisions. Full disclaimer · Editorial policy