At 7.00% on a $400,000 balance, an interest-only mortgage costs $2,333 a month — $328 less than the $2,661 monthly payment on a standard 30-year amortizing loan at the same rate. That gap in monthly cash flow explains why some retirees explore interest-only structures when portfolio income runs tight. The monthly saving is real. So is the math on what you give up: zero principal reduction, a higher payment when the IO period ends, and $30,120 in extra lifetime interest on this example loan.
These examples use 7.00% as a round number to make the arithmetic transparent. As of mid-June 2026, the Freddie Mac Primary Mortgage Market Survey 30-year fixed average is approximately 6.47%–6.52% — somewhat below these illustrations. Use the mortgage calculator to plug in the current week's rate and get real-dollar figures for your balance.
What Is an Interest-Only Mortgage?
An interest-only (IO) mortgage is a home loan where monthly payments cover only the accrued interest — not the outstanding principal. The balance stays at exactly the starting amount for the duration of the IO period. IO periods on US loans typically run 5–10 years, structured as part of an adjustable-rate mortgage, after which the loan recasts to a fully amortizing principal-and-interest payment for the remaining term.
Interest-only loans gained widespread use before the 2008 financial crisis. The Consumer Financial Protection Bureau's Ability-to-Repay / Qualified Mortgage rule, effective January 10, 2014, eliminated IO loans from the conforming "Qualified Mortgage" category (CFPB ATR/QM rule). They now exist as non-QM products offered by portfolio lenders and in the jumbo market. They don't qualify for conventional securitization through Fannie Mae or Freddie Mac.
How to Calculate an Interest-Only Payment
The IO payment formula is simpler than standard amortization. Monthly interest-only payment = outstanding balance × (annual rate ÷ 12). The balance never changes during the IO period, so every payment is identical:
IO Payment = Balance × (Annual Rate ÷ 12)
At 7.00% on $400,000: $400,000 × (0.07 ÷ 12) = $400,000 × 0.005833 = $2,333/month.
For comparison, the standard 30-year P&I payment uses the amortization (PMT) formula:
PMT = P × r(1+r)ⁿ ÷ [(1+r)ⁿ − 1]
where r = monthly rate (7.00% ÷ 12 = 0.005833) and n = number of payments (360 for 30 years). At 7.00%, the (1+r)ⁿ factor works out to 8.116, so the payment is $400,000 × 0.006654 = $2,661/month. The $328 monthly saving is the IO premium — the cost you defer to build cash flow now.
IO vs P&I Payment by Loan Balance
The table below shows both payment types at common retirement-size loan balances at 7.00% (illustrative rate — plug your actual rate into the mortgage calculator for your real number):
| Loan Balance | IO Payment (7.00%) | P&I (30-yr, 7.00%) | Monthly Saving |
|---|---|---|---|
| $200,000 | $1,167 | $1,331 | $164 |
| $300,000 | $1,750 | $1,996 | $246 |
| $400,000 | $2,333 | $2,661 | $328 |
| $500,000 | $2,917 | $3,327 | $410 |
| $600,000 | $3,500 | $3,992 | $492 |
| $800,000 | $4,667 | $5,323 | $656 |
IO payment = Balance × (7.00% ÷ 12). P&I = standard amortization at 7.00% for 360 months. All figures rounded to the nearest dollar. Rate is illustrative.
Why Retirees Consider Interest-Only Mortgages
Retirement income is fixed or slowly growing. A retiree drawing from a $500,000 portfolio at the 4% rule (Bengen, W., Journal of Financial Planning, October 1994) generates $1,667/month — less than the IO payment on a $300,000 mortgage at 7%. See the 4% withdrawal rate guide for the full math. That arithmetic creates real pressure. Three situations where IO structures attract serious attention:
Bridging to Social Security at 70
Delaying Social Security from full retirement age (67 for those born after 1960) to age 70 adds 8% per year in Delayed Retirement Credits — a 24% larger benefit for a 3-year delay (SSA.gov, Delayed Retirement Credits). That wait requires 3 years of income from savings. An IO mortgage reduces the housing payment during the bridge period, stretching the portfolio further. See the Social Security filing age guide for the breakeven math.
Asset-Rich, Cash-Flow-Constrained
A retired homeowner might carry a $400,000 mortgage balance on a home worth $850,000, earning $4,200/month from Social Security and a pension. Net equity is substantial, but the $2,661 P&I payment leaves little margin for healthcare costs, home maintenance, or unexpected expenses. Refinancing to an IO structure drops the payment to $2,333/month — a $328 improvement without liquidating any assets. The trade-off is that the equity cushion grows only through home price appreciation, not amortization.
Planned Short-Term Hold
Some retirees plan to sell in 3–7 years — downsizing, relocating, or moving to assisted living — and don't want a long-term P&I obligation. An IO loan on the current home reduces carrying cost during the transition. If the exit plan is to pay off the balance from sale proceeds anyway, building equity through amortization during the hold period has limited practical value.
What Happens When the IO Period Ends
The payment reset is where interest-only structures create risk. After a 5-year IO period on a $400,000 loan at 7.00%, the outstanding balance is still $400,000 — not a single dollar of principal was repaid. But the remaining term is now 25 years, not 30.
Amortizing $400,000 over 25 years at 7.00% produces a monthly P&I payment of $2,827 — $166 more per month than if you had started with a 30-year P&I loan on day one. In practice, IO mortgages are typically adjustable-rate, so the interest rate may also reset at the IO conversion date, adding further uncertainty to the recast payment.
| Period | Monthly Payment | Balance at End |
|---|---|---|
| IO period (months 1–60) | $2,333 | $400,000 (unchanged) |
| P&I reset (months 61–360) | $2,827 | $0 at month 360 |
| Standard 30-yr P&I from day 1 | $2,661 | $0 at month 360 |
All at 7.00% illustrative rate. The IO reset payment is $166 more per month than the standard P&I alternative.
The True Cost: Total Interest Comparison
Interest-only mortgages cost more in total interest than standard P&I structures, even though they save cash in the early years. The nominal math for the $400,000, 7.00% example:
- IO structure (5-year IO + 25-year P&I reset):
- IO period: $2,333/mo × 60 months = $139,980 (100% interest, zero principal)
- P&I reset: $2,827/mo × 300 months = $848,100; minus $400,000 principal = $448,100 in interest
- Total interest paid: $588,080
- Standard 30-year P&I from day 1:
- $2,661/mo × 360 months = $957,960; minus $400,000 = $557,960 total interest
- Extra lifetime interest from IO structure: $30,120
The IO approach saves $19,680 in the first 5 years ($328 × 60 months) but generates $30,120 in extra lifetime interest — a nominal net cost of $10,440 for choosing IO over standard P&I. Whether the monthly savings offset that depends on what the $328/month does during the IO period. Invested at the same 7.00% rate in a diversified portfolio, $328/month over 60 months compounds to approximately $23,470 — using the future value of an annuity: FV = 328 × [((1.005833)^60 − 1) ÷ 0.005833] = $23,470. See the compound interest calculator to run this with different return assumptions. Still $6,650 short of the extra lifetime interest in nominal terms, though the time value of future savings shrinks that gap in real-dollar terms.
Run your actual balance and rate through the mortgage calculator to see whether the savings-vs-interest math works in your specific situation.
Alternatives to an Interest-Only First Mortgage
HELOC with an Interest-Only Draw Period
A home equity line of credit (HELOC) typically requires interest-only payments during a 10-year draw period before converting to P&I repayment. If you need to reduce monthly cash outlays on existing home equity without refinancing your primary mortgage, a HELOC can achieve a similar cash-flow effect — without replacing the original loan structure or resetting the amortization clock on the full balance. See our home equity loan vs HELOC guide for the mechanics and cost comparison.
Refinancing to a Longer Amortization
A retiree with 15 years remaining on a mortgage can sometimes achieve a comparable payment reduction by refinancing to a new 30-year P&I loan at a lower rate — while still building equity from payment one. Use the mortgage refinance calculator to compare the break-even timeline against an IO structure before committing.
Tax Treatment of IO Mortgage Interest
Every dollar of an IO payment is interest — no principal component. That makes the IO structure maximally useful for deductibility purposes, subject to IRS Publication 936 limits: mortgage interest is deductible on acquisition debt up to $750,000 for loans originated after December 15, 2017, or $1,000,000 for loans originated before that date (IRS Publication 936). On a $400,000 balance at 7.00%, the annual interest is $28,000 — fully deductible for qualifying retirees who itemize. In the first year of a standard P&I loan at the same rate, approximately $4,070 of the $31,932 in annual payments goes toward principal, leaving roughly $27,862 in deductible interest — about $138 less than the IO deduction that year. The IO advantage widens in later years of a P&I loan as the principal component grows and the deductible interest shrinks.
Retirees managing taxable income carefully — particularly those doing partial Roth conversions or managing bracket boundaries — may find the higher deductible interest useful during the IO period, if they itemize rather than taking the standard deduction. Verify with a tax professional whether itemizing makes sense for your situation.
When IO Makes Sense vs When It Doesn't
IO may make sense when:
- You have a documented exit strategy (sell in 5–7 years, or pay off the balance from a liquid asset)
- The IO period aligns with a specific bridge (Social Security delay, pension vesting, portfolio recovery)
- You've modeled the recast payment and confirmed it fits future income
- The monthly savings are invested at a rate that at least approaches the mortgage rate
IO is likely the wrong choice when:
- IO is the only path to affordability — the reset payment is an unfunded obligation
- The mortgage is adjustable and rates may rise at the IO conversion date
- You're banking on home price appreciation to cover the unchanged balance at sale (prices can fall)
- Retirement income is fixed with no capacity to absorb $166/month more in 5 years
If retirement income math is the constraint, start with the retirement calculator to see what your portfolio and Social Security generate against your expected expense load — then layer in the IO decision once you know the full picture.
Frequently Asked Questions
What is an interest-only mortgage payment?
An interest-only payment covers only the monthly interest on the outstanding balance, with no principal repayment. Formula: Balance × (Annual Rate ÷ 12). On a $400,000 loan at 7.00%, that's $2,333/month — $328 less than the $2,661 standard P&I payment — but the balance stays at $400,000 indefinitely during the IO period.
Can you get an interest-only mortgage in retirement?
Yes, though IO loans are limited to non-QM lenders, portfolio banks, and some jumbo loan programs. The CFPB's 2014 Qualified Mortgage rule excludes IO loans from the conforming market, so conventional lenders won't offer them. Qualifying requires documented retirement income (or significant documented assets), strong credit, and a low debt-to-income ratio. Expect a slightly higher rate than a standard conforming mortgage.
What happens at the end of the interest-only period?
The loan recasts to a fully amortizing P&I payment for the remaining term. On a $400,000 loan with a 5-year IO period at 7.00%, the 25-year P&I recast payment is $2,827/month — $166 more per month than if you had amortized on a 30-year schedule from day one. The outstanding balance remains at $400,000 (no principal was repaid during IO).
How does an interest-only mortgage affect total interest paid?
On a $400,000 loan at 7.00%, choosing a 5-year IO period followed by 25-year P&I repayment generates $588,080 in total interest — $30,120 more than a straight 30-year P&I structure ($557,960). The IO approach saves $19,680 in the first 5 years, but the deferred amortization during that period and the shorter repayment window afterward produce the higher lifetime cost.