Mortgage Interest vs. Inflation: Your Real Borrowing Cost
In short
At a 7.03% mortgage rate and 3.4% inflation, the real cost of borrowing is about 3.5%. How to calculate it, and whether to prepay or invest.
Your real mortgage rate is roughly your fixed rate minus inflation. With the average 30-year fixed rate at 7.03% (Freddie Mac, September 24, 2026) and consumer prices up 3.4% over the past year, a new loan costs about 3.5% a year in real terms. That matters most when you decide whether to pay the loan down early or invest.
How to Calculate Your Real Rate
New loan: 1.0703 ÷ 1.034 − 1 = 3.51%
Older 3% loan: 1.03 ÷ 1.034 − 1 = −0.39%
A borrower who locked in 3% a few years ago currently has a slightly negative real rate. The lender is being repaid in dollars that buy less each year. Inflation changes, so this number moves; your fixed rate does not.
Here is how the real rate shifts if inflation cools to 2%:
| Fixed mortgage rate | Real rate at 3.4% inflation | Real rate at 2% inflation |
|---|---|---|
| 3.00% | −0.39% | 0.98% |
| 5.00% | 1.55% | 2.94% |
| 7.03% | 3.51% | 4.93% |
Lower inflation makes the same fixed rate more expensive in real terms. Anyone borrowing at today's rates on the assumption that inflation will stay high is making a bet, not a plan.
Why a Fixed Payment Shrinks Over Time
The payment on a $350,000, 30-year loan at 6.5% is $2,212.24 and never changes. If prices rise 3% a year, that payment is worth $1,646.11 in current dollars after 10 years ($2,212.24 ÷ 1.03^10) and $1,224.86 after 20.
The debt does not get smaller on its own. What shrinks is its weight in your budget, and only if your income rises along with prices. If your pay stays flat, the payment feels just as heavy in year 20 as in year 1.
Worked Example: Prepay $10,000 or Invest It?
$350,000 loan at 6.5%, $10,000 to spare at the start
Prepay: the $10,000 saves 6.5% a year for sure. Ten years later your balance is $19,121.84 lower than it would have been ($10,000 × (1 + 0.065 ÷ 12)^120). Keep making the normal payment and the loan ends after 331 payments instead of 360, saving $54,712.57 in interest.
Invest: at a hypothetical 8% a year, $10,000 grows to $21,589.25 in 10 years ($10,000 × 1.08^10), before tax. That is $2,467.41 more than the prepayment, but the return is not guaranteed and could be lower or negative.
Adjusted for 3.4% inflation, the prepayment's $19,121.84 is worth $13,687.50 in current dollars. Inflation reduces both sides, so it does not change which one wins. The real question is whether a possible extra $2,467 is worth taking market risk.
Find Your Own Real Rate
- Get your rate. It is on your monthly statement or your Closing Disclosure. Use the interest rate, not the APR, for this calculation.
- Get current inflation. The Bureau of Labor Statistics publishes the 12-month change in the Consumer Price Index each month. It was 3.4% for the year to August 2026.
- Apply the formula. Divide 1 plus your rate by 1 plus inflation and subtract 1. For a 6% loan: 1.06 ÷ 1.034 − 1 = 2.51%.
- Compare. Set that against the real return you expect from the alternative, such as a savings account's rate minus inflation.
The Tax Deduction Helps Fewer People Than You Might Think
Mortgage interest is deductible only if you itemize. For 2026, the standard deduction is $16,100 for single filers and $32,200 for married couples filing jointly. Interest is deductible on up to $750,000 of mortgage debt taken out after December 15, 2017.
In the first year, the $350,000 loan above charges $22,634.82 of interest. A married couple with few other deductions would still take the $32,200 standard deduction, so the interest gives them no extra tax benefit. For many borrowers, the real after-tax rate is simply the full rate minus inflation.
When Each Choice Tends to Fit
- Prepaying fits if you have no higher-rate debt, already have an emergency fund, value a guaranteed return and plan to stay in the home.
- Investing fits if you have a low fixed rate, a long time horizon, and can hold through market drops without selling.
- Neither comes first if you carry credit card debt (22.15% average APR for accounts charged interest in early 2026, per the Federal Reserve) or are missing an employer 401(k) match.
What This Doesn't Cover
This guide assumes a fixed-rate loan and ignores taxes on investment gains, adjustable-rate mortgages, private mortgage insurance and state taxes. The 8% return is a hypothetical, not a prediction. A fee-only financial planner or CPA can compare prepaying and investing using your own tax bracket and goals.
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Sources: Freddie Mac Primary Mortgage Market Survey; BLS Consumer Price Index, August 2026; IRS 2026 inflation adjustments (standard deduction); IRS Publication 936 ($750,000 limit).
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