30-Year vs 15-Year Mortgage: Real Numbers, Real Trade-offs

The 30-year vs 15-year mortgage decision sounds like a simple trade-off: pay more each month, save big on interest. The actual math is more interesting, and the answer depends on three numbers most calculators do not show you side by side: the rate spread between the two products, the total interest you pay over the life of the loan, and what your monthly payment difference could earn if you invested it. Let us run all three.

The current rate spread is about 0.65 percentage points

As of April 23, 2026, Freddie Mac's Primary Mortgage Market Survey (PMMS) reports the 30-year fixed-rate mortgage averaging 6.23% and the 15-year fixed averaging 5.58% — a spread of 0.65 percentage points [1]. The spread has hovered between 0.65 and 0.87 percentage points over the past two years, narrowing slightly in late 2025.

Why is the 15-year cheaper? Two reasons. The 15-year loan exposes the lender to less duration risk (a shorter window for interest rates to drift), and borrowers who can afford a 15-year payment are typically lower-credit-risk on average. The spread is not a discount — it is a smaller risk premium.

Apples to apples: a $300,000 loan, both terms

Let us run a $300,000 loan with a 20% down payment ($60,000 down, $240,000 borrowed) at the rates above:

	30-year @ 6.23%	15-year @ 5.58%
Monthly P&I	$1,475	$1,971
Total interest paid	$290,856	$114,817
Total cost (P&I)	$530,856	$354,817
Difference vs 30-year	—	$176,040 less

The 15-year saves $176,040 in interest over the life of the loan but costs $497 more per month. Run your own numbers in the mortgage calculator — every loan is different, and small rate differences move these totals significantly.

The opportunity cost the lender does not mention

Here is where the simple "$175K saved!" pitch gets complicated. If you take the 30-year and invest the $498/month difference, you might come out ahead. Or you might not. It depends entirely on your investment return.

Suppose you invest $497/month for 15 years (until the 15-year mortgage would have been paid off) at a 7% annualized return — roughly the long-run real return of a diversified U.S. stock portfolio. Future value formula: FV = PMT × ((1+r)^n − 1) ÷ r, where r is monthly return (7% ÷ 12) and n is 180 months.

That investment grows to roughly $157,847. You would still be behind the 15-year option ($176,040 saved vs $157,847 invested), and you would still owe 15 more years of mortgage payments. The 15-year wins in this scenario — but the gap is smaller than the headline interest-savings number implies, and the 15-year only wins reliably under disciplined investing.

If you would otherwise let that $498/month leak into lifestyle inflation — a more expensive car, a bigger restaurant budget, an extra subscription bundle — the 30-year is dramatically worse. The forced savings of the 15-year is a feature, not a cost. For households without a strong investing habit, the 15-year almost always wins.

When the 30-year is the right call

The 30-year is the right call in three specific scenarios:

1. You are buying at the edge of affordability. If the 15-year payment is more than ~28% of your gross monthly income, you are tight on cash flow and one job loss away from a missed payment. Take the 30-year and pay extra principal voluntarily — you can always increase your payment, but you cannot easily decrease it.
2. You have higher-yield uses for the cash. Maxing out a 401(k) with employer matching (50-100% instant return) beats prepaying a 6.23% mortgage on a risk-adjusted basis. So does paying off any debt with an APR above 7-8%.
3. You are confident about long-run market returns. If you will reliably invest the difference at 8%+ real returns over 15+ years, the math flips and the 30-year + invest combination wins. This requires discipline most households do not have.

When the 15-year is the right call

1. You have a long horizon and want forced savings. Most households save more aggressively when their mortgage forces it. The 15-year acts like a high-yield savings vehicle (paying 5.58% guaranteed) that you cannot easily raid for impulse spending.
2. You are within 15 years of retirement. Entering retirement debt-free dramatically reduces the income you need to retire. The 15-year aligns the payoff with the start of retirement income.
3. You are risk-averse. A 15-year loan locks in a low rate AND a fixed payoff date. The 30-year's "invest the difference" math depends on returns that are not guaranteed. If you would not take a margin loan to invest in stocks, you should not be conceptually similar by holding a longer mortgage to free cash for investing.

The hybrid: 30-year with extra principal payments

There is a middle path that gets too little attention: take the 30-year loan, then voluntarily pay it down on a 15-year schedule. The loan payoff calculator will show you the extra monthly payment required to hit any payoff target.

The downside: you pay the higher 30-year rate (currently 0.65 percentage points more), so this approach costs more interest than a true 15-year loan. On a $240,000 loan, that's roughly $20,000-$25,000 in extra interest over the life of the loan, even if you make exactly the same payments.

The upside: if you lose your job or face a medical emergency, you can drop back to the 30-year minimum payment without refinancing. That insurance policy might be worth $25,000 to households without a robust emergency fund.