15-Year vs 30-Year Mortgage: The Math Most People Get Wrong

Everyone knows the 15-year mortgage 'saves you money.' Personal finance people say it all the time. And it's technically correct — you will pay less total interest on a 15-year loan than a 30-year loan. Significantly less.

But the comparison is incomplete, and people make expensive decisions based on the incomplete version. The real question isn't 'which loan has a lower total interest number.' The real question is whether the money you'd save in interest is a better use of those dollars than what you'd do with the payment difference.

Sometimes it is. Sometimes it isn't. And the math on this is worth actually doing rather than trusting the vibes of 'shorter is smarter.'

As of March 2026, Freddie Mac's Primary Mortgage Market Survey has the 30-year fixed averaging 6.11% and the 15-year averaging 5.50%. Bankrate's broader survey shows slightly higher averages — 6.35% for 30-year and 5.72% for 15-year. Let's use numbers in that range and look at a real $350,000 purchase.

Let's set the scenario: $350,000 home, 20% down ($70,000), so you're borrowing $280,000. You're avoiding PMI either way with 20% down. Let's run the tape on both loans.

30-year fixed at 6.35%: - Monthly payment (P&I): $1,743 - Total paid over 30 years: $627,480 - Total interest paid: $347,480

15-year fixed at 5.72%: - Monthly payment (P&I): $2,314 - Total paid over 15 years: $416,520 - Total interest paid: $136,520

The interest difference: $347,480 minus $136,520 = $210,960. So yes — the 15-year saves you roughly $211,000 in interest. That's a real number. Not hypothetical. Not 'maybe if rates move.' That's the hard math.

But here's what you gave up to get there: $571 more per month. Every single month for 15 years. That's $102,780 in extra payments over the life of the 15-year loan.

Wait — $102,780 in extra payments to save $210,960 in interest? That sounds like a clear win for the 15-year. And in the purest, most literal sense, it is. But we haven't asked what happens to the $571/month difference if you choose the 30-year and invest it instead.

Here's the fork in the road that the simple interest comparison completely ignores.

If you take the 30-year mortgage, you have $571/month in your pocket that you wouldn't have with the 15-year. What if you invested that $571 every month?

At a 7% average annual return (conservative for a diversified stock index fund over long periods): - After 15 years: $571/month at 7% = approximately $183,000 - After 30 years: $571/month at 7% = approximately $689,000

At an 8% return: - After 15 years: ~$195,000 - After 30 years: ~$789,000

So by year 15 — the point where the 15-year mortgage is fully paid off — the 30-year borrower who invested the difference has roughly $183,000–$195,000 in investments. The 15-year borrower has $0 in those investments but a fully paid-off house.

The 15-year mortgage isn't obviously better anymore. It's a tradeoff between guaranteed interest savings and potential investment growth. The crossover depends on your investment returns vs your mortgage rate.

Here's the rule of thumb that actually holds up: if your mortgage interest rate is higher than your expected after-tax investment return, paying off the mortgage faster wins. If your expected after-tax investment return is higher than your mortgage rate, investing the difference wins.

At 5.72% (15-year rate), you're getting a guaranteed 5.72% return on every extra dollar of mortgage paydown. Is your investment portfolio going to beat 5.72% reliably after taxes? Maybe — but it's not guaranteed. At 6.35% mortgage rate, the comparison gets even tighter. Both scenarios are defensible depending on your assumptions and your personality.

One thing that makes the 15-year genuinely attractive right now: the rate spread between 15 and 30-year mortgages is meaningful. About 0.60–0.65 percentage points currently.

That spread matters a lot. If both loans had the same rate, the 15-year would just be an accelerated paydown — and that's something you can replicate by making extra principal payments on a 30-year. But because the 15-year rate is actually lower, you're getting a discount that you can't replicate with prepayments alone.

The 15-year at 5.72% is a genuinely better deal per dollar borrowed than the 30-year at 6.35%. You're not just paying faster — you're paying less per dollar of outstanding balance.

Where this gets interesting: if you take the 30-year but commit to paying the equivalent of 15-year payments, you'll pay the loan off in roughly 15 years. But you'll pay the higher 6.35% rate the whole time instead of 5.72%. On $280,000, that rate difference costs you additional interest even if the amortization timeline is similar.

And if something happens — job loss, medical emergency, any shock — the 30-year's lower required payment gives you breathing room. The 15-year's higher required payment does not. You can always pay more on a 30-year. You cannot pay less than the minimum on a 15-year without consequences.

There's a profile where the 15-year is the clearly correct choice, and it's not everyone.

You're within 15–20 years of retirement and want the mortgage gone. If you're 50 years old, a 30-year mortgage doesn't pay off until you're 80. A 15-year pays off at 65 — potentially right around when your income changes. Eliminating the housing payment heading into retirement is valuable in ways that compound calculations don't fully capture.

You have high income stability and the higher payment doesn't stress your cash flow. If the $571/month difference between 15 and 30 represents a rounding error in your budget — maybe you earn $200,000 a year and have low other debts — the guaranteed interest savings look very attractive. The 'invest the difference' argument requires discipline that not everyone maintains.

You're buying in a high-appreciation market and want to build equity fast. More equity faster gives you more options — refinancing, accessing equity, selling. The 15-year pays down principal faster than the 30, which matters in markets where transaction costs (realtor fees, etc.) eat into equity on a shorter hold.

You have other retirement assets already funded. If you're already maxing your 401k and IRA, the 'invest the difference' option has diminishing marginal returns (less tax-advantaged space available). Paying off the mortgage becomes relatively more attractive when your investment accounts are full.

You hate debt philosophically. The math doesn't always win over emotion. Some people genuinely sleep better with less debt and a paid-off home as the goal. That's legitimate. Money is psychological too.

The 30-year is the right answer for more situations than the personal finance community usually acknowledges.

You're earlier in your career with variable income or growth potential ahead. The $571/month lower payment gives you flexibility to invest, change jobs, absorb emergencies, or fund career transitions. The optionality has real value.

You're buying in your 20s or 30s. Time is your biggest asset for compounding. A 30-year mortgage with the payment difference consistently invested in index funds over 30 years produces enormous wealth. Locking into a 15-year payment when you're 28 and have 30+ years of investing ahead of you is arguably leaving money on the table.

You have higher-rate debt. If you have student loans at 7%, car loans at 7.5%, or credit cards at 20% — none of that gets paid off by choosing a 15-year mortgage. The 30-year with the payment difference directed at higher-rate debt first is unambiguously the better financial decision.

You haven't maxed tax-advantaged accounts. Every dollar that goes to a 401k or Roth IRA gets tax advantages that accelerated mortgage payments do not. If your choice is between putting $571/month more toward the mortgage vs maxing your Roth IRA, the Roth IRA almost certainly wins on after-tax math.

Your job or income isn't guaranteed. Freelancers, business owners, commissioned salespeople, anyone with volatile income — the lower required payment is insurance. You can always pay extra in good months. You can't reduce your required payment in bad months.

Under current tax law, mortgage interest is deductible — but only if you itemize deductions, and only on the first $750,000 of mortgage debt.

The standard deduction is $30,000 for married filing jointly in 2026. Most homeowners don't have enough other deductions to push past the standard deduction threshold, which means most homeowners can't deduct their mortgage interest in practice. The Tax Cuts and Jobs Act made this the reality for a big chunk of the population.

If you are one of the people who can itemize — typically higher earners with significant other deductions — the mortgage interest deduction slightly favors the 30-year, because you're deducting more interest (you pay more of it). In the 24% tax bracket, a 6.35% mortgage becomes an effective after-tax rate of roughly 4.83%. At that effective rate, the 'invest the difference' argument gets even stronger, since you're essentially paying 4.83% guaranteed vs. potentially 7–8% in a diversified portfolio.

But for most people, this calculation doesn't change the outcome much. Run the numbers with your actual tax situation if you're close to the line.

If you take a 30-year mortgage and invest the $571/month difference at 7%, when does the 15-year borrower come out ahead in total net worth?

Here's the honest answer: on pure paper math with disciplined investing and 7% returns, the 30-year borrower wins for most of the loan's life. The 15-year borrower's paid-off house doesn't generate returns by itself. The 30-year borrower's investment portfolio keeps compounding.

But this analysis assumes perfect discipline — that the $571/month actually gets invested every month for 30 years. In the real world, people spend the payment difference. They buy cars. They go on vacations. The investment portfolio that was supposed to beat the mortgage interest never materializes.

This is where the 15-year makes a strong behavioral finance argument. The mortgage forces savings. You cannot not make the payment. Forced savings through mortgage principal reduction is not optional the way voluntary investment contributions are.

If you have high financial discipline, invest consistently, and can earn above 6% — take the 30-year and invest the difference. The math likely favors you.

If you know yourself well enough to know the money will get spent if it's not forced into the mortgage — take the 15-year. Honest self-assessment here is worth more than running spreadsheets.