15-Year vs 30-Year Mortgage: True Lifetime Cost (2026)

$308,601. That is how much extra interest a $400,000 borrower pays by choosing a 30-year mortgage over a 15-year one, using illustrative current-range 2026 rates. The 30-year's lower monthly payment is real and useful — but the lifetime cost gap is enormous, and most comparisons stop before they tell you the one number that actually decides the question: the break-even investment return.

This is a full head-to-head. We run both terms at three loan sizes — $300,000, $400,000, and $500,000 — with complete amortization math, then settle the real debate (take the 30-year and invest the difference, or take the 15-year and own faster?) with a side-by-side portfolio model and the exact crossover rate.

Rates used (illustrative, current 2026 range): 30-year fixed at 6.40%, 15-year fixed at 5.60%. These are representative figures in the range Freddie Mac's Primary Mortgage Market Survey (PMMS) has reported through 2026 — the 15-year has historically run roughly 0.50 to 0.80 percentage points below the 30-year. Confirm today's exact numbers at the PMMS link before locking a loan. Every figure below is computed, not estimated.

How We Computed These Numbers

Every payment uses the standard fixed-rate amortization formula:

M = P × [ r(1 + r)ⁿ ] / [ (1 + r)ⁿ − 1 ]

where M is the monthly payment, P is the loan principal, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments (180 for a 15-year loan, 360 for a 30-year). Total interest is simply (M × n) − P. This is the same formula every lender and the Investopedia amortization reference uses. The opportunity-cost section uses the future-value-of-an-annuity formula, FV = PMT × [((1 + i)^m − 1) / i], to compound the invested payment difference.

These figures cover principal and interest only. They exclude property taxes, homeowner's insurance, PMI, and closing costs — for the full picture on a representative loan see our true cost of a $400K mortgage breakdown, which layers in PMI and down-payment effects across 47 scenarios.

The Core Head-to-Head: $300K, $400K, $500K

Here is the complete comparison across three loan sizes. The pattern is identical at every level — the 15-year roughly triples your principal-paydown speed for about a 30% higher payment.

The single most striking row is “interest as % of principal.” On the 30-year, you pay the bank $1.25 in interest for every $1 borrowed. On the 15-year, you pay 48 cents. That ratio is constant across loan sizes because it is driven entirely by rate and term, not the dollar amount. Want your exact figures at your real rate? Run them in the MoneyLens mortgage calculator.

The Monthly Payment Delta — and the Interest Saved

The decision lives in the gap between two numbers: how much more the 15-year costs each month, versus how much it saves over the life of the loan.

Read the last column carefully: every extra dollar of monthly payment on the 15-year buys about $392 of lifetime interest savings, and shaves a full 15 years off your payoff. On a pure dollars-out, dollars-saved basis, that is one of the highest guaranteed returns in personal finance. The catch is liquidity — that $788/month on the $400K loan is committed, not optional. You cannot dial it back in a bad month the way you can with voluntary extra payments on a 30-year. If qualifying for the higher payment is in doubt, check the 28/36 limits with our affordability calculator first.

Equity Builds Far Faster on the 15-Year

Because more of every 15-year payment goes to principal, equity accumulates on a completely different curve. On a $400,000 loan, here is the principal balance remaining at key milestones:

At the 15-year mark, the 15-year borrower owns the home outright while the 30-year borrower still owes $289,044 — more than 72% of the original loan. This is the heart of the “own faster” argument, and it is the same reason early extra payments are so powerful, a dynamic we quantify in the pay off mortgage or invest analysis.

The Real Debate: Take the 30-Year and Invest the Difference?

Here is where most articles wave their hands. The honest answer requires modeling both borrowers as investors. We use the $400,000 loan and assume both people have the same total budget — whatever the 15-year payment is — and invest every dollar they do not send to the mortgage.

• 30-year borrower: Pays $2,502/month to the bank, invests the $788 difference every month for the full 30 years.
• 15-year borrower: Pays $3,290/month for 15 years (investing nothing extra), then — mortgage-free — invests the entire $2,502 freed-up payment for the final 15 years.

Both commit exactly $3,290/month for 30 years. The only difference is timing. Here is the resulting investment portfolio at year 30 across a range of returns:

The crossover is 5.2%. Below it, the 15-year wins — paying off in half the time and then investing a much larger payment for 15 years beats the slow drip. Above it, the 30-year wins, because the 30-year borrower's smaller contributions compound for a full 30 years instead of 15, and time-in-market dominates. The long-run S&P 500 nominal return has historically averaged roughly 10% (about 7% after inflation), which puts the math on the 30-year-and-invest side of the line for a disciplined investor.

But notice what the model assumes: that the 30-year borrower actually invests the $788 every single month for 30 years and never touches it. Behaviorally, most people do not. The $788 quietly becomes a nicer car or a bigger vacation. The 15-year forces the savings through a mandatory payment — that is its hidden advantage, and why it remains the right call for anyone who is honest about their own discipline. Model your own assumptions in the compound interest calculator.

The Tax Angle: Is the 30-Year's Bigger Deduction Worth It?

A common argument for the 30-year is the larger mortgage-interest deduction. Per IRS Publication 936, interest on up to $750,000 of home-acquisition debt is deductible — if you itemize. Two problems gut this argument:

• Most people don't itemize. The 2017 tax law roughly doubled the standard deduction, and the Tax Policy Center and CFPB homebuying guidance both note the large majority of filers now take the standard deduction and get zero mortgage-interest benefit.
• Even if you itemize, the math is bad. Paying the bank an extra $308,601 in interest (the $400K example) to deduct it at, say, a 24% marginal rate “saves” you about 24 cents per dollar spent. You are still down 76 cents on every dollar. No one should pay $1 of interest to save 24 cents of tax.

The deduction slightly narrows the lifetime-cost gap for high-income itemizers, but it never reverses it. Treat it as a minor rebate, not a reason.

The Hybrid: 30-Year Loan, 15-Year Payment

There is a middle path that captures most of the upside of both. Take the 30-year for its lower required payment — preserving the ability to drop back during a job loss or emergency — then voluntarily pay extra principal to hit a 15-year payoff.

The cost of this flexibility is the rate. You keep the 30-year's 6.40% rather than the 15-year's 5.60%, so paying off a $400,000 loan in 15 years at 6.40% costs about $217,000 in interest — roughly $25,000 more than a true 15-year loan at 5.60%. That $25,000 is the price of optionality. For households with variable income or thin emergency reserves, it is often worth paying. Model the extra-payment payoff month by month in the extra payments calculator or see the full schedule in the amortization calculator.

How Rate Spread Changes the Answer

The 15-year's advantage depends partly on how much lower its rate is. The 0.80-point spread we used (6.40% vs 5.60%) is typical, but it widens and narrows with market conditions. The wider the spread, the more decisively the 15-year wins on interest. If you are shopping, the 15-year/30-year spread you are quoted is itself a data point — a wide spread is a small bonus argument for the 15-year. Track where rates are heading in our 2026 mortgage rate forecast, and remember that shopping multiple lenders for a better rate on either term is the highest-ROI hour in the whole process.

So Which Should You Choose?

The decision reduces to three questions:

1. Can you comfortably afford the higher payment — ideally keeping total housing cost under 28% of gross income even at the 15-year payment? If not, the 30-year is the responsible choice; do not stretch into a payment that leaves no margin.
2. Will you actually invest the difference, every month, for decades? If yes, and you expect long-run returns above ~5.2%, the 30-year-and-invest play is mathematically optimal. If you are honest that you won't, the 15-year's forced savings wins.
3. How much do you value guaranteed, risk-free progress? The 15-year's interest savings are certain. The 30-year-and-invest edge depends on uncertain market returns. Paying off in 15 years is the lower-variance bet, and being mortgage-free at 50 instead of 65 has a value no spreadsheet fully captures.

For most disciplined high-earners with stable income, the 15-year — or the hybrid — is the cleaner, lower-risk win. For aggressive investors with reliable habits and a long horizon, the 30-year-and-invest can finish ahead. There is no universally correct answer, only the right answer for your income stability, your discipline, and your tolerance for market risk. And if you are weighing this against simply buying less house, start with how much house you can actually afford.

Limitations of This Analysis

All figures use the 6.40% / 5.60% illustrative rates and standard fixed-rate amortization; your quoted rates will differ — always verify against current Freddie Mac PMMS data. Investment-return scenarios are illustrative; actual market returns vary widely and past performance does not guarantee future results. The opportunity-cost model ignores investment taxes, mortgage-interest deductions, and the difference in risk between a guaranteed payoff and a volatile portfolio — all of which favor the 15-year on a risk-adjusted basis. This article is educational and not personalized financial advice. Once you have chosen a term, sanity-check the take-home income that has to support the payment with the Pay.thicket.sh take-home pay calculator — the after-tax number is what actually funds the mortgage.