15-Year vs 30-Year Mortgage: The Full Math

Choosing between a 15-year and 30-year mortgage is one of the biggest financial decisions you will make as a homeowner. The math is straightforward, but the right choice depends on your cash flow, risk tolerance, and opportunity cost. Here is the complete breakdown with real numbers.

The Side-by-Side Comparison: $400,000 Loan

Using current 2026 average rates:

• 30-year fixed at 6.5%: Monthly payment of $2,528. Total interest paid over the life of the loan: $510,177. Total cost: $910,177.
• 15-year fixed at 5.9%: Monthly payment of $3,355. Total interest paid over the life of the loan: $203,909. Total cost: $603,909.

The difference is staggering. The 15-year mortgage costs $827 more per month but saves you $306,268 in interest over the life of the loan. You also own your home free and clear 15 years sooner.

Run your own numbers with our mortgage calculator to see exact payments at today's rates.

Why the Interest Difference Is So Massive

Two factors drive the gap:

1. Lower rate. 15-year mortgages typically carry rates 0.5-0.75% lower than 30-year mortgages. Lenders offer a discount because they get their money back faster and face less risk. That 0.6% rate difference alone saves tens of thousands.

2. Less time for interest to accrue. On a 30-year mortgage, most of your early payments go toward interest. In the first year of a $400K loan at 6.5%, you pay $25,827 in interest and only $4,507 in principal. On the 15-year at 5.9%, you pay $23,329 in interest and $16,931 in principal. The 15-year builds equity dramatically faster from day one.

After 5 years of payments:

• 30-year: You've paid $151,680 in total payments, reduced the principal by $25,684, and still owe $374,316
• 15-year: You've paid $201,300 in total payments, reduced the principal by $95,476, and owe $304,524

The 15-year borrower has $69,792 more in home equity after just 5 years.

When the 30-Year Mortgage Makes Sense

Despite costing more in total interest, the 30-year mortgage is the right choice in several situations:

Tight Cash Flow

If the $827/month difference between the two payments would strain your budget, the 30-year is safer. An uncomfortably high mortgage payment leaves you one unexpected expense away from financial stress. The 30-year gives you breathing room for car repairs, medical bills, and job transitions.

Investing the Difference

If you take the 30-year and invest the $827/month payment difference in the stock market at a 7% average return, after 15 years you would have approximately $264,000 in your investment account. The 15-year borrower would have a paid-off house but $0 in that investment account.

After 30 years (when both scenarios are compared at the finish line), the invest-the-difference strategy can come out ahead — especially if investment returns exceed the mortgage rate. At 7% investment returns vs. 6.5% mortgage rate, the math slightly favors investing. At 10% returns, it heavily favors investing.

The catch: this only works if you actually invest the difference. If you spend it, the 30-year is strictly worse.

Tax Deduction Value

Mortgage interest is deductible if you itemize (and the SALT cap increase to $40,000 makes itemizing more attractive in high-tax states). The 30-year generates more deductible interest, particularly in the early years. For someone in the 24% bracket, the first-year tax savings on a 30-year is roughly $6,198 vs. $5,599 on the 15-year. This narrows the effective cost gap, though it doesn't close it.

When the 15-Year Mortgage Wins

You Can Comfortably Afford It

If the $3,355 payment is well within your budget (generally under 25% of your gross income), the 15-year is a clear win. You save $306,000 in interest and build equity at nearly 4x the rate. For a household earning $160,000+, the 15-year payment is very manageable.

Check what you can comfortably afford with our home affordability calculator.

Near Retirement

If you are 50 and buying a home, a 30-year mortgage means payments until age 80. A 15-year mortgage gets you to a paid-off home by 65, exactly when most people want to reduce fixed expenses. Entering retirement with no mortgage payment dramatically reduces the savings you need.

You Want Guaranteed Returns

Paying off a 6.5% mortgage is equivalent to earning 6.5% on your money, risk-free. The stock market averages 7-10% but with significant volatility and no guarantees. If you are risk-averse, the guaranteed "return" of paying off your mortgage faster is compelling.

The Hybrid Approach: 30-Year With Extra Payments

There is a middle path that many financial planners recommend: take the 30-year mortgage for its lower required payment and flexibility, but make extra payments toward principal as if you had a 15-year.

If you take the $400K loan at 6.5% (30-year) and add $827/month in extra principal, you pay off the loan in about 15.5 years — nearly identical to the 15-year mortgage timeline. You pay more total interest than the 15-year (because of the higher rate) but less than the full 30-year term.

The advantage: if you hit a rough patch financially — job loss, medical emergency, major home repair — you can drop back to the required $2,528 payment. With a 15-year mortgage, you are locked into the $3,355 payment regardless of circumstances.

The disadvantage: you need discipline. The flexibility cuts both ways — it is easy to stop making extra payments and never restart. And you will pay a slightly higher interest rate than you would on a 15-year.

The Bottom Line

If you can comfortably afford the 15-year payment and value certainty over flexibility, go 15-year. If you prefer lower required payments and the option to invest the difference, go 30-year. If you want the best of both worlds and trust your discipline, take the 30-year and make extra payments.

Whatever you choose, run the numbers first. Use our mortgage calculator to compare scenarios, check your home affordability to make sure the payment fits your budget, and if you are considering a refinance from an existing loan, model that too.