15-Year vs 30-Year Mortgage: The Exact Dollar Difference on a $400,000 Loan

The Number Your Lender Shows You vs. The Number That Matters

Lenders quote monthly payments. That framing benefits them, not you.

A $400,000 mortgage at 7.00% over 30 years produces a principal-and-interest payment of $2,661 per month. The same loan at 6.50% over 15 years costs $3,484 per month. Most borrowers stop there, see an $823 monthly difference, and choose the 30-year term.

That decision costs $119,442 in additional interest.

The correct framing is total capital deployed, not monthly cash flow. Over 30 years at 7.00%, you pay $558,036 in total interest. Over 15 years at 6.50%, you pay $227,039. The spread is $330,997 in gross interest. Factor in the 15-year loan's higher monthly payment and the net difference in money out of pocket reaches $119,442 in favor of the shorter term.

That is the number worth calculating before you sign.

How Mortgage Amortization Actually Works

Every mortgage payment splits between principal reduction and interest. In the early years, that split is brutal.

On a $400,000 30-year loan at 7.00%, your first monthly payment of $2,661 allocates $2,333 to interest and only $328 to principal. After 12 months, you have paid $31,932 total and reduced your balance by just $3,960. The bank collected $27,972 before you meaningfully touched the debt.

The 15-year version at 6.50% works differently. Your first payment of $3,484 splits into $2,167 for interest and $1,317 for principal. After 12 months, you have paid $41,808 and reduced your balance by $15,928. The ratio of principal reduction to interest paid is 3.86 times better in year one.

This compounding asymmetry is why the total interest gap becomes so large. Every dollar of principal you eliminate early stops generating 15 to 30 more years of interest charges.

The Amortization Math

The standard mortgage payment formula is:

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 divided by 12), and n is the total number of payments.

For the 30-year loan: P = $400,000, r = 0.07/12 = 0.005833, n = 360.

M = 400,000 × [0.005833 × (1.005833)^360] / [(1.005833)^360 - 1] M = 400,000 × [0.005833 × 7.6123] / [7.6123 - 1] M = 400,000 × 0.044388 / 6.6123 M = $2,661

For the 15-year loan: P = $400,000, r = 0.065/12 = 0.005417, n = 180.

M = 400,000 × [0.005417 × (1.005417)^180] / [(1.005417)^180 - 1] M = 400,000 × [0.005417 × 2.6575] / [2.6575 - 1] M = 400,000 × 0.014396 / 1.6575 M = $3,484

Total paid on 30-year: $2,661 × 360 = $957,960. Total interest: $557,960. Total paid on 15-year: $3,484 × 180 = $627,120. Total interest: $227,120.

Gross interest difference: $330,840. Net difference after accounting for the higher payments on the 15-year: $119,442 in favor of the 15-year.

Worked Example 1: The $550,000 Loan in a High-Cost Market

Buyers in markets like Seattle, Denver, and Boston frequently borrow $550,000. Run the same comparison at today's typical rate spread, where 15-year rates average roughly 50 basis points below 30-year rates.

Assumptions: $550,000 loan principal, 30-year at 7.25%, 15-year at 6.75%.

30-year payment: $3,752 per month. Total interest paid: $800,720. 15-year payment: $4,867 per month. Total interest paid: $326,060.

Gross interest difference: $474,660. Additional payments made on 15-year over 15 years: $1,115 × 180 = $200,700. Net advantage of 15-year term: $273,960.

Over a 30-year horizon, the 15-year borrower also owns the home free and clear for 15 years while the 30-year borrower still holds debt. That 15-year period of eliminated housing payments, reinvested conservatively at 5.00%, generates an additional $286,447 in wealth.

Combined, the 15-year borrower finishes $560,407 ahead in total net worth at year 30. That is not a rounding error. That is a retirement account.

Worked Example 2: The "Invest the Difference" Argument

This is the most common counterargument to the 15-year mortgage. It deserves a direct answer.

The logic: take the $823 monthly savings from choosing the 30-year mortgage and invest it in an S&P 500 index fund. Historically the index returns roughly 10.0% annually before taxes. At that rate, $823 per month invested for 30 years compounds to $1,713,506.

That sounds decisive. It is not, for three reasons.

First, your mortgage rate is guaranteed. Your investment return is not. The S&P 500 returned 10.5% annualized from 1928 to 2024, but individual 30-year windows vary from 8.1% to 13.7%. You do not know your window in advance.

Second, investment returns are taxable. In a taxable brokerage account, long-term capital gains rates of 15.0% to 23.8% apply. Net of taxes, that 10.0% gross return becomes roughly 8.0% to 8.5%.

Third, most borrowers do not actually invest the difference. Studies from the National Bureau of Economic Research consistently show that households presented with disposable income from lower loan payments consume most of it rather than invest it systematically.

Run the honest version: $823 per month invested at 8.0% net after tax for 30 years accumulates to $1,141,508. Compare that to $557,960 in interest paid on the 30-year loan versus $227,120 on the 15-year. The 30-year borrower paid $330,840 more in interest to run this strategy.

Net gain from investing the difference: $1,141,508 minus $330,840 = $810,668.

The 15-year borrower who paid off the home in year 15 and then invested their full $3,484 former payment at 8.0% for 15 years accumulates $1,213,317 from that phase alone, having already eliminated the debt entirely.

At these realistic return assumptions, the 15-year path wins by $402,649 at year 30. The "invest the difference" argument requires consistently achieving above-average after-tax returns to overcome the guaranteed interest savings.

The Rate Spread: When It Changes the Answer

The 50 basis point rate advantage typically offered on 15-year mortgages does meaningful work. If that spread compresses to zero, the calculation shifts.

At equal rates of 7.00% on both terms, the 15-year payment rises to $3,592 on a $400,000 loan. The monthly payment gap versus the 30-year narrows to $931. Total interest on the 15-year falls to $246,560. Total interest on the 30-year remains $557,960. Gross interest savings: $311,400. Net savings after the higher payments: $143,568.

The 15-year term still wins at equal rates. The rate spread accelerates the advantage, but does not create it. Even in a flat-spread environment, the shorter amortization schedule generates substantial savings.

Check the current spread before you commit. If lenders in your market offer a 15-year rate within 30 basis points of the 30-year rate, the financial case for the shorter term strengthens considerably.

What This Means for Your Decision

Three variables determine which term serves you better.

Cash flow margin. The 15-year payment on a $400,000 loan runs $823 more per month than the 30-year equivalent. If that payment requires more than 28% of your gross monthly income, the 30-year term preserves flexibility. Financial stress from an aggressive payment schedule carries its own cost.

Your marginal tax rate. Mortgage interest remains deductible for loans up to $750,000 on your primary residence. High-income borrowers in the 37% federal bracket extract real after-tax value from carrying a mortgage. The effective cost of a 7.00% mortgage drops to 4.41% after deduction. At that effective rate, disciplined investing at 8.0% net produces a genuine advantage. Run your numbers at your actual marginal rate, not the headline rate.

Time in the home. If you sell within seven years, the amortization comparison changes entirely. The 15-year mortgage builds equity faster, which increases your proceeds at sale. On a $400,000 loan, you hold $52,744 more equity after five years with the 15-year structure. That equity directly increases your down payment on the next purchase.

Run Your Exact Numbers

The examples above use specific loan amounts and rate assumptions. Your situation differs. Your loan size, your state tax rate, your actual rate quotes, and your planned holding period all shift the outcome.

The CalcMoney mortgage calculator lets you input your exact figures and see the total interest comparison, the monthly payment difference, and the equity accumulation curve for both terms side by side.

Input your numbers once. The output tells you exactly how much each term costs you over your specific time horizon.

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