15-Year vs 30-Year Mortgage: Total Cost Comparison

Monthly Payment Comparison — Same $400k Loan at 6.5%

The monthly payment on a fixed-rate mortgage is calculated from the standard amortization formula:

M = P × [r(1+r)^n] / [(1+r)^n − 1]

Where:
P = loan principal ($400,000)
r = monthly interest rate (annual rate ÷ 12)
n = number of payments (years × 12)

15-year at 6.5%: r = 0.065/12 = 0.0054167; n = 180

(1.0054167)^180 = 2.6541
M = 400,000 × [0.0054167 × 2.6541] / [2.6541 − 1]
M = 400,000 × 0.008692
M ≈ $3,477/month

30-year at 6.5%: r = 0.0054167; n = 360

(1.0054167)^360 = 7.0449
M = 400,000 × [0.0054167 × 7.0449] / [7.0449 − 1]
M = 400,000 × 0.006314
M ≈ $2,528/month

Monthly payment difference: $3,477 − $2,528 = $949/month

The 15-year payment is 37.5% higher monthly — a meaningful cash-flow commitment.

Total Interest Paid Comparison — The Real Cost

Total interest = (Monthly payment × number of payments) − Principal

The 30-year borrower pays roughly $284,000 more in interest for the flexibility of the lower monthly payment. That is a real cost — not a theoretical one.

Computed Total-Interest Table: Four Rate Scenarios × Two Terms

All calculations use a $400,000 loan principal. Monthly payments and total interest computed from the amortization formula above.

Why 15-Year Rates Are Typically Lower

Lenders charge less for 15-year mortgages because the repayment period is shorter, meaning less time for the borrower to default or for rates to move against the lender. The typical spread between 30-year and 15-year fixed rates has historically been 0.5–0.75 percentage points, per Freddie Mac's Primary Mortgage Market Survey (PMMS).

At approximate early-2026 market rates, a borrower might see 30-year rates in the mid-6s and 15-year rates in the high-5s to low-6s. This spread makes the 15-year even more attractive than the same-rate table above shows — because you'd typically get a lower rate on the 15-year, not the same rate as modeled.

Cash-Flow Trade-Off: What to Do with the ~$950/Month Difference

The central question is whether the $950/month you keep by choosing the 30-year mortgage can be invested to outperform the mortgage interest savings of the 15-year.

If invested at 7% average annual return (rough historical S&P 500 real return approximation):

FV = 950 × [(1.005833)^360 − 1] / 0.005833
(1.005833)^360 = 8.116
FV = 950 × 1220.0
FV ≈ $1,159,000

After 30 years, investing the $950 monthly difference at 7% produces approximately $1,159,000 — materially more than the $284,000 in interest savings from the 15-year mortgage.

The catch:This requires actually investing that $950 every month without exception for 30 years and achieving a consistent 7% return. Most people don't do this. Markets don't return 7% every year. And the mortgage interest savings are guaranteed; the S&P return is not.

For people with strong investment discipline and a long-term horizon, the 30-year plus invest-the-difference can mathematically outperform. For people who want a guaranteed outcome — own the home free-and-clear — the 15-year is structurally simpler.

The “Make Extra Payments on a 30-Year” Alternative

Can you replicate the 15-year outcome by taking a 30-year mortgage and making extra payments?

The short answer: Yes, approximately. If you consistently pay $3,477/month on a 30-year mortgage (the 15-year payment amount), you would pay it off in roughly 15 years and pay similar total interest to the 15-year mortgage.

The advantage: If income drops temporarily (job loss, parental leave, illness), you can revert to the minimum $2,528 payment. A 15-year loan has no such flexibility.

The disadvantage:The higher rate on the 30-year means a bit more total interest even if you pay it off at the same pace. And behavioral reality: many borrowers who “intend to make extra payments” don't do so consistently.

Tax Considerations

The mortgage interest deduction (IRC §163(h)) allows itemizers to deduct interest on up to $750,000 of acquisition debt (for loans originating after December 15, 2017, per TCJA — confirmed under current law). This applies to both 15-year and 30-year mortgages.

The difference: a 30-year mortgage generates substantially more deductible interest, particularly in the early years when more of each payment goes to interest. A 15-year borrower has lower total interest and a faster transition to primarily principal payments.

Practical impact for most borrowers: The mortgage interest deduction requires itemizing. With the 2026 standard deduction at $32,200 (MFJ), many borrowers — especially those without large state income taxes — do not benefit from itemizing. Use the mortgage interest deduction calculator to assess whether your interest level clears the itemization bar.

Fannie Mae Conforming Loan Limit

The 2025–26 conforming loan limit set by Fannie Mae and Freddie Mac is $766,550 for most areas (higher in designated high-cost areas). Loans above this limit are “jumbo” loans and may have different rate dynamics. The $400,000 example in this article is a conforming loan.

When 15 Wins / When 30 Wins

Choose the 15-year when:

• You can comfortably afford the higher payment without straining cash flow
• You are within 15–20 years of retirement and want to own free-and-clear
• You value certainty over flexibility
• You are not yet maximizing tax-advantaged retirement accounts (401k, IRA) — those should typically come first
• The 15-year rate is materially lower than the 30-year rate

Choose the 30-year when:

• The higher 15-year payment would strain cash flow or eliminate your emergency fund
• You have high-interest debt to pay down — those dollars work harder against 18–25% credit cards than against a 6.5% mortgage
• You are disciplined investors who will actually invest the payment difference
• You have variable income (self-employed, commission-based) and need payment flexibility
• You plan to sell within 7–10 years — in that horizon, the total interest savings of the 15-year are smaller

Frequently Asked Questions

Compare Your Specific Scenario with the Mortgage Calculator

The numbers above use a $400,000 loan at 6.5%. Your loan amount, rate, term, and tax situation are specific to you. The mortgage calculator computes your exact monthly payment, amortization schedule, and total interest for any combination of inputs. The mortgage interest deduction calculator shows whether your interest generates any tax value above the standard deduction.