15- vs 30-Year Mortgage: The Real Math

A 15-year mortgage on a $400,000 loan at 6.5% costs $3,484.43 a month and $227,197.30 in total interest. The 30-year version is $2,528.27 a month — almost a thousand dollars less — but you pay $510,177.95 in interest. Same loan, same rate: the 30-year costs $282,980.65 more over its life.

That is the trade in one sentence: the 15-year loan saves a fortune in interest and forces a much higher monthly payment. Everything else in this decision is about whether your budget and your other goals can absorb that payment.

Every number here comes from the same amortization engine the calculator runs. Put the two terms side by side yourself on the comparison page.

The headline numbers

All figures below are fixed-rate loans at 6.5%, the same rate on both terms, so the comparison is clean. (In the real world the 15-year usually gets a lower rate — more on that below.)

$400,000 loan          15-year @ 6.5%     30-year @ 6.5%
Monthly P&I            $3,484.43          $2,528.27
Total interest         $227,197.30        $510,177.95
Total paid             $627,197.30        $910,177.95
Interest saved (15y)   $282,980.65        —

The pattern scales almost perfectly with loan size:

Loan size    15y payment   30y payment   Interest saved by 15y
$300,000     $2,613.32     $1,896.20     $212,235.49
$400,000     $3,484.43     $2,528.27     $282,980.65
$500,000     $4,355.54     $3,160.34     $353,725.81

Compare a $400k loan at 15 vs 30 years.

Why the 15-year saves so much

It is not magic — it is time. Interest accrues on the outstanding balance every month, so the faster the balance falls, the less total interest the loan can ever charge. A 15-year loan crushes the balance roughly twice as fast, so it spends half as long accruing interest and the principal portion of every payment is far larger from day one.

On the 30-year $400k loan, your first payment is $2,528.27, of which only $361.61 goes to principal — the other $2,166.67 is pure interest. On the 15-year, the first payment is bigger ($3,484.43) but $1,317.76 of it hits principal immediately. You are buying equity instead of renting money. Watch the split shift month by month in the amortization schedule.

The part most calculators skip: the 15-year rate is lower

Lenders price 15-year loans at a discount — typically 0.5 to 0.875 percentage points below the 30-year rate — because the loan is less risky and pays back faster. That widens the gap further. Here is the same $400k loan with a realistic spread: 6.5% on the 30-year, 5.75% on the 15-year.

$400,000 loan          15-year @ 5.75%    30-year @ 6.5%
Monthly P&I            $3,321.64          $2,528.27
Total interest         $197,895.26        $510,177.95
Total paid             $597,895.26        $910,177.95
Interest saved (15y)   $312,282.69        —

The lower rate knocks the 15-year payment down to $3,321.64 and the lifetime interest to $197,895.26. Now the 30-year costs $312,282.69 more, and the monthly premium for the 15-year is only $793.37 instead of $956.16. Always quote both terms with a lender — the spread is real money. When you have two quotes, drop them into the calculator one at a time to see the exact split.

When the 30-year is the smarter choice

Lower lifetime interest is not the only thing that matters. The 30-year wins when:

• Cash flow is tight. The lower required payment is a safety margin. You can lose a job, have a kid, or face a medical bill without risking the house. A 15-year payment you can barely make is a fragile position.
• You have higher-return uses for the difference. The ~$800–950/month you save with the 30-year can go into a 401(k) match (an instant 100% return) or index funds (historically 7–10%). If your mortgage rate is below ~5%, investing the difference usually wins on expected value.
• You want flexibility. A 30-year loan can be paid like a 15-year loan whenever you choose — and you can stop any month you need to. A 15-year loan can never be paid like a 30-year.

That last point is the one people underrate, so it gets its own section.

You can get most of the benefit without locking in the payment

Take the 30-year loan, then send the difference as extra principal each month. On a $400k loan, paying an extra $956.16/month on the 30-year pays it off in exactly 15 years — the same payoff date as the 15-year loan, with total interest of $227,197.30.

The catch: at the same rate, the do-it-yourself 15-year and the real 15-year land in the same place. The real 15-year only pulls ahead because of its lower rate. So the honest framing is:

• Want the lowest possible interest and you qualify for the 15-year's lower rate and the payment is comfortable → take the real 15-year.
• Want the 15-year payoff but value the escape hatch of a lower required payment → take the 30-year and overpay. We walk through the exact extra needed in How to Pay Off a 30-Year Mortgage in 15 Years.

How to decide in five minutes

1. Get both a 15- and 30-year rate quote from your lender. Note the spread.
2. Open the comparison page and enter the 30-year in Scenario A, the 15-year (with its real lower rate) in Scenario B.
3. Look at the monthly payment difference. Can you make the 15-year payment every month, in a bad month, without raiding your emergency fund? If no, the decision is made — take the 30-year.
4. If yes, look at the interest saved. Decide whether that savings beats what the monthly difference would earn invested.
5. Either way, you can accelerate later. Nothing is permanent except the required payment.

The bottom line

At 6.5%, a 15-year mortgage saves roughly $210k–$355k in interest across common loan sizes, at the cost of a payment that is about 35–40% higher. The lower rate 15-year loans usually carry widens that savings further. But the 30-year's lower required payment is real insurance, and you can always overpay it to capture most of the benefit on your own terms.

Run your actual numbers on the calculator, or put both terms head to head on the comparison page. For the overpay-it-yourself route, read How to Pay Off a 30-Year Mortgage in 15 Years.