15-Year vs 30-Year Mortgage: The Real Cost of the Lower Payment
The 15-year mortgage saves a six-figure sum and most people should not take it. Both things are true; here is the arithmetic that reconciles them.
The choice looks like a rate comparison but behaves like a cash-flow decision. The 15-year loan forces wealth-building through a high mandatory payment; the 30-year preserves optionality at a steep interest price. The correct answer depends on your income stability and discipline more than on the spreadsheet — but the spreadsheet comes first.
Same loan, two terms
$340,000 borrowed. The 15-year rate is shown 0.5% lower, reflecting typical pricing (15-year loans price better because the lender's risk window is shorter).
| 15-year @ 6.0% | 30-year @ 6.5% | Difference | |
|---|---|---|---|
| Monthly payment | $2,869.11 | $2,149.03 | +$720.08/mo |
| Total interest | $176,440.38 | $433,651.26 | $257,210.88 saved |
| Payoff | Dec 2040 | Dec 2055 | 15 years sooner |
The $257,210.88 interest gap is real, and it is why the 15-year is the mathematically correct default for households that can absorb the payment. The question is what "absorb" means.
The 30-year-with-extra alternative
Take the 30-year loan but pay it on the 15-year's schedule — add $720.08/month as extra principal:
| Path | Mandatory payment | Actual payoff | Total interest |
|---|---|---|---|
| 15-year loan | $2,869.11 | Dec 2040 | $176,440.38 |
| 30-year + $720.08/mo extra | $2,149.03 | Nov 2041 | $205,417.41 |
Within $28,977.03 of the 15-year's cost — a small premium for a big option: in any bad month, you can drop back to the $2,149.03 contractual minimum without penalty or paperwork. The 15-year loan offers no such lever; missed payments there are delinquencies.
Decision rules that survive contact with reality
- Take the 15-year if the payment is under ~20% of gross monthly income, your job is stable, retirement savings are on track, and you would not realistically invest the difference anyway.
- Take the 30-year and self-amortize if income is variable, you are early-career with rising earnings, or you value the flexibility premium more than the ~$28,977.03 it costs.
- Never take the 30-year at its contractual minimum if you can afford more — that is the $257,210.88 mistake in the table above.
Model all three paths with your quoted rates on the mortgage calculator with extra payments — the extra field makes the 30-year-with-extra comparison one input away.