15-year vs 30-year mortgage: payment, total interest, and when each wins

Quick answer: at the same note rate and principal, a 15-year amortization raises your contractual payment and cuts total interest versus a 30-year. A 30-year is not “more expensive” because the rate is higher—it is usually more expensive because you pay interest across twice as many accrual periods unless you prepay aggressively.

Same-rate example ($400k @ 6.5%)

Fixed rate 6.5% APR, fully amortizing, $400,000 principal, monthly P&I only (no taxes, insurance, HOA, or MI). Rounded to the nearest dollar for scanning—recompute your exact offer in the mortgage calculator or loan calculator.

That ~$956/month gap is what you are “buying” with the 30-year: lower contractual obligation today in exchange for roughly $283k more interest if both loans run full term with no prepayment.

Payment budget vs lifetime interest

Lenders qualify you on housing payment + debts (see DTI for mortgage qualification). If a 15-year payment consumes too much of stable net cash flow after taxes, emergency fund contributions, and retirement, it can be the wrong tool even though the interest math looks attractive.

Use a hard rule: after choosing a term, simulate losing 10–20% of household income for six months. If the 15-year version fails that test but the 30-year passes, the 30-year is the more realistic contract—then you deliberately buy down interest with extra principal when months are strong.

Why term dominates total interest

Interest accrues on the outstanding balance. A 15-year schedule pays principal faster, so the balance drops sooner and each month’s interest charge shrinks earlier. A 30-year keeps principal higher for longer, so interest compounds in slow motion across 360 accrual periods. The mechanism is the same as explained in how loan interest is calculated—term is not a cosmetic label; it reshapes the amortization curve.

Liquidity: what the lower 30-year payment buys you

The 30-year’s lower required payment is best thought of as liquidity insurance: you can bank the difference, fund repairs, cover a job transition, or avoid carrying a balance on a 20%+ APR credit card. The insurance has a premium: scheduled interest, unless you systematically prepay.

If you know you will not prepay (behavior, cash discipline, competing goals), model the honest outcome: full-term 30-year cost from the table above, not a hypothetical early payoff.

30-year term + scheduled extra principal

If you want most of the interest savings of a 15-year but need a lower contractual floor, take the 30-year and set a standing extra principal that targets your desired payoff date. Our loan calculator shows months and interest saved when you add monthly or lump-sum prepayments—compare that path to the native 15-year payment.

Watch for prepayment penalties (uncommon on agency conforming loans but still present on some portfolios) and verify how your servicer applies overpayments (reduce term vs reduce payment).

Checklist before you lock

• Print or export amortization for both terms at the rate you were actually quoted, not the teaser.
• Add taxes, insurance, MI, HOA to the household budget—not just P&I.
• Stress income −10% / −20%: does the 15-year still clear reserves?
• If choosing 30-year “for flexibility,” set a calendar reminder to verify extra payments quarterly.
• If you may move before ~5–7 years, weight closing costs + equity pace more than ultra-long interest minimization.

Rate structure matters too: if you are comparing adjustable products, read fixed vs variable and the ARM cap ladder before you let term dominate the conversation.

Methodology & limits

Example payments use a standard fixed-rate amortizing loan (ordinary annuity) consistent with the engine behind CalnexApp calculators. Your Loan Estimate / Closing Disclosure may differ due to rounding, per-diem interest, buydown points (points & fees), or ARM discounts not modeled here.