When a 30-year fixed and a 15-year fixed sit side by side on a lender’s rate sheet, the spread looks small. In May 2026, Freddie Mac’s PMMS reports about 6.85% for a 30-year and 6.00% for a 15-year — less than a full percentage point apart. The mistake most first-time buyers make is comparing the rate. The right comparison is the lifetime interest, and on a typical $400,000 loan that gap is north of $300,000. The decision is rarely about rate; it is almost always about cash flow.
$400,000 loan: the numbers most people don’t see
Run the standard amortization on a single loan amount with both terms. Same principal, same property, two different futures.
| Item | 30-year fixed | 15-year fixed |
|---|---|---|
| Rate (PMMS, May 2026) | 6.85% | 6.00% |
| Monthly P&I | $2,621 | $3,375 |
| Lifetime interest | $543,498 | $207,723 |
| Total cost | $943,498 | $607,723 |
| Difference vs 30Y | — | -$335,775 |
That last row is what most online calculators bury below the fold. Switching from a 30-year to a 15-year on the same $400K loan saves more money over the life of the mortgage than the price of a small starter home. The monthly payment goes up about $754 — meaningful, but a fraction of the lifetime savings. You can model both scenarios in the interest tool and add them to the compare panel; the diff line shows monthly delta and lifetime interest delta on the same row.
Why the math is not linear
Lifetime interest scales worse than the term. Doubling 15 to 30 years more than doubles the interest because of how amortization front-loads it. On the 30-year, the very first payment is split roughly $2,283 interest / $338 principal — over 87% of the first month is interest. On the 15-year at a slightly lower rate, the first payment splits $2,000 interest / $1,375 principal — only 59% interest. The shorter term builds equity dramatically faster.
By month 60 (year 5):
- 30-year: $20,690 paid in principal, $137,565 paid in interest, balance $379,310
- 15-year: $86,000 paid in principal, $116,500 paid in interest, balance $314,000
After five years, the 15-year borrower has paid down four times as much principal. If life intervenes — job loss, divorce, a forced sale — the 15-year homeowner walks away with substantially more equity.
When 30-year still wins
There are real scenarios where 30-year is the right choice and 15-year would be reckless.
- Tight cash flow with no buffer. If $750 extra per month would put you below three months of liquid emergency savings, the 30-year preserves the buffer. A foreclosure during a job loss costs more than $300K in long-term financial harm.
- Investment alpha is real for you. A disciplined index investor can plausibly earn 7–9% long-run on the $750/month difference. Over 30 years that’s a portfolio worth more than the interest saved by going 15-year. Most people are not that disciplined; the math depends on actually investing the difference, every month, for decades.
- Mortgage interest deduction matters. If you itemize and the deduction lands you in a meaningfully higher tax savings bracket, the 30-year’s larger deductible interest reduces your effective after-tax rate. This effect is muted by the 2017 standard deduction increase — most filers no longer itemize — but it still applies for high-income, high-property-tax households.
- You’ll move within 7 years. Most of the lifetime interest gap accrues in years 15–30 of a 30-year loan. A buyer who plans to relocate by year 5 or 7 captures little of the 15-year’s savings while paying $750/month more for the privilege.
The “extra payment on a 30-year” workaround
The most common compromise: take the 30-year, then pay extra principal each month to mimic a 15-year amortization. This works, with two caveats.
The first is the rate. You keep the 30-year’s 6.85% rather than the 15-year’s 6.00%. Even paying $754 extra each month, the higher rate adds $20,000 to $40,000 in lifetime interest compared to a real 15-year. The flexibility costs roughly that much.
The second is discipline. The 15-year forces the higher payment; the 30-year + extra principal asks you to choose it every month, for 180 months. Behavioral economics suggests the forced version wins for most households. The flexible version wins for households with variable income (commission-based, freelance) where holding back optionality is itself the value.
Refinancing into a 15-year — when it pays off
Existing 30-year homeowners often eye a 15-year refinance. The break-even depends on three numbers: years already paid on the original loan, the rate gap, and closing costs. A rule of thumb that survives most stress tests:
Refinance to a 15-year only if (1) you’re at least 7 years into the original 30-year, (2) the new 15-year rate is at least 0.5 percentage points below your current rate, and (3) closing costs are below three years of monthly savings.
If all three hold, the refi typically pays off within 2–3 years. The interest tool’s compare panel handles this directly — enter your current loan in scenario A, the proposed 15-year in scenario B, and the refi break-even line shows the month at which the new lower payment plus principal acceleration overtakes the closing cost.
A practical decision framework
Before you commit to either term, run this checklist:
- Calculate the 15-year monthly payment in the interest tool. Is it more than 28% of your gross monthly income? If yes, 15-year is risky.
- Check your emergency fund. Does $750/month extra leave you with at least 3 months of liquid savings? If no, 30-year preserves the buffer.
- Estimate years in this home. Less than 7? The 15-year’s savings barely materialize.
- Test your discipline. Have you actually invested or saved a windfall before, monthly, for at least two years? If yes, 30-year + invested difference is viable. If no, the 15-year’s forced savings is your friend.
- Run the after-tax math. Does the mortgage interest deduction meaningfully exceed your standard deduction? If yes, the 30-year’s larger deductible interest carries weight.
The 30-year vs 15-year decision is a cash-flow decision dressed up as a rate decision. Most online calculators show you the rate spread and stop there. The interest tool puts the lifetime interest and the monthly payment on the same row, so the actual tradeoff — $750 a month for $335K — is the first thing you see, not the last.