The standard advice is simple: always take the 15-year mortgage if you can afford it. It's usually correct — but not always. Here's the actual math so you can decide for your specific situation.
The headline numbers
On a $350,000 mortgage at 6.5%:
| 30-Year | 15-Year | |
|---|---|---|
| Monthly payment | $2,212 | $3,040 |
| Total interest | $446,000 | $197,000 |
| Total cost | $796,000 | $547,000 |
| Interest savings | — | $249,000 |
The 15-year saves a quarter million dollars in interest. That's a compelling number.
But the monthly payment gap is $828. Before blindly choosing, you need to know what that $828 actually costs you in other opportunities.
The flip side: what you give up
That extra $828/month could be:
- Maxing out a Roth IRA ($583/month — $7,000/year at 30, growing at 7% = ~$600k by retirement)
- Extra payments on the 30-year mortgage itself (same effect as the 15-year, with more flexibility)
- A fully funded emergency fund (financial security has real value)
- Index fund investments (compounding starts sooner, even with smaller amounts)
The biweekly trick
Here's what most advisors don't mention: you can make the 30-year mortgage behave almost exactly like a 15-year without committing to the higher payment.
Take your monthly payment and divide by two — pay half every two weeks instead of monthly. This results in 26 half-payments = 13 full payments per year, not 12. That extra payment per year cuts about 4 years off a 30-year mortgage and saves roughly $30,000-40,000 in interest.
You get most of the 15-year benefit with the flexibility of the 30-year payment. If you hit a rough month, you can always fall back to the regular monthly amount.
When the 15-year actually wins
- You have a stable, high income with no other high-interest debt
- You are more than 10 years from retirement and need to accelerate mortgage payoff
- You are buying in a market where you plan to stay long-term
- You value the psychological win of owning your home outright faster
When the 30-year with biweekly is smarter
- You are self-employed or have variable income
- You want to keep maximum flexibility on where your money goes
- You are in your 20s or 30s with decades of compounding ahead in investments
- You plan to move within 5-10 years (pre-paying a mortgage you don't keep is a losing strategy)
The PMI variable
If you're putting down less than 20%, the calculation changes. PMI adds real cost — typically 0.5-1% of the loan per year. On a $350,000 loan with 10% down, that's $1,575-3,150/year until you hit 20% equity.
The 15-year gets you to 20% equity faster, which eliminates PMI sooner. Run the numbers with PMI included — it often closes the gap between the two mortgage types significantly.
The tool I built
I built a mortgage calculator that handles the full picture: principal, interest, property tax, insurance, HOA, PMI, and extra payments. You can model the 15-year vs 30-year side-by-side, see the full amortization schedule, and test the biweekly approach.
→ Mortgage Calculator — 15yr vs 30yr, biweekly, full amortization
No account. No tracking. Works on mobile.
The short answer
If you are financially stable, maxing out tax-advantaged retirement accounts, and staying in the home for 10+ years: take the 15-year.
If any of those conditions don't apply: take the 30-year and pay it biweekly. You'll lose some interest savings but gain flexibility that has real financial value.
The worst outcome is taking the 15-year mortgage at the edge of your budget and having to cash out investments or carry credit card debt to cover a gap month.
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