Switch to Biweekly Mortgage Payments, or Invest the Difference? One $380,000 Loan, Run Both Ways

Key takeaways

• On a $380,000, 6.75%, 30-year loan that is two years in, a biweekly schedule works out to an extra $205.39 a month toward principal — one-twelfth of the $2,464.67 payment.
• Kept up until payoff, that extra principal ends the loan 5 years and 3 months early and saves $101,102 in interest from today forward.
• At year ten, the extra-principal path holds $35,065 more home equity; investing the same $205.39 a month instead grows to $33,659 at 6% or $37,575 at 8%, before capital-gains tax.
• The loan's own rate decides the tilt: at the 7.40% new-loan average, the guaranteed side pulls ahead of the 6% case and roughly ties the 8% case.

Freddie Mac's Primary Mortgage Market Survey put the average rate on a new 30-year fixed mortgage at 7.40% for the week of October 15, 2026, with the 15-year fixed average at 6.71% the same week. Most homeowners asking whether to switch to biweekly mortgage payments, though, are not shopping for a new loan — they already have one, usually at a lower rate than today's new originations. So this comparison starts from a loan already in force: a $380,000, 30-year fixed mortgage taken out two years ago at 6.75%, and it runs the biweekly-payment path against the invest-the-difference path for the next ten years, using one consistent set of numbers rather than a rule of thumb.

Illustration of how total interest scales with rate and term (generic chart, not this article's specific loan). Image: Gallowolf, Wikimedia Commons, CC BY-SA 4.0.

$205.39: where the extra payment comes from, and the loan behind it

All the numbers below trace back to one loan: a $380,000 original balance, 30-year term, fixed at 6.75%, now 24 months into repayment. The standing monthly principal-and-interest payment on that loan is $2,464.67 (principal and interest only; it excludes taxes and insurance escrow, which don't change between the two paths compared here and so are left out). After 24 on-time payments, the balance has come down to $371,618. Every dollar figure past this point is calculated from that starting balance and that payment — not estimated.

A "biweekly" mortgage plan has the servicer (or a third-party biweekly-payment processor, which often charges its own setup or per-transaction fee) collect half the monthly payment every two weeks. Because a year has 52 weeks, that works out to 26 half-payments — the equivalent of 13 full monthly payments a year instead of 12. The extra payment is applied entirely to principal. For this loan, one-twelfth of $2,464.67 is $205.39, so the biweekly path is modeled here as: keep making the regular monthly payment, and also apply an extra $205.39 to principal every month. That is mathematically equivalent to the 26-half-payment schedule without requiring a third-party processor, and several servicers will accept extra-principal payments directly at no charge, which this comparison assumes.

26 half-payments, or 13 payments a year: what that does to this loan

The 13th payment is the whole mechanism. Sent to principal every month, it changes the payoff date and the lifetime interest bill like this.

Applying an extra $205.39 to principal every month, on top of the regular $2,464.67 payment, shortens the loan from its original 30-year amortization to about 22 years and 9 months — 5 years and 3 months earlier than the standard schedule. Run from today specifically: ten years from now (month 120 of this comparison), the remaining balance on this path is $272,648. Carried all the way to a zero balance rather than stopped at year 10, this path pays $355,410 in total interest from today forward, against $456,512 in total interest on the standard schedule — a lifetime interest saving of $101,102, but only if the extra payment continues every month without a gap until the loan is paid off. Missing months do not bank for later; this is not an average, it is what a continuous extra-principal schedule produces.

Year ten: $35,065 of equity against $33,659–$37,575 invested

The alternative is to leave the mortgage alone and invest the same $205.39. Both paths can then be read at the same point, month 120.

Under Path B, the mortgage is left exactly on its original 30-year schedule — no extra principal — and the same $205.39 a month instead goes into a taxable brokerage account in a broad index fund. Ten years out, the mortgage balance under this path is $307,713 (identical to what Path A's balance would be if the extra payments were never made, since Path B makes no extra payments). The investment account is the new variable. Using the 6% long-run planning assumption that NerdWallet's market-return explainer cites as a reasonable baseline after the S&P 500's roughly 10%-a-year, near-century average is adjusted down for a more conservative forward estimate, $205.39 invested monthly for 120 months grows to $33,659. Using an 8% annual return instead — within the range the same source's 10-year historical table implies, though at the higher end of it — the same contributions grow to $37,575. Both figures are investment growth before any capital-gains tax on the gains, and before accounting for the fact that markets do not compound in a straight line the way this calculation assumes.

Measure (at month 120, i.e., 10 years from today)	Path A — extra principal	Path B — invest the difference
Mortgage balance remaining	$272,648	$307,713 (unchanged from today's standard schedule)
Extra built up, relative to doing neither	$35,065 more home equity than the standard schedule	$33,659 invested at an assumed 6%/yr, or $37,575 at an assumed 8%/yr
Form of the gain	Illiquid — locked into the home, accessed only by selling, refinancing, or a home-equity loan	Liquid — sellable at will, but subject to capital-gains tax on the gains when sold
Certainty	Guaranteed: a dollar of extra principal always avoids 6.75% in future interest	Not guaranteed: 6%–8% is an assumption, and down years happen along the way

At this loan's 6.75% rate, the two paths land within about $1,400 of each other at a 6% investment assumption — Path A's extra equity ($35,065) is slightly ahead of Path B's invested total ($33,659). At an 8% assumption, Path B pulls ahead by about $2,500 ($37,575 versus $35,065). Both of those gaps are small compared with the $200,000-plus balances involved; this is a close call at this specific rate, not a lopsided one.

The two year-ten figures are close in dollars but are not the same kind of money. Four differences sit underneath the table:

• Taxes. Path B's $33,659–$37,575 is pre-tax growth in a taxable account; long-term capital-gains tax (0%, 15%, or 20% federally depending on income, plus any state tax) applies when the position is sold, which Path A's equity never faces since avoided interest is not taxable income. Holding the investment inside a tax-advantaged account such as an IRA would change this comparison, but contribution limits and withdrawal rules would then apply.
• Liquidity. A dollar of extra principal cannot be borrowed back without a new loan, a HELOC, or selling the home, each of which has its own cost and approval process. A dollar in the brokerage account can be sold in a day — a meaningful difference for anyone who might need the money for an emergency before year 10.
• Risk. The 6.75% interest avoided by Path A is contractual and does not fluctuate. The 6%–8% assumed in Path B is a range drawn from historical averages, not a promise; a decade that includes a prolonged downturn could land well below either figure, and this comparison does not model that case.
• Mortgage interest deduction. For a household that itemizes, part of the interest avoided by Path A would otherwise have been partly tax-deductible, which modestly narrows Path A's effective advantage; most filers now take the standard deduction, so this applies to a minority of households.

7.40% versus 6.75%: the number that tilts the answer

This loan's 6.75% rate is lower than what Freddie Mac's October 15, 2026 survey reports for a new 30-year fixed loan today (7.40%). That gap matters here because the "guaranteed return" from extra-principal payments is always equal to the loan's own rate, while the investment assumption (6%–8%) does not move with mortgage rates. On a hypothetical identical $380,000 loan opened today at 7.40% instead of 6.75%, the standard monthly payment would be $2,631.04 — $166.37 higher — and the extra-principal path would be avoiding interest at 7.40%, not 6.75%. At that rate, the guaranteed side of the comparison pulls further ahead of the 6% planning assumption and roughly ties the 8% upside case, which is the general pattern worth remembering: the higher a borrower's actual rate, the more the math favors extra principal over investing, all else equal.

Whichever way that rate points, a few practical checks come before the first extra payment:

1. Call the servicer and confirm, in writing, that extra payments are applied to principal immediately and not held until the next due date — some servicers default to the latter unless instructed otherwise.
2. Check for a prepayment penalty; these are rare on conventional loans originated after 2014 but still exist on some older or non-conforming loans.
3. If a third-party biweekly-payment service is being considered instead of self-directed extra payments, compare its setup and transaction fees against simply paying the extra $205.39 directly — the self-directed version in this article costs nothing extra.
4. Confirm whether an emergency fund already covers 3–6 months of expenses before directing money toward either path; both paths assume that question is already settled.
5. If choosing Path B, decide in advance which account (taxable brokerage, Roth IRA, 401(k)) the $205.39 will go into, since the tax treatment materially changes the after-tax comparison above.

Tip: Whichever path you pick, the comparison only holds if the extra payment actually leaves your account every month. Running the amortization yourself on a financial calculator makes the interest-saved figure concrete, and a budget planner is what keeps the difference from quietly getting spent instead. (These are Amazon Associate links — we may earn a small commission on qualifying purchases.)

Sources and disclaimer

30-year and 15-year fixed mortgage rates: Freddie Mac's Primary Mortgage Market Survey, week of October 15, 2026 (30-year average 7.40%, 15-year average 6.71%). Long-run and 10-year S&P 500 return figures and the 6% planning assumption: NerdWallet's average stock market return explainer. All payment, balance, and interest figures are calculated directly from the stated $380,000/6.75%/30-year loan terms using standard amortization formulas; they are not estimates or rules of thumb. This article is general information, not individualized financial, tax, or legal advice. Mortgage terms, fees, and tax treatment vary by lender, account type, and household; confirm your own servicer's rules on extra payments and consult a tax professional before choosing between these paths with your own money.

Related reading: Extra Mortgage Payments or a Maxed-Out Roth IRA First?, Mortgage Discount Points vs. Investing the Cash, 15-Year vs. 30-Year Mortgage in 2026.