Extra Mortgage Payments: How One Payment a Year Cuts Years Off Your Loan

Updated October 2026 · 6 min read · Small change, outsized effect — with two traps to avoid

Among all the "clever" money hacks, extra principal payments are one of the few that genuinely work — the math is boring, verifiable, and better than almost any alternative use of spare cash at today's rates. But the details of how you pay extra determine whether the money actually helps.

What one extra payment a year really does

Example: $300,000 loan, 30 years, 6.5%. Regular payment (principal + interest): $1,896. Add one twelfth of a payment ($158) each month — the practical version of "one extra payment a year":

Loan retires in roughly 24.5 years instead of 30 — about 5.5 years early.
Total interest drops from ~$382,000 to ~$305,000 — ~$77,000 saved.

Why so dramatic: every extra dollar goes entirely to principal, and every dollar of principal removed stops accruing 6.5% for the rest of the loan. Early in the loan, when payments are mostly interest, this leverage is at its maximum. Model your own numbers in the mortgage calculator.

The two ways to schedule it

Monthly thirteenth: add payment ÷ 12 to each monthly payment. Smooth on the budget, benefits start immediately, and it compounds — each month's extra dollar saves interest for every month after.
Annual lump: one full extra payment with a tax refund or bonus. Simpler to track, but the money sits idle until you send it — you lose months of "interest stopped" benefit on every dollar.

The monthly method wins mathematically; the annual method wins on simplicity. Either beats neither.

The trap: an unlabeled extra payment does nothing

This is where well-intentioned money gets wasted. Servicers may apply an unexplained extra amount as an early payment of next month's payment — same money, but treated as advance payment of interest you'd owe anyway, saving you nothing extra. Always:

Use the servicer's extra principal payment option (online portal or check memo: "apply to principal").
Verify the next statement shows principal reduced by the full extra amount.
Watch for biweekly payment programs that charge fees. The "free" version is just the monthly-thirteenth method — do it yourself for $0.

When extra payments are the wrong move

You have higher-rate debt. An extra $158 earning a guaranteed 6.5% is good; the same $158 killing a 24% credit card is great. Clear expensive debt first — see payoff ordering.
No emergency fund. Money sent to your mortgage is hard to get back. Home equity doesn't pay for a furnace — a savings buffer does.
You're skipping retirement matches. A 50% employer match is a 50% instant return. Nothing in mortgage math competes with that.
You expect to move soon. Extra principal builds equity you'll recover at sale either way — fine, but it's not "saving" if the loan dies early. Same logic as the points break-even.
You might benefit from investing instead. A guaranteed 6.5% "return" is attractive, but long-horizon market averages have historically exceeded it — that's the invest-the-difference argument from our 15 vs 30 comparison. Guaranteed vs historical: your risk tolerance decides.

A middle path that works for most people

Full employer retirement match first.
Emergency fund of 3–6 months second.
High-rate debt (>8–10%) third.
Then split surplus between extra principal and investing, according to how much certainty you want.

Run the exact timeline change from any extra amount in the loan calculator — watch the payoff date move, not just the interest.

Informational only — not financial advice.