A small extra payment toward your mortgage each month can save more than six figures in interest and cut years off the loan. The reason is the way mortgages are structured: early payments are mostly interest, so anything extra you throw at the principal pays outsized dividends. Here's exactly how much, with real numbers.
How mortgage amortization works
A fixed-rate mortgage is amortized: you pay the same total each month, but the split changes over time. Each month's interest is charged on the outstanding balance, which is highest at the start — so in the early years most of your payment covers interest and only a little reduces the balance. Every extra dollar of principal you pay early removes a dollar that would otherwise have accrued interest for decades.
Example: a $400,000 mortgage at 6.5% for 30 years
The monthly principal-and-interest payment is $2,528.27. Left to run its full 30 years, the loan costs about $510,180 in interest — more than the amount borrowed. Now add $200 to the principal every month:
| Scenario | Total interest | Payoff time |
|---|---|---|
| Standard payment | ~$510,180 | 30 years |
| + $200/month | ~$398,290 | ~24.4 years |
| You save | ~$111,900 | ~5.6 years (67 months) |
A $200 monthly extra — about $6.50 a day — saves roughly $112,000 in interest and clears the mortgage almost six years early. The bigger the extra payment and the earlier you start, the larger the saving.
The math: why a little does so much
At 6.5%, the monthly rate is about 0.5417%. An extra $200 of principal in month one avoids that 0.5417% every month for the rest of the loan — and avoids interest on the interest it would have generated. Each extra payment compounds in your favor, which is why the savings ($112k) dwarf the total extra you pay in (about $58,600 over the shortened term).
Three ways to pay extra
- Extra monthly payment: add a fixed amount to principal each month — the most flexible approach.
- Lump sum: apply a bonus, tax refund, or windfall directly to principal. A single $10,000 lump sum early in the loan can save many times that in interest.
- Biweekly payments: pay half your monthly amount every two weeks. That's 26 half-payments — 13 full payments a year instead of 12 — quietly adding one extra payment annually.
When NOT to pay extra
Paying down a mortgage is a guaranteed return equal to your interest rate. If you could reliably earn more elsewhere — say, in a diversified investment averaging more than your mortgage rate — investing the money may build more wealth. Also clear higher-interest debt first (credit cards at 20%+ beat any mortgage), keep an emergency fund, and capture any employer retirement match before overpaying the house. The right call depends on your rate, taxes, and risk tolerance.
Curious how the same compounding works in your favor when saving? See compound interest explained.
What extra payments do to your amortization schedule
Every mortgage payment is split between interest (charged on the current balance) and principal (what's left). Because the balance is highest at the start, early payments are mostly interest — on our $400,000 example, the very first payment puts about $2,167 toward interest and only $361 toward principal. An extra payment skips straight to principal, so it permanently removes that balance from every future interest calculation. View the amortization schedule and you'll see the crossover point — where principal finally exceeds interest in each payment — arrive years earlier once you add extra.
Lump sum vs. monthly extra: which wins?
Timing matters more than form. A one-time $10,000 lump sum applied in year one of our example saves more interest than the same $10,000 spread over many years, because it removes the balance sooner. But a sustained monthly extra is easier to budget and adds up to far more principal over time. The best results come from doing both when you can: start with any lump sum you have, then keep a monthly extra running.
| Strategy on the $400k loan | Approx. interest saved |
|---|---|
| $10,000 lump sum in year 1 | ~$28,000–$35,000 |
| $200 every month | ~$112,000 |
| Biweekly payments (13/yr) | ~$95,000 |
Extra payments vs. refinancing
If current rates are well below your mortgage rate, refinancing can cut interest without you contributing more each month — but it carries closing costs and resets the clock. Extra payments need no application, no fees, and no credit check, and you can stop any time. A common approach: refinance if you can lower your rate enough to cover closing costs within a couple of years, then direct the monthly savings into extra principal.
A note on the mortgage interest deduction
In the US, mortgage interest can be tax-deductible if you itemize, which slightly lowers the effective cost of that interest — and therefore slightly reduces the net benefit of paying extra. But most households now take the standard deduction and get no mortgage-interest benefit at all, and even for itemizers the deduction only returns a fraction of the interest. Don't let a partial tax break talk you out of guaranteed interest savings; run your own numbers.
How to start
- Decide on an amount you can sustain — even $50–$100/month compounds meaningfully.
- Tell your servicer in writing to apply extra funds to principal, and verify it on your next statement.
- Confirm there's no prepayment penalty (rare on modern conventional loans).
- Keep an emergency fund and clear higher-interest debt first — those usually beat overpaying the house.