Extra mortgage payments — how much you save

Updated May 4, 2026 · 8 min read

Every dollar of extra mortgage principal saves you the future interest that dollar would have accrued over the remaining loan. That sentence is correct but doesn’t help. Here’s the version that helps: the same extra dollar saves about 17× as much in month 1 as in year 25, and most homeowners aim extra payments in the wrong place.

This is the working-engineer guide to the math.

How a mortgage payment is split

Each month, your payment is divided into two parts: interest and principal.

• Interest = the lender charges you for borrowing the money you still owe. Computed monthly: balance × (APR ÷ 12).
• Principal = the part that actually pays down the balance. Whatever’s left after interest.

A new $400,000 mortgage at 7% APR has a fixed monthly payment of about $2,661. Month 1:

• Interest: $400,000 × (0.07 ÷ 12) ≈ $2,333
• Principal: $2,661 − $2,333 ≈ $328

That’s the answer to “where did my first payment go?” — 88% of it was interest. Only 12% bought down the loan.

Month 60 (year 5):

• Balance: ~$376,500
• Interest: ~$2,196
• Principal: ~$465

Month 240 (year 20):

• Balance: ~$229,200
• Interest: ~$1,337
• Principal: ~$1,324

Month 360 (final):

• Balance: ~$2,646
• Interest: ~$15
• Principal: ~$2,646

The principal portion grows over the life of the loan; the interest portion shrinks. This is the amortisation curve, and it’s the single most important shape to internalize about mortgages.

How extra payments change the schedule

On a standard fixed-rate mortgage, the required payment is set on day one and doesn’t change. Each month the lender works out interest on the balance you still owe, takes that from your payment first, and applies the rest to principal.

An extra payment marked for principal skips the interest step. It goes straight to the balance. Next month’s interest is worked out on that lower balance, so a bit more of your regular payment goes to principal. That repeats every month after.

Your required payment usually stays the same. What changes is the end date: the balance reaches zero sooner, and the months you no longer pay are where the interest savings come from. Some lenders will instead re-amortize (recast) the loan to lower the payment. Ask yours which it does.

Why early extra payments save so much more

Throw an extra $1,000 at your $400K mortgage in month 1:

• It immediately reduces your balance to $399,000.
• All future interest is computed on the lower balance.
• Over 30 years, that $1,000 saves about $7,000 of future interest at 7% — and shaves about 3 months off the loan because the amortisation table compresses.

Throw an extra $1,000 at the same mortgage in month 300 (year 25, 5 years to go):

• Balance was about $134,000.
• The $1,000 reduces it to $133,000.
• Over the remaining 5 years, that $1,000 saves about $420 in interest.

Same $1,000. About 17× the savings if you put it in early.

The reason is compounding. The $1,000 you save in month 1 doesn’t just save its own interest — it stops the interest on that interest from accruing for 30 years. By month 300, there’s only 5 years left for compounding to do its work.

This is the answer to “should I prepay the mortgage or invest?” If you’re early in the loan and your APR is comparable to or higher than your expected investment return, prepaying wins on a risk-adjusted basis. Late in the loan, the math reverses — there’s not enough remaining time for prepayment savings to compound.

The four ways to throw extra money at a mortgage

Each has a different effect. Understanding which is which matters.

1. Extra principal each month

Add $200 to every monthly payment with “apply to principal” in the memo. (Many lenders default to applying extras to next month’s payment unless you specify principal — read the small print.)

This is the canonical “pay off your mortgage faster” move. On a $400K, 30-year, 7% mortgage with $200 extra each month:

• Loan paid off in: 24 years 3 months (vs 30)
• Total interest saved: ~$126,600

5 years 9 months and about $126,600 for $200/month. Compounding does the work; you just need to start early.

2. One-time lump sums

Tax refund, year-end bonus, inheritance — apply it to principal once. Same compounding logic; the saved interest is multiplied by the remaining loan years.

A single $10,000 lump sum applied in year 1 of the example mortgage saves about $60,000 to $64,500 in future interest, depending on month 12 versus month 1, and shaves about 2.3 years off the loan. Same $10,000 applied in year 20 saves about $9,700 and shaves about 7 months.

The “what to do with a windfall” decision is mostly the same as the “monthly extras” decision: how early in the mortgage are we?

3. Biweekly payments

Instead of paying once a month, pay half the monthly amount every two weeks. Result: 26 half-payments per year = 13 full payments instead of 12. The “extra” 13th payment goes to principal.

On the example mortgage, biweekly payments (13 monthly payments a year) save about $137,000 in interest and shave about 6.2 years off the loan. Slightly more aggressive than $200/month: the extra payment is one full installment a year, about $2,661, not the same cash on a new calendar.

Caution: many lenders don’t offer true biweekly schedules. Third-party “biweekly programs” sometimes charge fees. The self-managed version is to set up an autopay for half the payment every two weeks, manually marked as principal-bearing, after confirming with your lender that they’ll apply it correctly.

4. Round up

Pay $2,700 instead of $2,661 — round up to the nearest $100. Or $2,750. The “extra $39” is a tiny amount; the impact compounds similarly to method 1 but at a smaller scale.

Worth noting: rounding up by even a small amount on a 30-year loan saves real money over the life of the loan, but it’s the smallest of the four methods. Best for people who want the discipline without the cash-flow change.

Worked example: $100 a month vs a $5,000 lump sum

A new $300,000 loan at 6% APR over 30 years (360 payments). The monthly rate is r = 0.06 ÷ 12 = 0.005.

Payment M = P × r ÷ (1 − (1 + r)^−n)
          = 300,000 × 0.005 ÷ (1 − 1.005^−360)
          ≈ $1,798.65

Total interest, no extras = 360 × 1,798.65 − 300,000 ≈ $347,515

These figures compound monthly, as US mortgages do. Canadian fixed-rate mortgages compound semi-annually, so the payment and interest will be slightly lower.

Extra $100 every month, from the first payment

Paying $1,898.65 a month, the number of payments becomes:

n = −ln(1 − r × P ÷ payment) ÷ ln(1 + r)
  = −ln(1 − 1,500 ÷ 1,898.65) ÷ ln(1.005)
  ≈ 312.9  →  313 payments (the last one smaller)

• Paid off in 26 years 1 month instead of 30 years (47 fewer payments).
• Total interest ≈ $294,168, so about $53,346 less.
• Extra money paid in: 312 × $100 = $31,200. The 313th payment is a final partial payment of about $1,788.87.

One $5,000 lump sum

Same loan, no monthly extras, with $5,000 added to principal alongside the 12th payment:

• Paid off in 28 years 9 months (15 fewer payments).
• Total interest ≈ $325,146, so about $22,368 less.

The same $5,000 added with the 240th payment (year 20) saves about $4,006 and 5 payments. Same money, about a fifth of the savings, because far fewer months of interest are left to avoid.

If your lender recasts after the month-12 lump sum instead of keeping the payment, the remaining $291,316 re-spread over 348 months gives a payment of about $1,768.30, roughly $30 lower, with the original end date.

For the same idea shown as $200 a month in year 1 vs year 25, see Why an extra $200/month in year 1 saves more than $200/month in year 25.

What “save $X” actually means

Two warnings about the savings numbers.

Future interest is in nominal dollars. Saving $100,000 over 30 years means $100,000 in mixed dollars across 2026 to 2056. In present-value terms — discounted at, say, 3% inflation — that’s about $50,000 of today’s purchasing power. Still a lot, but worth knowing.

Mortgage interest may be tax-deductible. In the US, the mortgage-interest deduction means each $1 of interest paid costs your real wallet ~$0.78 (at the 22% bracket). So saving $100K of mortgage interest is really saving ~$78K of after-tax money.

In Canada, the mortgage-interest on your principal residence is not deductible, so the “save $X” number is the full pre-tax amount — which is unusually high-impact among savings strategies.

In both countries, the after-investment-return comparison is what you should run for a careful “prepay vs invest” analysis. The mortgage calculator lets you see the prepayment savings; combine those with your expected investment-return to make the call.

When not to prepay

Three cases where extra principal isn’t the best use of the money:

1. You have higher-APR debt. Credit cards at 24% beat any mortgage prepayment math. Pay those first.
2. You don’t have an emergency fund. Liquid savings ($1K–$5K minimum, ideally 3 months of expenses) come before prepayment. Mortgage equity isn’t liquid — you can’t pay your car repair with home equity without taking on a HELOC at higher interest.
3. Your mortgage rate is below the safe-investment rate. A 3% mortgage and a 5% Treasury bond mean you should buy the bond. The prepayment “saves” 3%; the bond “earns” 5%. Net: +2%.

The 2020–2021 cohort of US homebuyers locked in 2.5–3% rates that are below current safe-bond yields. For them, prepaying isn’t mathematically optimal — the same dollar earns more in a money market fund than it saves on the mortgage.

When prepayment definitely beats investing

The opposite case: a 7%+ mortgage and an uncertain investment horizon. Every $1 of prepayment is a guaranteed 7% return for the remaining life of the loan. The S&P 500’s expected return is 7–10% on average over decades, with massive year-to-year variance. For risk-averse savers, the certainty of the prepayment return often beats the higher-but-uncertain investment return.

The 2025 cohort of new homebuyers locking in 7% mortgages is in exactly this situation. Prepayment is unusually attractive right now.

The rule of thumb

If your mortgage APR > your expected long-term investment return (after taxes), prepay. If APR < expected return, invest.

For most people in 2026 with mortgages from the last 3 years and diversified index funds, those numbers are close enough that either is fine. The decision is usually more emotional than mathematical: do you sleep better with less mortgage debt or with more invested assets?

That’s a real question, not a math question.

Frequently asked questions

Does an extra payment lower my monthly payment?

Usually not. On a standard fixed-rate loan the required payment stays the same and the loan ends sooner. A lower payment only happens if the lender agrees to recast (re-amortize) the loan.

Is $1,200 once a year the same as $100 a month?

Close, but monthly saves a little more because each $100 reaches principal sooner. On a $300,000, 6%, 30-year loan, $100 a month saves about $53,346 and 47 payments. $1,200 paid with every 12th payment saves about $51,290 and 45 payments.

Does paying extra change my interest rate?

No. The rate stays the same. Extra payments lower the balance the rate is applied to, which cuts the interest charged each month after.

Can my lender charge me for paying extra?

Some mortgages have prepayment penalties or limits on how much extra you can pay each year. Check your loan terms, or ask the lender, before sending large extra amounts.

How is “interest saved” worked out?

Add up all the interest on the original schedule, then all the interest on the schedule with extra payments, and take the difference. Both totals use the same rate and assume the rate doesn’t change.

Try it

The mortgage calculator at finance.tooljo.com supports extra-principal payments and shows you the savings + payoff date in real time. The debt calculator does the same for non-mortgage debt — and the math is identical, just with consumer APRs (much higher) and shorter loan lives (much shorter).

Both tools live in your browser. Nothing you enter is uploaded.