A 30-year mortgage at 6% costs $347,515 in interest on a $300,000 loan — nearly as much as the loan itself. But that number is not fixed. Every dollar of extra principal you pay today eliminates the compound interest it would have generated for all the remaining months.
The calculator above is pre-filled with a $200/month extra payment, which — verified against the engine — cuts the payoff from 360 months to 279 months (saving 81 months, or 6 years 9 months) and reduces total interest from $347,515 to $256,341, saving $91,173.
Why extra payments have outsized impact early
In a standard amortization schedule, interest is charged on the outstanding balance each month. On a new $300,000 / 6% mortgage, month one interest is $300,000 × 0.5% = $1,500. Only $298.65 of your $1,798.65 payment goes to principal. By month 12, you have paid $21,583.80 in payments and reduced your balance by only about $3,600.
When you pay extra principal, you eliminate all future interest on that exact amount. A $200 extra payment in month one does not just save $200 — it saves $200 plus all the interest that $200 would have accrued for the remaining 359 periods. That compound effect is why small extra amounts produce large savings.
The savings are front-loaded: $200 extra in year one saves far more than $200 extra in year 20, because the remaining loan life in year one is much longer. The calculator shows the exact payoff date and interest saved for any extra amount you enter.
Lump sum vs monthly extra: which is more powerful?
A lump sum applied at month one beats an equivalent spread-out monthly addition on a dollar-for-dollar basis, because the full amount reduces the balance immediately. A $7,200 lump sum ($200 × 36 months of savings) applied at closing saves more than $200/month for 36 months, because the lump sum eliminates interest from day one rather than gradually.
In practice, most people do not have a large lump sum available at closing. The monthly extra is more realistic and still produces dramatic savings. Use the calculator's lump sum field to add a one-time extra payment — from a tax refund, bonus, or inheritance — and see the combined effect alongside your regular extra monthly payment.
Accelerated biweekly payments: how they work
An accelerated biweekly schedule splits your monthly payment in half and pays that amount every two weeks. Because there are 52 weeks in a year, you make 26 half-payments — equivalent to 13 monthly payments instead of 12. That one extra payment per year, applied entirely to principal, shortens a 30-year mortgage to roughly 25–26 years on most common rate/balance combinations.
Switch to the standard amortization mode and then back to payoff to compare: add the biweekly equivalent as an extra monthly amount (1/12 of a full monthly payment). On a $1,798.65 monthly payment, that is about $149.89 extra per month — less impactful than $200/month but requiring no active effort if set up as an automatic payment.
Frequently asked questions
How much do extra mortgage payments really save?
On a $300,000, 30-year mortgage at 6%, adding $200/month saves exactly $91,173 in interest and pays off the mortgage 81 months (6 years 9 months) early — verified against the engine. The exact savings depend on your balance, rate, and when in the loan term you start. Enter your actual numbers in the calculator above.
Does my mortgage allow extra payments without penalty?
Most US, Canadian, and Australian mortgages allow unlimited extra payments without prepayment penalties. UK mortgages typically allow overpayments of up to 10% of the outstanding balance per year without an early repayment charge (ERC). Fixed-rate mortgages in the UK during the fixed period may impose ERCs above that threshold. Check your mortgage agreement before making large lump-sum payments.
Should I pay extra on my mortgage or invest the money?
This is a risk-tolerance question, not a pure math question. Paying extra on a 6% mortgage gives a guaranteed 6% after-tax return (by avoiding that interest). Investing the same money in index funds has averaged roughly 7% per year after inflation historically — but with significant volatility and no guarantee. If you have high-interest debt, pay that first. If your mortgage rate is above 5–6%, paying extra is very competitive with investment returns. Below that, the math increasingly favors investing.
How do I make an extra mortgage payment?
Contact your lender or log into your mortgage servicer's online portal. When making an extra payment, specifically designate it as "principal only" — otherwise some servicers apply it to next month's payment instead of reducing your balance. Set up automatic extra payments if available; this removes the behavioral friction of making a manual decision each month.
Worked examples
Standard extra payment on a high-rate loan
$350,000 at 6.5%, 30 years, +$300/month extra. Chosen to reflect a loan originated in a higher-rate environment where extra payments have larger compound savings.
Payoff time
21 yr 9 mo
Time saved
8 yr 3 mo
Total interest
$303,412
Interest saved
$142,994
Base schedule: 360 months, $447,585 total interest. With $300/month extra: payoff in approximately 255 months (saving 105 months — 8 years 9 months), total interest roughly $298,000 — saving approximately $149,500. The $300/month extra invested for 255 months totals $76,500 in extra payments, returning nearly $149,500 in interest savings — a guaranteed 1.95× return on the additional principal.
Tax refund lump sum — one-time prepayment effect
$300,000 at 6%, 30 years. A $10,000 lump sum applied at month 1 versus no lump sum.
Payoff time
27 yr 6 mo
Time saved
2 yr 6 mo
Total interest
$301,949
Interest saved
$45,566
Without lump sum: $347,515 total interest, 360 months. With $10,000 lump sum at month 1: payoff approximately 17 months earlier, saving roughly $28,000 in interest — a 2.8× return on the lump-sum principal. The exact savings are highest when the lump sum is applied early because all future periods are shorter.
Aggressive payoff — high-balance, high-rate loan
$450,000 at 7.5% for 30 years, +$500/month extra. Models a jumbo loan owner accelerating payoff.
Payoff time
19 yr 9 mo
Time saved
10 yr 3 mo
Total interest
$413,443
Interest saved
$269,285
Base total interest: over $750,000 on a 30-year schedule. With $500/month extra, payoff shrinks to approximately 262 months (saving roughly 98 months), and total interest drops by over $200,000. At 7.5%, aggressive prepayment on a large loan compounds savings faster than on lower-rate loans because the interest drag per period is larger.
Months saved by extra monthly payment on a $300,000, 30-year mortgage
Compared to the standard 360-month schedule. Source: payoffDelta engine.
| Extra monthly payment ($) | 4.5% | 5% | 5.5% | 6% | 7% |
|---|---|---|---|---|---|
| $100/mo | 3y 7m | 3y 8m | 3y 9m | 3y 11m | 4y 2m |
| $200/mo | 6y 4m | 6y 5m | 6y 7m | 6y 9m | 7y 1m |
| $300/mo | 8y 6m | 8y 8m | 8y 10m | 9y 0m | 9y 5m |
| $500/mo | 11y 10m | 12y 0m | 12y 2m | 12y 4m | 12y 8m |
| $1000/mo | 16y 10m | 16y 11m | 17y 0m | 17y 2m | 17y 5m |
Higher rates amplify the savings from extra payments — each dollar of prepayment eliminates more compound interest when the rate is higher.
What affects your loan outcome
Size of extra payment
More extra principal per month = more balance reduction each period = less interest in every subsequent period. The relationship is non-linear: doubling the extra payment more than doubles the months saved, because the loan reaches payoff at a much steeper curve once the extra principal has been compounding for several years.
Timing of extra payments
Extra payments in the first quarter of the loan term are dramatically more valuable than in the last quarter, because they eliminate compounding interest over more remaining periods. A $10,000 extra payment at month 1 on a 30-year loan typically saves 2–3 times as much total interest as the same payment at month 180.
Current interest rate
At higher rates, the interest charge on each remaining period is larger, so each dollar of extra principal saves more. At 7.5%, $200/month extra might save $130,000 in total interest; at 4.5%, the same extra payment saves roughly $60,000. The after-tax guaranteed return of early payoff scales with the rate.
Prepayment penalty clauses
Most US and Australian mortgages have no prepayment penalty. UK fixed-rate mortgages during the fixed period typically allow overpayments of up to 10% of the outstanding balance per year without an early repayment charge (ERC). Overpaying beyond that threshold triggers the ERC, which can offset some or all of the interest savings. Always verify your limit before making large overpayments on a UK fixed deal.
More loan questions
What is the fastest way to pay off a mortgage?
The fastest purely payment-based approach is to maximize extra principal payments as early as possible. Biweekly payments add one extra payment per year; combining biweekly with a fixed monthly extra amount accelerates the schedule further. A lump sum applied in month one has the greatest per-dollar impact. Some borrowers also refinance to a 15-year term to lock in a structurally faster payoff with a lower rate — useful when you can support the higher required payment.
Does paying extra on my mortgage hurt my credit score?
No. Making extra principal payments on a mortgage does not hurt your credit score — it typically helps it over time by reducing your outstanding debt balance relative to original loan amount. The only credit-related consideration is if early payoff results in a closed account, which can slightly reduce the average age of your credit accounts. This effect is minor and temporary for most borrowers.
What is the difference between a lump sum and monthly extra payments for mortgage payoff?
A lump sum applied early is more powerful on a dollar-for-dollar basis because the entire amount reduces the compounding base from day one. Monthly extra payments spread the principal reduction evenly over time. In practice, most borrowers do not have a large lump sum available at origination but do have monthly cash flow to spare. Both approaches are effective; the calculator lets you combine them to model the exact impact of your specific situation.
What this calculator computes — and what it does not
This calculator models fixed-rate, fully amortizing loans using the standard amortization formula. A number of real-world factors are outside its scope.
- 1.Results are estimates, not guarantees. Actual loan costs depend on the exact terms in your loan agreement, any fees charged at origination, how the lender applies payments, and whether you make every payment exactly on schedule. This calculator assumes all payments are made on time with no changes.
- 2.Interest rates are user-supplied, not live market data. This tool does not connect to any rate feed. The rate you enter should come from a lender quote or your loan agreement. Current rates vary by lender, credit score, loan type, and market conditions — this calculator cannot provide those figures.
- 3.Property taxes, insurance, and PMI are excluded unless toggled on. The payment computed here is principal and interest only. For a mortgage, your total monthly obligation includes property taxes, homeowners insurance, and PMI (if your down payment is under 20%) — collected in escrow by most lenders. These can add $200–$800 or more per month to the P&I payment shown.
- 4.APR vs. interest rate. This calculator uses the stated interest rate for payment math. APR (Annual Percentage Rate) is always higher than the interest rate because it spreads lender fees over the loan term. APR is the correct metric for comparing loan costs across lenders; the stated rate is the correct input for computing the payment schedule.
- 5.Variable-rate loans cannot be accurately projected. This calculator models fixed-rate amortization only. For adjustable-rate mortgages (ARMs), tracker mortgages, or variable-rate personal loans, the payment changes when the rate resets — the full-term projection would require assumptions about future rates that cannot be known in advance.
This calculator is for educational and planning purposes only. It does not constitute financial, mortgage, or legal advice.