What This Calculator Does
You got a bonus. You received a tax refund. You finally paid off your car and have an extra $300 per month. What if you put that money toward your mortgage or student loan? How much interest would you save? How many years earlier would you be debt-free? This calculator answers those questions in seconds.
The Extra Payment Calculator shows the impact of adding extra money to your regular loan payments. You can model both recurring extra monthly payments and one-time lump sum payments. The calculator compares your standard payment schedule against the accelerated schedule, showing the interest saved and the time shaved off your loan.
According to Freddie Mac market data, the average 30-year fixed mortgage rate in 2026 has been around 6.5%. On a $300,000 loan at that rate, adding just $200 per month in extra payments saves approximately $72,000 in interest and pays off the loan about 6 years earlier. Small amounts, applied consistently, produce large results.
Inputs Required
- Loan Amount: Your current loan principal or the amount you plan to borrow
- Interest Rate: Your annual interest rate (APR)
- Loan Term: Original term in years (10, 15, 20, or 30)
- Extra Monthly Payment: Additional amount you add to each monthly payment
- One-Time Extra Payment: A lump sum applied to principal at the start (bonus, tax refund, inheritance)
Outputs Provided
- Interest Saved: Total interest reduction from making extra payments
- Time Saved: Years and months shaved off your loan term
- Base Monthly Payment: Your standard payment without extras
- Total Interest Comparison: Side-by-side interest costs with and without extra payments
How the Calculation Works
The calculator first computes your standard monthly payment using the loan amortization formula. Then it simulates two scenarios in parallel.
Standard schedule: You pay the base monthly payment for the full term. Total interest equals (monthly payment x number of months) minus principal.
Accelerated schedule: The one-time extra payment is applied to principal immediately. Then each month, interest accrues on the remaining balance at the monthly rate, and the full payment (base plus extra) is applied. The simulation continues until the balance reaches zero.
Each month: Balance = Balance x (1 + r/12) - (Base Payment + Extra)
Interest that month = Balance x (r/12)
The difference in total interest between the two scenarios is your savings. The difference in the number of months is your time saved.
How to Use the Calculator
- Enter your loan amount. For an existing loan, use the current principal balance, not the original loan amount.
- Set your interest rate. Check your most recent statement for the exact rate.
- Select your remaining loan term. If you have 22 years left on a 30-year mortgage, set the term to match the remaining years.
- Enter how much extra you can add to each monthly payment. Start with a small amount like $100 to see the impact.
- If you have a lump sum to apply (bonus, tax refund), enter it in the one-time payment field.
For a full amortization schedule showing every payment, use our Amortization Calculator. To compare bi-weekly payments against monthly, try our Bi-Weekly Payment Calculator.
Example Calculations
Example 1: The $200 Monthly Extra
A homeowner has a $300,000 mortgage at 6.5% for 30 years. The base monthly payment is $1,896. They add $200 extra per month.
- Standard: 360 payments, total interest $382,600
- With $200 extra: approximately 277 payments (about 23 years), total interest approximately $310,500
- Interest saved: approximately $72,100
- Time saved: approximately 6 years and 11 months
That $200 per month totals $55,400 in extra payments over 23 years, but it saves $72,100 in interest. The return on those extra payments is effectively the loan's interest rate (6.5%), which is a guaranteed, tax-free return. Few investments can match that.
Example 2: The $10,000 Lump Sum
A borrower receives a $10,000 year-end bonus and applies it as a one-time extra payment on their $250,000 mortgage at 6.75% for 30 years. Monthly payment is $1,622.
- Standard: 360 payments, total interest $334,000
- With $10,000 lump sum: approximately 343 payments (about 28.5 years), total interest approximately $298,000
- Interest saved: approximately $36,000
- Time saved: approximately 17 months
A single $10,000 payment saves $36,000 in interest. That is a 3.6x return on the lump sum, because it reduces the principal that would have accrued interest for the remaining 28.5 years.
Real World Scenarios
The Raise Redirect
A teacher gets a $400 per month raise. Instead of lifestyle inflation, she redirects the entire raise to extra mortgage payments. Her $220,000 mortgage at 6.5% has 25 years remaining. The extra $400 per month saves approximately $88,000 in interest and pays off the mortgage about 9 years earlier. She will own her home free and clear at age 53 instead of 62.
The Tax Refund Strategy
A family receives an average tax refund of $3,000 each year. Instead of spending it, they apply it as a one-time extra payment each spring. On a $280,000 mortgage at 6.5% for 30 years, this annual $3,000 extra payment saves approximately $76,000 in interest and pays off the loan about 7 years early. The calculator lets them model this by entering $250 as a monthly extra ($3,000 / 12) to approximate the annual lump sum.
The Student Loan Acceleration
A recent graduate has $45,000 in student loans at 6.8% for 10 years. The monthly payment is $520. They add $150 extra per month by cutting dining out. The loan pays off in about 7.5 years instead of 10, saving approximately $6,800 in interest. Once the loan is gone, they redirect the $670 per month ($520 + $150) to retirement savings. Use our Debt Payoff Calculator for more debt strategies.
Common Mistakes to Avoid
- Not specifying extra payments go to principal: Some lenders apply extra payments to next month's interest instead of principal. This eliminates the benefit. Always specify that extra payments should be applied to principal reduction.
- Ignoring opportunity cost: If your loan rate is 3.5% and you can earn 7% in investments, paying extra on the loan costs you money long-term. Compare your loan rate to expected investment returns. As a rule of thumb, prioritize extra payments when your loan rate exceeds 5%.
- Not having an emergency fund first: Do not put every spare dollar toward extra loan payments if you have no emergency savings. Keep 3 to 6 months of expenses in savings before accelerating debt payoff. Use our Emergency Fund Calculator to set your target.
- Forgetting about prepayment penalties: Some loans charge fees for paying off early. Check your loan agreement. Most modern mortgages do not have prepayment penalties, but some auto loans and personal loans do.
Limitations of This Calculator
This tool assumes a fixed-rate, fully amortizing loan. It does not work for adjustable-rate mortgages, interest-only loans, or loans with balloon payments. The calculator does not account for escrow payments (property taxes and insurance), PMI, or HOA fees. It does not model prepayment penalties or lender-specific rules for applying extra payments. The one-time payment is applied at the beginning of the loan; if you apply it later, the savings will be smaller. For exact figures, contact your lender and request a payoff quote with extra payments applied.
Authoritative Research and Resources
- Freddie Mac Primary Mortgage Market Survey - Weekly survey of mortgage rates providing current benchmark rates to use in the calculator.
- CFPB: Paying Mortgage Early - The Consumer Financial Protection Bureau explains how extra payments reduce interest and what to confirm with your lender.
For related tools, try our Amortization Calculator for full schedules, our Bi-Weekly Payment Calculator for the bi-weekly strategy, or our Mortgage Payoff Calculator for comprehensive early payoff planning.