AtlasCalcsUtility Calculation Suite
2026-09-21 7 min read

Mortgage Prepayment Calculator

The interest and the years saved by paying extra for a mortgage. Live calculator with worked steps, charts and a full amortisation schedule.

Calculator

Home Price Help
$

The purchase price of the property.

Down Payment(%)
%
Annual Interest Rate(%)
%
Loan Term(years)
years
Extra Payment Each Period
$
One-Time Extra Payment
$
One-Time Payment Made At Payment #

This prepayment & payoff calculator runs on the standard amortisation model: interest is charged on the outstanding balance each month, and anything extra you enter is applied to principal immediately.

Amortization schedule

Balance Interest Payment
View:
YearInterestPrincipalEnding Balance
1$25,435.91$9,540.65$310,459.35
2$24,644.04$5,332.52$305,126.82
3$24,201.44$5,775.12$299,351.70
4$23,722.11$6,254.45$293,097.25
5$23,202.99$6,773.57$286,323.68
6$22,640.79$7,335.77$278,987.91
7$22,031.92$7,944.64$271,043.27
8$21,372.52$8,604.04$262,439.24
9$20,658.39$9,318.17$253,121.07
10$19,884.99$10,091.57$243,029.50
11$19,047.39$10,929.17$232,100.33
12$18,140.28$11,836.28$220,264.05
13$17,157.87$12,818.69$207,445.36
14$16,093.93$13,882.63$193,562.72
15$14,941.67$15,034.89$178,527.84
16$13,693.79$16,282.77$162,245.06
17$12,342.32$17,634.24$144,610.83
18$10,878.69$19,097.87$125,512.96
19$9,293.58$20,682.98$104,829.98
20$7,576.90$22,399.66$82,430.32
21$5,717.74$24,258.82$58,171.50
22$3,704.27$26,272.29$31,899.21
23$1,523.68$28,452.88$3,446.33
24$29.45$3,446.33$0.00

Results

Optimal

Interest Saved

$147,360

Versus the scheduled payment alone.

Time Saved

6 years 10 months

82 fewer payments.

Payoff Without Extra Payments

30 years

New Payoff Time

23 years 2 months

Total Extra Principal Paid

$46,700

Return on Each Extra Dollar

315.55%

Interest avoided per dollar prepaid.

New Total Interest

$377,937

Original Total Interest

$525,297

Live results update as you type — currently showing interest saved of $147,360 on 12 payments a year. Replace the defaults with your own quote for an exact comparison.

Visualizations

Payments Before vs After

360Scheduled
278With extras

Interest Paid vs Avoided

Interest still paid72%
Interest avoided28%

Balance With Extra Payments

Formula

Any structure that delays principal repayment keeps the balance higher for longer, and because interest is charged on the balance, the total cost rises. Conversely, anything that accelerates repayment — a shorter term, an extra principal payment, a faster frequency — lowers the average balance and therefore the finance charge.

See the worked steps below for the exact relationship used

Variables

SymbolNameUnitDescription
AAmountcurrencyBalance or amount involved in the calculation
iPeriodic ratedecimalAnnual rate ÷ periods per year
nNumber of periodscountTerm in payments

Explanation

Any structure that delays principal repayment keeps the balance higher for longer, and because interest is charged on the balance, the total cost rises. Conversely, anything that accelerates repayment — a shorter term, an extra principal payment, a faster frequency — lowers the average balance and therefore the finance charge.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Run the base schedule with the contractual payment only.

Baseline interest = P × n − principal
Payment:$2,348.05
Payments:360
Result: $525,297
Step 2

Re-run the schedule with the extra amount applied to principal each period.

Balance(k) = Balance(k−1)(1 + i) − (P + extra)
Extra:$150.00
One-time:$5,000
Result: 278 payments instead of 360
Step 3

The difference between the two finance charges is the saving.

Saved = baseline interest − new interest
Intermediate: $525,297 − $377,937
Result: $147,360 over 6 years 10 months fewer payments
Final Answer
$147,360

Worked Examples

See how the calculator works with different scenarios.

Scenario

The mortgage prepayment calculator run on the values above, showing the full calculation path rather than only the answer.

Inputs

Rate:8
Years:30

Calculation

1. Run the base schedule with the contractual payment only. → $525,297 2. Re-run the schedule with the extra amount applied to principal each period. → 278 payments instead of 360 3. The difference between the two finance charges is the saving. → $147,360 over 6 years 10 months fewer payments

Result

Interest Saved: $147,360

Interpretation

Extra $150.00/month plus $5,000 one-off saves $147,360 of interest and 6 years 10 months.

Understanding Your Result

Your Result: Interest Saved: $147,360

Typical range: $100 a month extra on a $320,000 loan at 6.5% saves roughly $60,000 of interest and 4–5 years of term.

What This Means

Any structure that delays principal repayment keeps the balance higher for longer, and because interest is charged on the balance, the total cost rises. Conversely, anything that accelerates repayment — a shorter term, an extra principal payment, a faster frequency — lowers the average balance and therefore the finance charge.

Why It Matters

Extra $150.00/month plus $5,000 one-off saves $147,360 of interest and 6 years 10 months.

Professional Interpretation

Extra $150.00/month plus $5,000 one-off saves $147,360 of interest and 6 years 10 months.

How this is calculated

Any structure that delays principal repayment keeps the balance higher for longer, and because interest is charged on the balance, the total cost rises. Conversely, anything that accelerates repayment — a shorter term, an extra principal payment, a faster frequency — lowers the average balance and therefore the finance charge.

What drives the result

Any structure that delays principal repayment keeps the balance higher for longer, and because interest is charged on the balance, the total cost rises. Conversely, anything that accelerates repayment — a shorter term, an extra principal payment, a faster frequency — lowers the average balance and therefore the finance charge.

Recommendations

  • •Confirm the servicer applies extra funds to principal, not as an early next instalment.
  • •Prepay only after clearing higher-rate debt and funding the emergency reserve.

Educational Content

Introduction

Loan structure determines when cash leaves your account and how much flexibility you keep. Interest-only periods, balloons, deferred starts, graduating payments, refinancing and biweekly schedules all change the timing of principal repayment without changing the formula behind it. This page prices the structure, not just the rate.

Concept Explanation

Any structure that delays principal repayment keeps the balance higher for longer, and because interest is charged on the balance, the total cost rises. Conversely, anything that accelerates repayment — a shorter term, an extra principal payment, a faster frequency — lowers the average balance and therefore the finance charge.

Applications

▸Choosing between a lower payment now and a lower total cost
▸Testing whether refinancing or restructuring is worth the fees
▸Planning around a balloon, a reset date or the end of a promotional period
▸Deciding how much extra to pay and whether it is better used to invest

Benefits

  • •Prices structures that are hard to compare by eye
  • •Shows the payment step-up or lump sum you must plan for
  • •Quantifies the break-even point on any restructuring cost
  • •Separates cash-flow relief from genuine savings

Limitations

  • •Assumes rates, income and repayment behaviour stay as entered
  • •Lender-specific rules (recast fees, minimum payments, penalties) are not automatic
  • •Tax treatment of interest, penalty and forgiven amounts is not included
  • •Real-life events such as a sale or refinance can change the outcome entirely
  • •Prepayment handling: Extra amounts are applied immediately to principal while the scheduled payment stays unchanged, so the term shortens rather than the payment shrinking.

Best Practices

  • •Match repayment to the life of the asset being financed
  • •Write down the exit plan for any balloon or reset before signing
  • •Compare structures on total cost over the horizon you will actually hold
  • •Keep a cash buffer instead of pushing every dollar into principal

Common Misconceptions

Myth: A lower payment is always a win
Truth: Lower payments usually mean more periods and more interest unless the rate falls enough to compensate.
Myth: Structures are set in stone
Truth: Many loans allow recasting, extra payments or rate renegotiation — ask before assuming.

Practical Tips

  • •Match repayment to the life of the asset being financed
  • •Write down the exit plan for any balloon or reset before signing
  • •Compare structures on total cost over the horizon you will actually hold

Common Mistakes

A lower payment is always a win
Lower payments usually mean more periods and more interest unless the rate falls enough to compensate.
Structures are set in stone
Many loans allow recasting, extra payments or rate renegotiation — ask before assuming.

Assumptions

This calculator makes the following assumptions to provide estimates:

Figures as entered: The result follows directly from the values you supplied.

Limitations

Be aware of these limitations when using this calculator:

Limitation: Assumes rates, income and repayment behaviour stay as entered
Limitation: Lender-specific rules (recast fees, minimum payments, penalties) are not automatic
Limitation: Tax treatment of interest, penalty and forgiven amounts is not included
Limitation: Real-life events such as a sale or refinance can change the outcome entirely

Frequently Asked Questions

Answers to common questions about this calculator.

Who uses the mortgage prepayment calculator?
It is used by home buyers comparing mortgage offers, by owners weighing a refinance or equity move, and by loan officers and advisers sanity-checking a payment, ratio or fee structure. It sits in the Prepayment & Payoff group of the Mortgage & Home Finance section of Atlascalcs.
How accurate is the mortgage prepayment calculator?
Every figure follows directly from the inputs using standard amortisation mathematics, so it is accurate to the quality of the values you enter. Lenders round payments and apply their own day-count conventions, so expect small differences against a formal statement or payoff quote.
How current are the rates and thresholds used here?
The calculator uses the figures you enter, so update the rate to your actual offer or current market quote. Ratio thresholds, poverty guidelines and programme rules reflect the conventions described in the references and change periodically.
Does rounding change the results?
Every calculation runs on full precision and rounds only for display. Lenders round payments to the nearest cent or dollar, so expect a small difference between these figures and a formal statement or payoff quote.
Should I use the payment or the total cost to compare offers?
Use the payment to judge cash flow and the finance charge or APR to judge cost. When two offers differ on both, the all-in cost over the period you expect to hold the loan is the decisive measure.
Can I use this calculator for a business or commercial loan?
Yes. The payment, amortisation, ratio and cost calculators apply the same amortisation mathematics to commercial loans; the difference is in the fees, covenants and collateral rules, which you should enter explicitly.
What information should I have ready before applying?
Your exact rate and term, the fee schedule, any required add-ons, and your gross income and existing monthly obligations. With those five numbers every figure on this page can be tied back to a lender quote.

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Related Guides

Deepen your understanding with these educational resources.

Glossary

Technical terms used in mortgage calculations explained in plain language.

Principal
The amount borrowed, excluding interest and fees. Payments applied to principal reduce the balance and therefore future interest.
Amortisation
Repaying a debt through a schedule of payments that gradually shifts the split from interest to principal until the balance reaches zero.
APR (Annual Percentage Rate)
The yearly cost of credit expressed as a rate, including interest and most upfront finance charges.
Finance charge
The total dollar cost of credit, including interest and certain fees.

Authoritative References

Trusted sources and standards referenced by this calculator.

Applicable Standards

Level-payment (simple-interest) amortisation, payments assumed in arrearsAPR computed by discounting the actual payment stream to net proceedsDebt ratios computed on gross income for lender comparisonsMoney rounded for display only; all calculations run at full precision

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Page Information

Last Updated:

2026-09-21

Version:

2.0.0

License:

Free to use for educational and professional purposes.