Mortgage Prepayment Calculator
How it Works
01Current Mortgage
Enter balance, rate, term, and compounding method
02Add Prepayments
Recurring + optional lump sum applied to principal
03Time Saved
Compare original vs accelerated payoff timeline
04Interest Saved
Exact savings figure, side-by-side PDF report
How to Calculate Mortgage Prepayment Savings
A mortgage prepayment is any amount paid toward the principal balance beyond the scheduled monthly payment. Because interest is calculated on the current balance, every extra dollar applied to principal saves you future interest — and shortens the overall payoff timeline. This calculator simulates month-by-month amortization with and without your prepayment plan, then shows the exact time and interest saved.
Supports both recurring prepayments (monthly, bi-weekly, accelerated bi-weekly, weekly, accelerated weekly, or annual) and one-time lump sum prepayments on any specific date. Combine both — for example, a small prepayment with every paycheck plus a tax-refund lump sum each spring — for realistic planning.
💡 Prepayment vs Extra Payment vs Overpayment
Different terms, same concept. A prepayment is any principal payment beyond what the amortization schedule requires. Make sure to instruct your lender to apply it to principal only — otherwise some lenders default it toward the next scheduled payment, which doesn't produce the same savings.
Supports 30+ currencies (USD, GBP, EUR, CAD, AUD, BDT, INR, PKR, and more) plus multiple compounding conventions (US/UK monthly, Canadian semi-annually, quarterly, annual, continuous) for cross-border accuracy.
How to Use the Mortgage Prepayment Calculator
How the Prepayment Math Works
The regular monthly payment stays the same: M = P × r(1+r)n / ((1+r)n − 1). Prepayments add on top — they don't reduce the scheduled monthly amount, just accelerate the payoff.
Each month: (1) interest charged = balance × monthly rate; (2) principal portion of regular payment = monthly payment − interest; (3) balance reduces by principal; (4) scheduled prepayment reduces balance further; (5) repeat until balance hits zero. Compared against the same simulation with no prepayments.
Non-monthly prepayments convert to monthly-equivalent: bi-weekly × 26/12 ≈ 2.17, weekly × 52/12 ≈ 4.33, annual × 1/12 ≈ 0.083. This lets the simulation run on a consistent monthly cadence while accurately reflecting your actual prepayment rhythm.
Lump sums are applied on the exact 'Paid on' month. Earlier lump sums have dramatically larger impact because interest charges on the reduced balance for every subsequent month compound the savings. A $10,000 lump sum in year 1 saves far more than the same $10,000 in year 10.
Example: $250,000 at 5% Over 30 Years
How different prepayment strategies compare on the same baseline:
| Strategy | Payoff In | Total Interest | Interest Saved |
|---|---|---|---|
| No prepayment (baseline) | 30 yrs | $233,139 | — |
| $50/month prepayment | 27 yrs 1 mo | $203,310 | $29,829 |
| $200/month prepayment | 22 yrs 3 mo | $157,050 | $76,089 |
| Accelerated bi-weekly | 25 yrs 5 mo | $194,300 | $38,839 |
| $15,000 lump sum at year 1 | 27 yrs 4 mo | $190,400 | $42,739 |
Notice how a $200/month prepayment saves $76K — more than the $72K in total prepayments themselves, meaning the strategy effectively earns a return equal to your mortgage rate. This is why mortgage prepayment is considered a guaranteed, risk-free investment at your mortgage's rate.
Who Uses This Calculator?
Technical Reference
Key Takeaways
Mortgage prepayments are one of the highest-return, lowest-risk financial moves available to homeowners. Every dollar paid to principal earns a guaranteed return equal to your mortgage interest rate — typically 4-7% in current markets, with no volatility, taxes, or complexity.
The most effective strategy isn't always dramatic. Even $50-100 of extra principal per month — roughly the cost of a weekly coffee habit — compounds into $30-70K of interest savings and 3-5 fewer years of debt over a typical 30-year mortgage.
Related: Mortgage Rate Calculator, Mortgage Interest Calculator, Mortgage Points Calculator. More in the Math & Science Calculators Collection.
Frequently Asked Questions
What is a mortgage prepayment?
Any principal payment made beyond what your amortization schedule requires. Can be a one-time lump sum (tax refund, bonus) or a recurring amount added to each monthly payment. Reduces your balance faster, saves future interest, and shortens the loan.
Are there penalties for mortgage prepayment?
Depends on the loan:
- Most US conventional mortgages: No prepayment penalty. Prepay any amount, any time.
- Some older or subprime loans: May have penalty clauses, usually limited to the first 3-5 years.
- Canadian mortgages: Often limited to 10-20% of original balance per year without penalty. Larger prepayments may trigger "interest rate differential" penalties, especially for fixed-rate loans.
Always check your mortgage documents before making large prepayments.
Should I prepay my mortgage or invest the money?
Rule of thumb: compare your mortgage rate to expected after-tax investment returns. Mortgage at 3% = easy to beat in the market. Mortgage at 7% = hard to beat consistently without risk. Many advisors recommend a blend: invest enough to capture employer 401(k) match first, then split remaining surplus between investing and prepayment based on your risk tolerance.
What's the difference between prepayment and recasting?
Prepayment applies extra principal, which shortens the loan but keeps the same monthly payment. Recasting re-amortizes the loan after a large prepayment, lowering your monthly payment over the remaining term (while keeping the original end date). Some lenders offer recasting for a small fee ($150-500). Recasting is useful if you want lower monthly obligations; prepayment is usually better for pure interest savings.
Does paying bi-weekly save money?
If done as "accelerated bi-weekly" (paying half your monthly amount every 2 weeks), yes. 26 half-payments per year = 13 full monthly equivalents, one more than the 12 you'd pay with monthly billing. That one extra payment per year typically saves 4-6 years on a 30-year mortgage and $30-60K of interest on a typical $250K loan.
Should I tell my lender the payment is for principal?
Yes — explicitly. Many lenders default extra payments to the next month's payment (not useful for saving interest). Most online payment portals have a principal-only option; use it. For paper checks, write "Apply to Principal" on the memo line. This ensures the extra reduces balance immediately.
Is the interest I save from prepayment taxable?
No — saving interest isn't income, so it's not taxed. However, in some jurisdictions (like the US), mortgage interest is tax-deductible, which reduces the effective benefit of prepayment. If your marginal tax rate is 30% and mortgage rate is 5%, the effective after-tax rate is ~3.5%. This lowers the "return" on prepayment compared to taxable alternative investments. Consult a tax professional for your specific situation.
Author Spotlight
The ToolsACE Team
Our specialized research and development team at ToolsACE brings together decades of collective experience in financial engineering, data analytics, and high-performance software development.
Disclaimer
The results provided by this tool are for informational purposes only and do not constitute financial, tax, legal, or investment advice. Always seek the advice of a qualified financial advisor, accountant, or legal professional regarding your specific situation.