What this tool does
Compare a fixed-rate monthly loan with recurring or one-time extra principal payments. Keep your scheduled payment unchanged while testing an earlier payoff.
How to use it
- Enter the outstanding principal, annual interest rate (not APR) and remaining term in whole years.
- Enter extra principal each month and an optional payment number for a lump sum. Payment 0 disables the lump sum. Add the total fee confirmed with your lender.
- Calculate, then expand the schedule or use the complete CSV or print view. Changing an input clears all previous results.
Formula and method
Monthly rate r = annual interest rate ÷ 100 ÷ 12; n = years × 12. Scheduled payment = principal × r ÷ [1 − (1 + r)^(−n)], or principal ÷ n at 0%. Interest uses the opening balance. Scheduled principal, recurring extra and then the one-time extra reduce that balance. Net savings = original interest − extra-payment interest − the entered total fee.
Worked example
For a 300,000 loan at 5% for 30 years, test 300 extra each month and a 10,000 payment in month 24.
Useful for
- Testing recurring overpayments
- Comparing a lump sum
- Reviewing fees before paying
Limits and notes
Planning estimate, not a lender payoff quote or financial advice. Use one currency throughout. Fixed rate, monthly end-of-period payments; no taxes, insurance, daily accrual, variable rates, recasting, investment returns or country-specific rules. Ask your lender or a qualified adviser to check your contract, extra-payment instructions and fees before acting.
Frequently asked questions
What do the payment numbers mean?
Payment 1 is the first monthly payment in this model, not a calendar date. The lump sum applies after scheduled and recurring principal in the selected month, capped at the balance. It is not used if the loan is already repaid.
Does the principal column already include the extra amount?
Yes. Principal includes all principal repaid in that month. Extra is the portion of principal paid above the scheduled amount; do not add it to Payment or Principal again.
How are fees and rounding handled?
The entered total fee is subtracted once from net savings and is not added to schedule payments. Calculations retain internal precision; displayed amounts and CSV cells are rounded to two decimals independently, so adding displayed rows can differ slightly from the summary.
Can I use this for any country or loan?
Only when this fixed-rate monthly model fits your loan. It does not infer local rules, prepayment rights or lender approval. Confirm the figures with your lender or a qualified adviser.
Source and method review: CFPB — Your Home Loan Toolkit