Why the original amortization schedule is not the actual loan history
The original schedule assumes that scheduled payments arrive on schedule. If borrowers pay late, make extra principal payments, or miss installments, the outstanding principal and accrued interest may diverge from that projection. The correct method depends on the note's terms, payment application rules, and day-count basis.
Work through dated payments in order
- Start from a known verified principal balance and transaction date.
- Count elapsed calendar days to the first payment using the loan's agreed date inclusions and exclusions.
- Accrue interest using the contract's rate and day-count basis.
- Apply the received payment according to the contract, recording unpaid interest, fees, and principal separately.
- Use the resulting principal and new date to calculate the next interval.
Two-payment example using Actual/365 simple interest
Starting principal: $10,000 at 12% per year. After 30 days, the interest is $98.63. A $300 payment reduces principal by $201.37, to $9,798.63. Another payment arrives 45 days later: interest is approximately $144.97; a $250 payment reduces principal by about $105.03 to $9,693.60. This illustration assumes no other fees, no interest capitalization, and interest-first allocation.
| Elapsed days | Payment | Interest | Principal paid | Ending principal |
|---|---|---|---|---|
| 30 | $300.00 | $98.63 | $201.37 | $9,798.63 |
| 45 | $250.00 | $144.97 | $105.03 | $9,693.60 |
Calculate with your actual dates
Use the free irregular-payment loan calculator to test one repayment scenario. For a repeatable saved history and payoff documentation, see Loan Reconciliation & Payoff Pro.
Common errors
Do not treat every late payment as though it arrived on the installment due date. Do not apply the entire payment to principal when accrued interest is owed. Do not assume all loans use Actual/365—many notes use monthly or 360-day conventions. Check contractual rules before issuing an official payoff quote.
Repeatable Windows software
Evaluate programs designed to save and document recurring work.
Browse Windows software →Updated October 24, 2026