In an equal-month amortization model, interest equals the unpaid principal times the annual note rate divided by 12. A daily simple-interest model instead uses the outstanding principal, annual note rate, day count and contractual year basis. On $10,000 at 15%, one modeled 30-day period using a 365-day basis accrues $123.29, while one equal-month period accrues $125.00. Your agreement determines the actual method.
Why can actual interest differ from a monthly example?
A monthly table divides a year into 12 equal periods. A daily calculation counts days. Alliant’s simple-interest guidance describes its daily calculation using principal, annual rate and days since the last payment, with a 365-day basis or 366 in a leap year. That is a named provider’s explanation, not proof that every lender uses the same contract method.
Ask your actual servicer which balance, day-count convention, posting date and payment-allocation rules apply. Do not substitute a fee-inclusive APR for the note interest rate and then count the same fee again.
How do different period lengths split the same payment?
Each independent illustration starts at $10,000 principal, uses a hypothetical 15% annual note rate and applies the same $346.65 payment after the stated period. No interim transactions, other fees or compounding of unpaid interest are modeled. The rows are alternative single periods, not consecutive installments or complete loan schedules.
| Elapsed days | Year basis | Payment | Accrued interest | Principal paid | Closing principal |
|---|---|---|---|---|---|
| 28 | 365 | $346.65 | $115.07 | $231.58 | $9,768.42 |
| 30 | 365 | $346.65 | $123.29 | $223.37 | $9,776.63 |
| 31 | 365 | $346.65 | $127.40 | $219.26 | $9,780.74 |
| 30 | 366 | $346.65 | $122.95 | $223.70 | $9,776.30 |
Formula: interest = opening principal * annual note rate / 100 * days / year basis; principal paid = payment - interest; closing principal = opening principal - principal paid. The 366-day row changes only the denominator to illustrate a different year basis; use the contract’s actual rule.
Does paying five days earlier always save the same amount?
No. If the principal stayed at $10,000 and the simple daily rate used 15% over 365 days, five days of interest would be $20.55. A different balance produces a different amount. Moving a payment can also change the next period’s balance and day count, so that five-day figure is not a full-term savings forecast.
Confirm how extra funds are applied and how the payoff quote accounts for arrival dates. The early-repayment guide explains a separate extra-payment model. Do not change required payments or automatic-payment instructions based only on this example.
How is the interest part of a payment calculated?
In this monthly fixed-rate model, interest for a period equals the unpaid balance times the annual interest rate divided by 12. A $10,000 opening balance at 15% produces $125 of first-month interest. The rest of the estimated $346.65 payment reduces principal.
| Month | Payment | Interest | Principal paid | Balance after payment |
|---|---|---|---|---|
| 1 | $346.65 | $125.00 | $221.65 | $9,778.35 |
| 2 | $346.65 | $122.23 | $224.42 | $9,553.92 |
| 3 | $346.65 | $119.42 | $227.23 | $9,326.69 |
| 4 | $346.65 | $116.58 | $230.07 | $9,096.62 |
| 5 | $346.65 | $113.71 | $232.95 | $8,863.68 |
| 6 | $346.65 | $110.80 | $235.86 | $8,627.82 |
Why does the interest portion get smaller?
As principal is repaid, the next period begins with a smaller balance. At the same rate and payment, less goes to interest and more to principal. Late payments, different accrual methods, or fees can change a lender’s actual schedule.
How do monthly and daily interest methods differ?
This example uses equal monthly periods. A lender may instead accrue interest daily using actual payment dates and the day-count convention in its agreement. Ask which method applies before using an estimate as a payoff quote.
For these fixed-payment examples, M = P × r ÷ [1 − (1 + r)−n]. P is the loan principal, r is the annual rate divided by 12 and by 100, n is the number of monthly payments, and M is the monthly payment. At a zero rate, M = P ÷ n. Total repayment = M × n; total interest = total repayment − P.
Calculations retain full precision until display, then round to cents. Multiplying the displayed rounded payment can differ by a few cents from the displayed total. A lender may round each instalment and adjust the final payment. Fees, daily accrual, odd first periods, late payments, and prepayments can change actual results.
Can extra principal payments change total interest?
They may reduce future interest if they reduce the balance earlier. Check prepayment charges and payment-allocation rules first.
Sources supporting this explanation
- Alliant: its published simple-interest method uses principal, elapsed days and a 365/366-day year basis
- CFPB: the note interest rate differs from APR that can include other credit costs
Sources checked 2026-10-29. Examples are hypothetical BYL calculations or planning worksheets;they are not observed lender prices,offers or approval criteria.