Finance & Investment

Simple-Interest Loan Schedule with Actual Payment Dates

Build a daily simple-interest repayment schedule, apply payments to interest and principal, and distinguish it from interest on an unchanged lump sum.

By Uddhav Gopani, FounderPublished 25 September 2026Reviewed 29 September 2026

Simple-Interest Loan Schedule with Actual Payment Dates

Define the accrual convention and payment dates

A daily simple-interest loan schedule calculates interest on the outstanding principal for the time since the previous balance-changing event. It therefore needs payment dates as well as payment amounts. This example assumes a 365-day denominator, a constant annual rate, interest paid before principal and no fees. These are explicit model assumptions, not universal terms for every loan. A contract may use another day-count convention or payment-allocation rule. Establish those details before comparing a calculated schedule with an actual lender statement.

Calculate the first thirty-day interval

Begin with principal of 10,000 and an illustrative annual rate of 7.3%. Over 30 days on the stated 365-day basis, interest is 10,000×0.073×30/365=60. If the payment at the end of that interval is 200, it covers 60 of interest and reduces principal by 140. The new principal balance is 9,860. A schedule row should show all five pieces: opening principal, days, interest, payment and closing principal. Keeping interest and principal allocation separate prevents treating the entire 200 payment as debt reduction.

Continue from the reduced principal

For the next interval of 31 days, use 9,860 as the interest base, not the original 10,000. Interest is 9,860×0.073×31/365=61.132. With another 200 payment, principal reduction is 138.868 and closing principal is 9,721.132 before rounding. If the chosen schedule rounds interval interest to cents before applying the payment, interest is 61.13 and closing principal is 9,721.13. State the rounding policy. Carrying unrounded amounts and rounding every line are different conventions and can create small cumulative differences.

Use the local tool for one unchanged-principal interval

The Simple Interest Calculator uses I=P×r×t, with an annual percentage and time in years. It does not accept dated repayments or automatically update principal after each payment. To check the first interval, use principal 10000, rate 7.3 and time equal to 30/365 years, approximately 0.08219178082. For the second, use principal 9860 and time 31/365. A separate worksheet must allocate each payment and carry forward the new balance. The tool's total amount includes principal plus that interval's interest; it is not a complete multi-payment amortization schedule.

Explain why changing a payment date matters

At the original 10,000 balance and 7.3% rate, one day of interest on the stated basis is 2. A payment after 31 days rather than 30 therefore faces 62 of interest instead of 60, before other changes. With a fixed 200 payment, that leaves 138 rather than 140 for principal. Later balances then differ as well. This is why equal nominal monthly payments do not imply identical interest charges for months with different accrual periods. The actual dates and contract convention determine the calculation.

Do not assume unpaid interest automatically becomes principal

If a payment is smaller than accrued interest, some interest remains unpaid. Whether and when that amount is capitalized into principal depends on the agreement and applicable rules. It is incorrect to add it to principal automatically while claiming to reproduce any simple-interest loan. Maintain separate principal and unpaid-interest columns when necessary. Also distinguish simple interest on a reducing balance from a precomputed-interest arrangement. A formula using the original principal for the entire term and dividing the result into payments describes a different structure from the balance-by-balance schedule developed here.

Reconcile the schedule with actual records

Use the opening statement balance and actual payment dates, then compare each calculated interest allocation and closing balance with the next statement. Fees, late charges, payment reversals and allocation to future instalments can explain differences that the simplified example omits. An early principal reduction generally lowers later interest under the stated balance-based method, but the effect depends on how the payment is applied. Keep the day-count basis and rounding policy visible so a mismatch can be investigated without changing assumptions merely to make the final balance agree.

Use the Simple Interest Calculator

Open Simple Interest Calculator

Source and scope

CFPB: simple interest on an outstanding loan balance