Can you calculate a full year of interest after withdrawing some principal?
A constant-principal formula only applies while the balance stays unchanged. Split the timeline at deposits, withdrawals or principal repayments, then calculate each segment separately.
Simple interest →Identify intervals with an unchanged balance
Use each transaction date as a boundary. Within a segment, apply its principal, applicable rate and duration, then add the segment interest amounts. If interest is credited and itself begins earning interest, the account also requires compounding rather than a pure simple-interest treatment. A single annual average can hide the timing that determines what was actually earned.
Work through two half-year segments
Assume 10,000 is held for the first half of a year. Then 4,000 is withdrawn, leaving 6,000 for the second half. At 4% simple annual interest and exactly 0.5 years per segment, the first period earns 200 and the second 120, for a total of 320. Applying 4% to the original 10,000 for the entire year would give 400 and overstate interest by 80.
Use real dates for account reconciliation
Two calendar half-years do not necessarily contain the same number of days. For an actual account, follow its day-count convention and transaction dates. The simple-interest calculator can be run separately for each unchanged-balance segment, but it does not import account transactions or automatically combine a complex ledger. Charges, tax deductions and changing rates should be recorded explicitly rather than attributed to a mysterious calculation discrepancy.
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