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Simple Interest / Late Fee Calculator
Calculate interest owed on a balance using a simple annual rate over days, months, or years.
How simple interest is calculated
Simple interest = principal × (annual rate ÷ 100) × time in years. Days convert to years by dividing by 365, months by dividing by 12. This is simple interest — it doesn't compound, which matches how most flat late fees and short-term invoice interest are calculated.
Example: a $1,000.00 invoice at a 12% annual rate, 30 days overdue, accrues about $9.86 in interest, for a $1,009.86 total.