Work out the interest and late fee owed on an overdue invoice — from the amount, the annual rate you charge, and how many days late it is.
Interest = amount × annual rate ÷ 365 × days overdue, plus any flat late fee. Late-payment terms must be stated in your contract or invoice to be enforceable, and many jurisdictions cap the rate — check local rules. General information only, not legal advice.
If your contract allows it, overdue invoices can accrue interest — typically a stated annual rate applied daily, plus an optional flat late fee. It compensates you for the delay and gently encourages prompt payment. The total owed grows each day the invoice stays unpaid.
Interest = amount × rate ÷ 365 × days late
No — it must be in your agreed terms, and many places cap the maximum. A common range is 1–1.5% per month (12–18%/yr). Check local law and state it on invoices up front.
A modest flat fee plus daily interest is often more effective than either alone — it signals seriousness while the interest keeps the pressure on.
Related: simple interest calculator · days sales outstanding calculator · all free calculators.
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Yes. The Late Payment Interest Calculator is completely free, runs in your browser, and never asks for a signup, email, or download. Enter your numbers and the result updates instantly.
It uses standard formulas and recalculates the moment you change a value, so the math is exact for the inputs you enter. Treat the result as a reliable planning estimate; for decisions with legal or tax consequences, confirm the figures with a qualified professional.
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Last reviewed: September 2026. Free to use, no signup — Late Payment Interest Calculator by ToolWise Digital.
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