Invoice late fee calculator: charge monthly interest or flat penalties on overdue invoices from the due date to today. Free, editable rates, no signup.
Optional flat admin/collection fee added on top.
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Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for how this tool works.
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Invoice late fee calculator: charge monthly interest or flat penalties on overdue invoices from the due date to today. Free, editable rates, no signup.
At 12 months, compounding adds $22.68 to a $1,200 invoice.
An invoice late fee calculator is a free, browser-based tool that instantly computes the late fee on overdue invoices based on the invoice amount, the number of days past due, and your chosen daily or monthly interest rate. No signup is required , everything runs locally in your browser and nothing is stored or transmitted.
How does this invoice late fee calculator protect my privacy?
This tool is entirely no-signup and runs in your browser. No data is ever sent to a server, no files are saved, and no personal or financial information is collected. You can calculate late fees with full confidence that your invoice details remain private.
Can I use this invoice late fee calculator for any invoice type?
Yes. The invoice late fee calculator works for any invoice , whether it's for freelance services, business-to-business projects, or rental agreements. Simply enter the invoice amount, the overdue days, and your desired late fee rate, and it will compute the total fee and updated balance immediately.
BoringToolsKit. “Invoice Late Fee Calculator.” boringtoolskit.com/invoice-late-fee-calculator/. Free to reference in articles, syllabi, and answer posts with a link.
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A worked example at $1,200 and 45 days
Take an invoice of $1,200 that is 45 days past its due date, with a contract late fee of 1.5% per month on the overdue balance and simple interest as the method. First the calculator converts days to months: 45 / 30.4 = 1.480263 months. It rounds up to 30.4 rather than 30 because that is the average length of a calendar month (365.25 / 12), so short months like February do not inflate the fee.
Simple interest is balance x rate x months: $1,200 x 0.015 x 1.480263 = $26.64. If the same contract said compound instead, the fee would be $1,200 x (1.015^1.480263 - 1) = $26.74, about 10 cents more over a period this short. Compounding only pulls away on longer delinquencies: at 12 months the same inputs give $216.00 simple versus $238.68 compounded, a $22.68 gap. The final line, updated amount due, is $1,200 + $26.64 + $0.00 in custom fees = $1,226.64. Add any flat reactivation or collection charges the contract allows in the custom fees field and the tool totals them separately from the percentage-based fee.
What this estimate cannot see
The calculation assumes the fee accrues uniformly from day one, but most contracts do not work that way. A typical term is '1.5% per month on balances unpaid after 30 days', which means the fee here starts on day 31, not day 1. On the $1,200 invoice that turns 45 days of accrual into roughly 15 days, about $8.84, a third of what straight-line math suggests. Read the grace period before trusting the output.
Statutory caps also bite hard. In New York, for example, the default interest rate on judgments is 9% per year; charging 1.5% monthly (18% annualized) may be unenforceable in the absence of an explicit contractual term, and some states cap late fees outright regardless of contract language. A 0.5% monthly rate (6% annualized) on the same $1,200 over 45 days yields only $8.88. The tool also ignores partial payments: if the client paid $500 on day 20, the correct fee base drops to $700, and the calculator has no field for that. Treat the result as a planning number, then check it against the exact clause and your state's usury and late-fee rules before invoicing.
