Freelance calculators
Invoice Late-Fee Calculator
Prorate a monthly or annual late-fee rate over the number of days overdue. Your inputs stay in this browser.
Live calculator
Enter your numbers
What this calculator measures
This calculator prorates a stated simple rate across overdue days so you can put a dollar figure on a late invoice. It is useful when the contract already allows a monthly or annual fee and you need the amount before you send a reminder — not as proof the delay only cost that amount.
The metric is misleading when you treat the fee as the cost of waiting. $105.80 on a $4,600 invoice at 1.5% per month for 46 days is a nudge. The real cost is chase hours, a cheaper panic job you took to cover rent, and a year-to-date rate that can no longer claim those billed hours until the check clears. It is also misleading as legal advice. Some places cap or prohibit fees unless the original contract allows the method you used.
The common manual mistake is to apply an annual rate as if it were monthly, or to compound when the agreement says simple. 18% a year on $4,600 for 46 days is $4,600 × 0.18 × (46/365) ≈ $104.25, close to 1.5% monthly in this window — but 1.5% monthly treated as 18% without proration, or compounded monthly, is a different number and may be unenforceable. People also start counting from the invoice date instead of the due date.
Rafi invoiced $4,600 on 3 March, Net 14. On 2 May it is 46 days late. At 1.5% monthly simple, the fee is $4,600 × 0.015 × (46/30) ≈ $105.80. He also spent six hours chasing payment ($570 at a $95 floor) and turned down a $1,800 week. The fee does not buy those hours back. A 40% deposit on day one would have left $2,760 at risk and covered most of April's costs. Issue the fee if the contract allows it. Do not let $105.80 talk you into starting phase two unpaid.
Formula
Inputs and variables
- Invoice amount
- Enter the money amount used by this formula.
- Late-fee rate
- Enter the percentage used by this formula.
- Rate period
- Enter the value used by this formula.
- Days late
- Only charge a fee allowed by the signed contract and applicable law.
How to use the calculator
- Choose a display currency when money fields are present.
- Enter values that cover the same period, scope, and currency.
- Review the result, formula, and worked example before using the estimate in a decision.
- Change one assumption at a time to understand what drives the result.
Worked example
- 1Rafi's $4,600 invoice is 46 days late at 1.5% monthly.
- 2$4,600 x 0.015 x 46/30 ≈ $105.80.
- 3That is the contractual interest, if enforceable — not the cost of six chase hours or the work he turned down.
How to interpret the result
Charge only terms agreed in the original contract and permitted where you work. This tool prorates simple interest; it does not compound. Price the delay in hours and in the work you took instead, not only on the fee line.
Common mistakes and limits
- Adding a fee that was not in the signed terms.
- Treating an annual rate as a monthly rate, or the reverse.
- Compounding when the agreement specifies simple interest.
- Counting days from the invoice date instead of the contractual due date.
Frequently asked questions
How much should I charge for a late invoice?
Only the rate and method in the signed contract, if local law allows it. This calculator prorates that rate over overdue days. A 1.5% monthly fee on $4,600 for 46 days is about $106 — useful as a nudge, rarely enough to repair chase time or panic work you took while waiting.
Does a late fee make me whole?
Usually not. The fee is interest on the invoice, not a refund of hours spent collecting or of cheaper work you accepted to cover rent. Stop unpaid extra work, require deposits on the next phase, and recount overdue invoices as unavailable for rent.
Is this the same as compound interest?
No. The tool uses simple proration: invoice × rate × (days ÷ 30 or 365). If your contract compounds, this number will be low. If your contract is simple and you compound anyway, the fee may not be enforceable.
When do I start counting overdue days?
From the contractual due date — Net 14 means day 15 is day one late — or from the date the applicable statute uses. Do not start from the invoice date unless that is what the terms say.
Should I send a late fee on a $900 invoice?
Do the arithmetic first. At 1.5% monthly for a few weeks the fee can be a rounding error, and the relationship cost of sending it may exceed the dollars. On a large invoice the same rate is at least a conversation. Deposits prevent more damage than a small fee repairs.