Late Payment / Overdue Invoice Calculator
Calculate what a late-paying client owes you — a flat fee plus interest for the days overdue. A clear number to put on your reminder (check your contract and local limits).
Open the free Late Fee Calculator →How it works
A late fee on an overdue invoice usually has two parts: an optional flat fee that covers the cost and hassle of chasing payment, and interest that grows the longer the invoice goes unpaid. This late payment interest calculator adds both so you get one figure to put on your reminder.
The interest piece uses a simple, transparent formula. If your contract states an annual rate, the interest is:
interest = invoice amount × annual rate × (days overdue / 365)
If your rate is quoted per month (very common for freelancers), the overdue invoice interest calculator instead uses:
interest = invoice amount × monthly rate × (days overdue / 30)
The total your client owes is then: original invoice + interest + flat fee. Count the days overdue from the day after the payment due date — if your terms are Net 30, day one of overdue is day 31. Everything runs in your browser, so your invoice numbers are never uploaded.
Worked example
Say a client owes you $2,000 and the invoice is now 45 days overdue. Your contract allows a $35 flat late fee plus 1.5% interest per month.
- Interest = $2,000 × 0.015 × (45 / 30) = $2,000 × 0.015 × 1.5 = $45.00
- Flat fee = $35.00
- Late charges total = $45.00 + $35.00 = $80.00
- New balance owed = $2,000 + $80.00 = $2,080.00
So your reminder can state: original invoice $2,000, plus $45.00 interest (45 days at 1.5%/month), plus a $35.00 late fee, for a total of $2,080.00 due now.
Frequently asked questions
- How do I calculate the late fee on an overdue invoice?
- Multiply the unpaid invoice amount by your interest rate, scaled to how long it is overdue. For an annual rate, the interest is amount × annual rate × (days overdue / 365). For a monthly rate, it is amount × monthly rate × (days overdue / 30). Then add any flat late fee your contract allows. The total a client owes is the original invoice plus that interest plus the flat fee.
- How much late fee can I charge on an invoice?
- There is no single universal figure. Many freelancers charge a flat fee of 25 to 50 dollars plus monthly interest of 1 to 2 percent (roughly 12 to 24 percent a year), but the enforceable amount depends on what your signed contract states and the legal cap in your country, state or region. A fee that is far above local limits or not in your contract may be unenforceable, so always check before charging.
- What is a typical late payment interest rate for freelancers?
- A common range is 1 to 2 percent per month, which is about 12 to 24 percent per year. Some businesses tie the rate to a statutory reference rate plus a margin. Use whatever rate your contract specifies; if it is silent, your right to charge interest may be limited, so set a clear rate in writing for future work.
- Should I charge a flat fee or interest, or both?
- Both are common. A flat fee (for example 35 dollars) covers your admin cost of chasing payment, while a percentage interest charge grows the longer the invoice stays unpaid and discourages clients from sitting on large bills. This calculator lets you add a flat fee, a percentage rate, or both, as long as your contract allows it.
- When does an invoice count as overdue?
- An invoice is overdue the day after the payment due date on the invoice or in your contract. If your terms are Net 30, the clock starts 30 days after the invoice date, and day one of overdue is day 31. Count the number of full days past the due date and enter that as the days overdue.
- Are late payment fees enforceable?
- They are generally enforceable when the fee or interest rate is clearly stated in a contract the client agreed to, and stays within any legal cap for your jurisdiction. Fees that were never agreed in writing, or that exceed local usury or late-payment limits, can be challenged. This tool gives an estimate only; verify the legal specifics for your country before sending a demand.