Estimate the interest portion of an overdue balance before you prepare a reminder, follow-up, or updated invoice.
Estimate overdue interest
This tool uses simple daily interest from the invoice principal, annual rate, and number of days overdue. It does not decide whether a fee is allowed.
Used for display only. No currency conversion is performed.
Estimated Interest Accrued0.00
Estimated New Total Owed0.00
How this is calculated: Interest Accrued = (Principal × (Annual Rate / 100) / 365) × Days Overdue.
Important: This calculator is for estimation only. Late fees, interest, collection charges, and reminder wording may be limited by local law or by the written agreement with your client. Confirm your terms before charging a late fee.
How To Use The Late Payment Calculator
Use this calculator when an invoice is overdue and you want to estimate a possible late fee or interest amount before sending a reminder. Enter the overdue balance, the agreed annual rate or fee basis, and the number of days late. The calculator gives you a simple estimate only; it does not decide whether a fee is allowed.
Late fees should match the agreement you gave the customer and the rules that apply in your location. When in doubt, send a polite reminder first and keep the wording factual.
Check the original invoice terms before using any fee.
Use the unpaid balance, not the original total if partial payment was received.
Keep reminder language professional and avoid threats or confusing math.
Save a copy of the invoice, reminder, and any payment response.
The Formula, In Plain English
The calculator treats an overdue balance the way simple interest works: it converts the annual rate you agreed with the customer into a daily rate, multiplies that by the number of days the invoice has been late, and applies it to the unpaid principal. Nothing compounds — yesterday’s interest does not earn interest today.
In notation: Interest = Principal × (Annual Rate ÷ 100 ÷ 365) × Days Overdue. The second output simply adds that interest back: New Total Owed = Principal + Interest.
Two inputs decide almost everything. The principal should be the amount still unpaid, not the original invoice total, and the days overdue should be counted from the day after the due date printed on the invoice — not from the invoice date and not from the day you noticed.
Estimated interest 9.86 · new total owed 1,009.86.
Thirty days at 12% a year is under ten units of currency on a thousand. That is usually the honest answer to “should I bother?” — the reminder matters more than the fee.
Example figures — not real client data
A partly paid invoice, 75 days late
Original invoice: 4,800.00. Customer already paid 1,800.00.
Unpaid principal = 4,800.00 − 1,800.00 = 3,000.00. Enter 3,000.00, not 4,800.00.
Estimated interest 110.96 · new total owed 3,110.96.
Charging interest on the full 4,800.00 would have overstated the fee by about 66 — the single most common mistake with late-fee math.
Edge Cases, Rounding, And Input Limits
Negative numbers are ignored. Any input below zero is treated as zero, so a mistyped minus sign produces a zero result rather than a nonsense credit.
Rounding happens only at display. The interest is calculated at full precision and shown to two decimal places, so a long period will not drift the way day-by-day rounding would.
365 days, always. The daily rate uses a 365-day year even in a leap year. If your written terms specify a 360-day year or a monthly percentage instead, this tool will not match your contract.
A flat fee is not interest. If your terms say “25 flat after 14 days,” there is no rate to enter — just add the flat amount as a line item in the invoice generator.
The currency box is cosmetic. It prefixes the output for readability. No conversion happens, and only the first ten characters are used.
What This Calculator Does Not Do
It does not decide whether you are allowed to charge a late fee. That depends on what your written terms say, what the customer agreed to, and what the rules are where you operate — caps on interest, notice requirements, and consumer-versus-business distinctions all vary, and none of them are built into this tool. It also does not compound interest, prorate part-days, apply grace periods, handle multiple invoices at once, or produce a legally formatted demand. It is arithmetic that helps you decide what to write; it is not a ruling that you may write it.
In our own event-services operation we have never actually charged a late fee. Reminders are triggered by the payment tracker — an event coming up with the advance unpaid, or a completed event with a balance outstanding — and a polite note has resolved it every time. The fee clause exists so the terms are complete, not because it gets used. Most small service businesses find the same thing, which is why the late payment reminder guide is usually the more useful page.
How This Differs From The Other Calculators Here
Deposit / balance calculator answers what is still owed after payments received. Start there, then bring that unpaid figure here as the principal.
Sales tax calculator handles tax on the original sale. Interest on a late payment is a separate charge from the tax on the work; do not stack them in one field.
Late fee calculator — checks performed after any change
Input
Expected result
Why it matters
1,000 principal · 12% · 30 days
Interest 9.86 · total 1,009.86
Baseline simple-interest path
1,000 principal · 0% · 30 days
Interest 0.00 · total 1,000.00
A zero rate must not produce a fee
0 principal · 18% · 90 days
Interest 0.00 · total 0.00
Empty invoice must not generate interest
1,000 principal · 12% · 0 days
Interest 0.00 · total 1,000.00
Not yet late means nothing accrues
−5,000 principal · 12% · 30 days
Interest 0.00 · total 0.00
Negative input is clamped, not honoured
3,000 principal · 18% · 75 days
Interest 110.96
Long-period precision and display rounding
These cases are re-run by hand in the browser whenever the calculator changes. The wider testing routine is described on how we test.
Privacy: this calculator runs entirely in your browser. Amounts you type are never sent to a server, never stored on our systems, and disappear when you close the tab. No account, no signup, nothing retained.
Formula, worked examples, and limitations last reviewed by Khurram Naeem Jaswal, operator of ClearPaperwork. Spotted an error? Tell us.
Calculator FAQ
Can I charge a late fee on any invoice?
Not automatically. A late fee generally needs to have been agreed in advance — stated on the estimate, the invoice, or a signed agreement — and some jurisdictions cap the rate or require notice before it applies. If the terms you sent did not mention a fee, adding one after the fact is usually a conversation, not a right. Check your own written terms first, and confirm local rules if the amount is significant.
Should I send a reminder before adding a fee?
Almost always. In practice a polite, factual reminder resolves most late invoices without any fee at all. A good sequence is a short nudge a few days after the due date, a firmer note referencing the terms about two weeks later, and only then a conversation about interest. The reminder guide has wording for each stage.
What number goes in the principal field?
The amount still unpaid, not the original invoice total. If a customer paid part of the balance, subtract that first — the deposit and balance calculator will do it for you — and bring the remaining figure here.
Does it compound the interest?
No. It uses simple daily interest, so interest never earns interest. If your written terms specify monthly compounding or a 360-day year, this calculator will not match your contract and you should follow the contract.
How do I handle a flat late fee instead of a percentage?
There is nothing to calculate — add the flat amount as its own line item on the invoice, labelled clearly, so the customer can see exactly what changed and why.
Does this calculator collect my invoice data?
No. Everything runs in your browser. Nothing you type is transmitted or stored, and closing the tab discards it.