When an invoice sits unpaid past its due date, both sides of the transaction start asking uncomfortable questions. The vendor wants to know how much it is reasonable to charge for the delay. The customer wants to know how the bill grew and whether the number on the revised statement is even legal. This late fee calculator answers both in seconds: it takes the original invoice amount, the number of days past due, the monthly penalty rate, and any flat late charge, then returns the interest portion, the fixed fee, the capped total, and the final amount owed.
Most commercial late fees combine two parts. The first is a percentage charge on the overdue balance, usually quoted as a monthly rate — 1.0% to 1.5% per month is the standard range written into net-30 and net-60 terms. The second is a flat administrative fee, often $25 to $50, added once once an invoice crosses into delinquency. This calculator models both at the same time, and it lets you subtract a grace period and apply a legal cap so the result stays inside what courts and state usury statutes will actually enforce.
How to Use
- Enter the Invoice Amount — the principal that was originally due on the bill.
- Type Days Past Due — the number of days between the due date and today (or the date you are assessing the fee as of).
- Set the Penalty Rate as a monthly percentage. 1.5% per month is the most common commercial default; many leases and contracts use 1.0%, and credit-card-style terms run higher.
- Add a Fixed Late Fee if your contract charges a flat administrative amount on top of the interest. Leave it at zero if your terms are interest-only.
- Expand the advanced options to set a Grace Period — the number of days after the due date during which no penalty accrues — and a Max Fee Cap as a percentage of the invoice, which is how many states (and many judges) keep late charges from ballooning beyond the original debt.
The result card shows how many of the overdue days are actually chargeable, the interest portion, the fixed fee, the total late charge, the effective monthly rate implied by the final number, and the full amount the customer now owes.
The Late Fee Formula
Late fees on trade credit are almost always simple (non-compounding) interest prorated by the day. The engine uses:
Late Fee Interest = Invoice Amount × (Monthly Rate ÷ 100) × (Billable Days ÷ 30)
Where Billable Days is the overdue count minus the grace period, floored at zero. The fixed late fee is then added on top — but only if at least one day is actually chargeable. If the invoice is still inside the grace period, the entire late fee is zero, even if a fixed fee is defined.
For example, on a $10,000 invoice that is 30 days overdue at 1.5% per month with no grace period: the monthly penalty is $10,000 × 1.5% = $150, and because the billable window is exactly one month, the prorated interest is $150. Add a $25 flat fee and the total late charge is $175, bringing the amount due to $10,175.
If the invoice is only 15 days overdue, the interest is halved: $10,000 × 1.5% × 15/30 = $75, plus the same $25 fixed fee, for a total late charge of $100. This day-proration is why two invoices with the same monthly rate can produce very different bills — the difference is entirely in how long they have been outstanding.
Grace Periods and Legal Caps
A grace period is a free buffer window. If your terms say "net 30, 5% service charge after day 15," then an invoice paid on day 20 accrues nothing: the first 15 days are forgiven, and the calculator subtracts them before applying the rate.
Many US states also cap late fees as a percentage of the overdue balance — commonly 5% of the invoice, sometimes 10%. The optional Max Fee Cap input enforces that ceiling automatically: if the computed interest plus fixed fee exceeds the cap, the result is truncated to the cap. This keeps your billing consistent with what a small-claims or district-court judge will actually collect on.
Examples
Example 1 — The textbook $10,000 invoice. A vendor sends a $10,000 net-30 invoice with terms of 1.5% per month and a $25 flat late fee. The customer pays 30 days late. Billable days = 30. Interest = $10,000 × 1.5% × 30/30 = $150. Total late fee = $150 + $25 = $175. Total due = $10,175.
Example 2 — Inside the grace period. The same $10,000 invoice carries a 15-day grace period. The customer is 10 days past due. Billable days = max(0, 10 − 15) = 0. No interest accrues, no fixed fee triggers, and the amount due stays at $10,000 — even though the invoice is technically late.
Example 3 — A six-figure receivable. A supplier is owed $1,000,000 and the invoice is 30 days overdue at 1.5% per month with a $25 fixed fee. The interest alone is $1,000,000 × 1.5% = $15,000, plus $25, for a total late charge of $15,025. The calculator handles balances at this magnitude without rounding drift, so the number you send to accounting matches the number the customer sees.
Example 4 — A state-imposed cap. A $10,000 invoice is 60 days overdue at 1.5% per month with a $25 fixed fee. Uncapped, the interest is $10,000 × 1.5% × 60/30 = $300, plus $25, for $325. But the state caps late charges at 2% of the invoice ($200). The calculator truncates the total to $200, and the customer owes $10,200 — not $10,325.
FAQ
How is a late fee on an invoice calculated?
The interest portion is the invoice amount multiplied by the monthly penalty rate, then prorated by the number of overdue days divided by 30. A flat fixed fee is added on top. For example, a $10,000 invoice 30 days overdue at 1.5% per month accrues $150 of interest, plus any fixed fee your terms specify.
What is a typical late fee percentage?
On commercial invoices in the United States, 1.0% to 1.5% per month (equivalent to roughly 12% to 18% per year) is the most common term. Residential leases and consumer contracts are more tightly regulated and often capped at 4% to 5% per month by state statute.
Are late fees legal?
Late fees are enforceable only if they are disclosed in the original contract or invoice terms before payment is due, and only if they are not "penal" rather than a reasonable estimate of damages. Courts regularly refuse to enforce late charges that exceed a few percent of the overdue balance, which is why many businesses voluntarily cap fees at 5% of the invoice.
What is a grace period, and how does it affect the fee?
A grace period is a window after the due date during which no penalty accrues. If your terms give a 10-day grace period and the invoice is paid on day 8 past due, the late fee is zero. In this calculator, enter the grace period under "Grace Period" and it is subtracted from the overdue days before interest is applied.
Can late fees be charged on late fees?
Almost never. The standard practice — and the one this calculator models — is simple interest on the original unpaid balance. Compounding a late fee onto a prior late fee is considered an illegal penalty in most jurisdictions and is not what commercial customers expect.
How do I avoid late fees as a customer?
Negotiate net-30 or net-60 terms up front, pay within the discount window if one is offered (e.g., 2/10 net 30), and communicate early if a payment will slip. Most vendors will waive a one-time late fee for a long-standing customer who asks politely before the invoice ages past 60 days.
Disclaimer
This late fee calculator is provided for informational purposes only and does not constitute legal, accounting, or tax advice. Late-fee rules vary by state, contract, and industry; some jurisdictions impose statutory caps or require written disclosure before a fee can be charged. Always confirm your terms against your signed agreement and consult a qualified attorney or accountant before assessing, waiving, or disputing a late charge.