Invoice Late Fee Calculator

The number that actually gets invoices paid is not the total — it is the amount the debt grows by each day. This works that out, along with the fee itself, and writes the reminder email for you with the figures already in it.

Invoice amount

Invoice number

Days past due

Counted from the due date.

Grace days

Before the fee starts.

Fee structure

Whatever your written terms actually say.

Monthly rate %

1% to 1.5% is the commercial norm.

Compound monthlyOnly if your terms say so. If they are silent, simple interest is the defensible reading.

Late fee accrued

$96.00

over 40 chargeable days

Growing by

$2.40

every further day

Total now owed

$4,896.00

invoice plus fee

If it keeps going unpaid

Past due	Late fee	Total owed
30 days	$60.00	$4,860.00
60 days	$132.00	$4,932.00
90 days	$204.00	$5,004.00
180 days	$420.00	$5,220.00

A reminder you can send

Firm, specific, and it gives them a way to say what is actually blocking payment — which is usually a missing PO number rather than an unwillingness to pay.

Subject: Invoice 2026-014 — 45 days past due

Hello,

Invoice 2026-014 for $4,800.00 was due 45 days ago and is still showing as unpaid on our side.

As agreed in our terms, a late fee of $96.00 has now accrued, and it increases by $2.40 each further day.

The current balance is $4,896.00. If payment has already gone out, please send me the reference and I will chase it at our end. If something is holding it up — a missing PO number, an approval, a query on the work — tell me what you need and I will get it to you today.

Thanks,

Calculated in your browser. Nothing you enter is uploaded. A late fee is only chargeable if it was in your written terms before the work started — this works out the amount, it does not create the right to charge it.

How to use this invoice late fee calculator

Enter the invoice amount and how many days past the due date it is. Count from the due date, not from the date you sent it.
Set your grace period. Many terms allow a few days before a fee starts; if yours do not, leave it at zero. Days inside the grace period do not accrue anything.
Pick the fee structure your written terms actually use — a percentage per month, an annual rate, a flat fee, or a flat fee plus a percentage. Use what the contract says, not what you wish it said.
Leave compounding off unless your terms explicitly say the fee compounds. If the wording is silent, simple interest is the reading you can defend.
Take the per-day figure into your reminder. "This grows by $2.40 a day" moves an accounts payable queue in a way that "you owe $4,896" does not.

What makes a late fee actually collectable

A late fee is a contract term, not a right that exists on its own. It has to have been agreed before the work started — in a signed contract, in accepted terms of business, or on a quote the client approved. Adding a fee to an invoice for work already done, when nothing beforehand mentioned one, means you are asking rather than charging. Most clients who are simply slow will pay it anyway; a client who disputes it has a good position, and you will not enjoy the conversation.

The usual commercial structure is 1% to 1.5% per month on the outstanding balance. That is 12% to 18% a year, which sounds mild monthly and substantial annually, and both framings are useful in different conversations. A flat fee suits small invoices where a percentage would be trivial — $40 on a $300 invoice is a real deterrent where 1.5% is $4.50 and deters nobody. A flat fee plus a percentage combines the two: a fixed cost for being late at all, then a running cost for staying late.

Compounding is a detail worth getting right because it is where terms and arithmetic diverge. If your contract says "1.5% per month" and nothing else, that is simple interest on the original balance. If it says the fee is charged "on the outstanding balance including previously charged fees", that is compounding. The difference is small over a couple of months and stops being small after a year, and the burden of showing that compounding was agreed sits with you.

Then there is the ceiling. Several states cap the interest chargeable on an unpaid debt, and the cap is frequently lower for consumers than for business-to-business work — a rate that is unremarkable on a commercial invoice can be unenforceable on one sent to a private individual. This page deliberately does not publish a state-by-state rate table, because the correct number depends on the type of transaction as much as the state, and a table that ignores that would give confident wrong answers. If you are charging above 18% a year, check your own state's rule before you send it.

Practically, the fee is rarely the point. Most late payment is not refusal; it is a missing purchase order number, an approver on holiday, or an invoice that landed in the wrong inbox. A fee gives your reminder a reason to exist and a deadline that means something, but the reminder should still make it easy for the client to tell you what is actually stuck. The generated text below does both on purpose.

Example

A freelancer sent invoice 2026-014 for $4,800 on Net 30 terms. Her terms of business, agreed before the project started, charge 1.5% per month on overdue balances after a five-day grace period, calculated as simple interest. The invoice is now 45 days past its due date and still unpaid.

Invoice amount	$4,800.00
Days past due	45
Grace period	5 days
Chargeable days	40
Rate — 1.5% per month	18.0% a year
Total now owed	$4,896.00

Late fee after 45 days: $96.00, growing $2.40 a day.

Forty chargeable days is 1.333 months, so the fee is $4,800 × 1.5% × 1.333 = $96.00. The five grace days are genuinely free — dropping them would have added $12.00, which is the kind of detail that decides whether a client argues with the invoice or just pays it.

The $2.40 a day is the figure to put in the email. It is small enough not to sound punitive and specific enough to be real, and it converts a vague obligation into something with a running meter. Accounts payable departments respond to running meters in a way they do not respond to totals.

Left alone the fee reaches $132.00 at 60 days, $204.00 at 90 and $420.00 at 180. On a $4,800 invoice, six months of lateness costs the client less than 9% — which is a useful reminder that the fee is a nudge, not a remedy. If an invoice has reached 90 days, the fee is not the thing that will fix it.

Had the terms compounded monthly instead, the fee at 45 days would be $96.24 rather than $96.00. Twenty-four cents. Over a year the gap widens, but for the ordinary case the argument about compounding costs more in goodwill than it earns in cash — which is a reason to write simple interest into your terms and stop thinking about it.

Change the structure to a $40 flat fee plus the same monthly rate and the total becomes $136.00. On small invoices that flat component does most of the work: on a $300 invoice, forty days of 1.5% is $6, and nobody has ever paid faster because of $6.

Limits and things to watch for

This calculates an amount. It does not establish your right to charge it — that comes from terms agreed with the client before the work began.
A month is treated as 30 days and partial months are prorated by day. Some contracts instead charge a full month's interest for "each month or part of a month", which produces a higher figure in the early days. Use whichever your wording says.
State law caps the interest chargeable on overdue balances in several states, and consumer transactions are frequently capped lower than business-to-business ones. No state-by-state table is published here, on purpose.
Federal government invoices are governed by the Prompt Payment Act, which sets its own interest rate and its own timetable. Private contract terms do not apply.
It does not model partial payments, disputed line items, or invoices in a foreign currency, all of which change what is actually outstanding.

Disclaimer. This calculator is general information, not legal advice. Whether a late fee is enforceable, and at what rate, depends on your written terms, the type of client and the law of your state. Confirm with a qualified professional before relying on any figure here, particularly for consumer invoices or amounts large enough to litigate.

Frequently asked questions

How much can I charge as a late fee?: The commercial convention is 1% to 1.5% a month, which is 12% to 18% a year. Above that you are into territory where state interest caps start to matter, and the cap is usually lower for invoices sent to individuals than for business-to-business work. Charging a defensible 1.5% that clients pay beats charging an aggressive 5% that gets disputed.
Can I add a late fee if my contract never mentioned one?: You can add it to the invoice, and many clients will pay without comment. But a client who challenges it is on solid ground, because a fee that was never agreed is not a contract term. The fix is forward-looking: put the fee in your terms now so that the next invoice is covered. Retrospective fees are a negotiating position, not an entitlement.
Should I actually charge it, or just threaten it?: Charging it consistently is what makes it work. A fee you waive every time is a fee your regular clients have learned to ignore, and the ones who pay late will keep paying late because there is no cost. A reasonable middle path is to apply it automatically and waive it on request for a client who is otherwise reliable — that keeps the deterrent and costs you a good relationship only when you choose to spend it.
What is a sensible grace period?: Three to seven days. It absorbs the ordinary friction of bank transfers and approval queues without making the due date meaningless, and it removes the awkward case where a client paid on time and the money arrived a day late. Longer than about ten days and the due date stops being the date anyone works towards.
What if the client just never pays?: The fee stops being the tool. Escalate on a schedule rather than by mood: a reminder at seven days, a firmer one with the fee stated at fourteen, a call at thirty, and a decision at sixty about whether to send a formal demand, use a collections service or file in small claims court. Small claims limits vary by state but usually cover ordinary freelance invoices, and the filing fee is modest. Deciding the schedule in advance is what stops the invoice quietly ageing.
Does charging a late fee damage the relationship?: Less than people fear, and less than the alternative. Chasing an invoice four times without ever imposing a consequence teaches the client that your deadlines are soft, and the resentment that builds on your side does more damage than a stated fee ever does. Clear terms applied evenly read as professional. Surprise fees read as punitive — which is the argument for putting them in the contract rather than the argument against having them.