Free Invoice Late Fee Calculator
Runs in your browser — nothing you type is sent anywhere.
This calculator is a planning tool, not financial or legal advice.
This invoice late fee calculator tells you two numbers: the fee an overdue invoice has earned so far, and the new total your client owes today. Enter the invoice amount and how many days it's overdue, then choose how your fee works – a flat amount, or a monthly percentage prorated by the day. The result updates as you type. What the calculator can't check is whether you're entitled to the fee at all – and that's the first thing to get right, because it's decided before the invoice is ever late.
The one precondition: the fee must exist before the delay
A late fee is only chargeable if it was agreed before the work – stated in the contract, the proposal, or on the invoice's payment terms from day one. That's the whole rule. A fee that first appears after the invoice goes overdue isn't a term, it's a surprise, and a client is generally free to ignore it. Enforceability varies by place and by contract even for properly disclosed fees – some jurisdictions cap what can be charged – so treat the fee as a documented agreement between you and the client, not as a right that exists by default. Practically, one line in your standard payment terms does the job: the fee, the trigger date, and how it accrues. If your invoices already carry that line, this calculator just does the arithmetic it promised.
How the math works
Flat fees need no math: the fee is the fee, whether the invoice is five days late or fifty. Percentage fees are where a late payment interest calculator earns its keep. The convention is a monthly rate applied pro rata by the day: take the invoice amount, multiply by the monthly rate, divide by 30 to get the daily charge, then multiply by the number of days overdue. Prorating matters because the alternative – charging a full month's fee the moment the due date slips – feels punitive at day three and understates at day fifty-nine. A daily accrual is easy to defend line-by-line, and easy for the client to verify with their own calculator, which is exactly what you want a disputed number to be. Choose between the two by invoice size: on small invoices a percentage rounds to pocket change and a flat fee carries the message better, while on large invoices a flat fee is symbolic and the percentage does the actual work of pricing the delay.
A worked example
A $2,000 invoice is 45 days overdue, and your terms state a 1.5% monthly late fee. The monthly fee on the full amount would be $2,000 × 0.015 = $30.00. Divided by 30, that's $1.00 of fee per day. At 45 days overdue: $1.00 × 45 = $45.00 in late fees, making the total now due $2,000 + $45.00 = $2,045.00. Written on the revised invoice, that's the original amount on its own line, and one added line – "Late fee – 1.5% monthly, prorated, 45 days past due – $45.00" – so the client can see the term, the math, and the total without a phone call.
Before you charge it: the gentler alternative
A fee recovers money; a reminder usually recovers it faster and keeps the client – most overdue invoices are forgotten, not refused, and a plain-worded past-due notice resolves the majority before any fee needs to exist. The fee is the tool for the client who ignored the reminder, not the first move. When you do apply it, apply it exactly as your terms describe – a fee charged inconsistently is a fee argued about.
When you're ready to send the updated bill, the fee goes on the same document as the original amount – one added line, one new total. Invosmith's generator lets you rebuild the invoice with the late-fee line in place and download the PDF in a minute, free and without a watermark: create an invoice free.
Invoice late fee FAQs
Can I add a late fee retroactively?
Generally, no. A fee your client never agreed to before the invoice went overdue is unlikely to be enforceable, and pressing it risks the relationship over a number you probably can't collect. Add the term to your contracts and invoices going forward; for the current overdue invoice, a firm reminder is the better tool.
How much late fee can I charge?
There's no universal number – caps vary by jurisdiction and by what your contract says, and some places limit interest-style fees significantly. The honest answer is: whatever your signed terms state, within your local rules. If you're setting terms now, keep the fee modest enough that a court and a client would both read it as compensation for delay, not a penalty.
What's a typical late fee percentage?
By convention, 1–1.5% per month is common on small-business invoices – that's an observation about practice, not advice, and your contract and local rules control what applies to you. Flat fees are also common on smaller invoices, where a percentage would round to almost nothing.
Does the late fee get its own line on the invoice?
Yes – always its own labeled line, never folded into the original amount. State the rate or flat amount, the days overdue, and the fee, then show the new total. A client who can audit the line in ten seconds pays it; a mysteriously grown total gets a phone call.
When should I waive the fee?
When the client pays promptly after one reminder, when the delay was your error, or when the relationship is worth more than the fee – which, for a good client's first slip, it almost always is. Waive it explicitly ("I've removed the late fee this once") so the gesture is visible and the term stays alive for next time.