Late Fee Calculator — Overdue Invoice Interest (% / Flat)
Work out the fee on an overdue invoice three ways — interest per month, a flat fee per month late, or both — and see what each does to the total owed.
Late Fee / Interest Calculator
Late fee charged
$225.00
Total amount owed
$10,225.00
A late fee is generally only collectable if it was agreed in advance — stated in the contract or printed on the invoice before the work, not added afterwards. 1.5% a month (about 18% a year) is a widely used contract rate, but many jurisdictions cap what you can charge and the rules differ for business and consumer debt. Check your local position before you set the rate, not after the invoice goes out.
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A fee you did not agree in advance is a request, not a debt
Start here, because it is the part most often got wrong. A late fee generally has to have been agreed before the work — written into the contract or printed on the invoice terms — rather than added once the payment is overdue. A charge invented after the fact is something you are asking for, and a client who declines it is usually on solid ground.
The rules vary by country and differ between business and consumer debt, so this is worth settling once in your standard terms rather than improvising per invoice. The practical version: decide your policy, put it on every invoice from the first one, and you never have to raise it as a new subject.
The model changes the client's incentive, not just the number
This is the genuinely useful thing the calculator shows, and it is easy to miss if you only look at the total.
The percentage model pro-rates by day. Interest accrues smoothly, so every day of delay costs something and paying sooner always saves money.
The flat model rounds up to whole months. That makes it a series of cliffs — and cliffs have flat tops, where delay is free.
Worked through on the defaults
Take a 10,000 invoice at 1.5% a month, with a flat fee of 500.
- Day 30: percentage charges 150. Flat charges 500.
- Day 31: percentage charges 155 — five pounds, rupees or dollars more. Flat charges 1,000, because one day past the boundary rounds up to a second whole month.
- Day 45: percentage charges 225. Flat still charges 1,000.
- Day 60: percentage charges 300. Flat still charges 1,000.
Read the flat column again: between day 31 and day 60 the bill does not move. A client who has already crossed into the second month has no financial reason whatsoever to pay before day 60, and every reason to let it drift to the last moment. The percentage column costs a steady 5 a day throughout, so paying on day 45 instead of day 60 saves 75 — a real, visible reward for settling.
That is the choice being made. The flat model punishes crossing a line; the percentage model prices the delay itself. Only one of them pays a client back for changing their mind on day 40.
The combined model
A fixed charge for the fact of being late plus interest for the length of it — 725 at 45 days on the same figures. The fixed part covers the real administrative cost of chasing, and the interest part keeps the daily incentive to settle intact. For most suppliers it is the sensible middle.
What actually gets invoices paid
The fee is rarely the mechanism. Terms on every invoice so it is never a surprise, a reminder a few days before the due date rather than after it, and a named person to send it to all move payment dates more than the charge does.
It is worth being honest about proportion too: for most small suppliers the relationship is worth more than the fee, and the fee's job is to make lateness slightly less comfortable than paying, not to become a revenue line. A policy applied consistently to everyone is also far easier to enforce than one you decide case by case, because nobody can argue they were singled out.
The other half of the same policy is the carrot: the payment terms calculator shows what an early-payment discount costs you to offer, annualised.
How to use the Late Fee / Interest Calculator
Takes about a minute. No signup, no download, your data stays in your browser.
- 1Open the tool. Scroll up to the Late Fee / Interest Calculator above — it loads instantly in your browser, no install needed.
- 2Enter your values. The fields come pre-filled with realistic defaults so you can see how it works — replace them with your own numbers.
- 3Read the result. The output updates instantly. Copy or share it — nothing is uploaded to a server, everything stays on your device.
Frequently asked questions
Common questions about the Late Fee / Interest Calculator.
Can I add a late fee that was not in my terms?
You can ask, but it is generally not enforceable. A late fee usually has to be agreed in advance — in the contract or in the invoice terms — rather than added once payment is overdue. The rules vary by country and between business and consumer debt, so the practical answer is to get your policy into your standard terms and apply it from the first invoice.
Percentage or flat fee — which should I use?
Percentage, in most cases, because it prices the delay rather than the boundary. On a 10,000 invoice at 1.5% a month, the flat model charges the same amount at day 31 as at day 60, so a client already past the first boundary gains nothing by paying sooner. The percentage model costs a steady 5 a day, so settling earlier always saves them money.
How is the percentage fee calculated?
It pro-rates by thirty-day month: the invoice amount times the monthly rate times days overdue divided by thirty. So 45 days at 1.5% on 10,000 is a month and a half of interest, or 225 — not one month. The flat model works differently and rounds days overdue up to the next whole month.
What should I set the rate to?
Whatever your terms say, and 1.5% a month is a widely used contract figure. Two constraints matter more than the number: many jurisdictions cap what can be charged, and the cap often differs for business and consumer debt. Check your local position before setting the rate rather than after an invoice has gone out.
Is charging a late fee worth the relationship risk?
Often the threat is worth more than the collection. For most small suppliers the ongoing relationship outweighs the fee, so the point is to make being late slightly less comfortable than paying — not to build a revenue line. Applying one policy to every client is also easier to defend than deciding case by case, since nobody can claim they were treated differently.
What works better than a late fee?
Sending the reminder before the due date rather than after it, putting your terms on every invoice so they are never a surprise, and addressing the invoice to a named person rather than an accounts inbox. These move payment dates more reliably than the charge does, and none of them costs you goodwill.
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