Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Total now due
$8,691.25
Late fees owed
$191.25
Interest portion
$191.25
Accruing per day
$4.25
Annualised rate
18.25%
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter invoice amount ($).
- 2Enter late fee rate (%).
- 3Enter rate period.
- 4Enter days overdue.
- 5Enter flat late fee ($).
- 6Read your total now due on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Late fees are usually stated as a monthly percentage or an annual rate applied daily. Check your contract and local law — some jurisdictions cap what you can charge.
Worked example
Using the values the calculator loads with:
Inputs
- Invoice amount: 8500 $
- Late fee rate: 1.5 %
- Rate period: Per month
- Days overdue: 45
- Flat late fee: 0 $
Results
- Total now due: $8,691.25
- Late fees owed: $191.25
- Interest portion: $191.25
- Accruing per day: $4.25
- Annualised rate: 18.25%
What each field means
Inputs
- Invoice amount ($)
- The invoice amount used in the calculation, measured in $. Starts at 8500 $ so you have a working example on load.
- Late fee rate (%)
- The late fee rate used in the calculation, measured in %. Starts at 1.5 % so you have a working example on load. Accepted range: 0–20 %.
- Rate period
- Pick the option that matches your situation — the maths changes per option. Choices: Per month, Per year.
- Days overdue
- The days overdue used in the calculation. Starts at 45 so you have a working example on load.
- Flat late fee ($)
- The flat late fee used in the calculation, measured in $. Starts at 0 $ so you have a working example on load.
Results
- Total now due
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Late fees owed
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Interest portion
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Accruing per day
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Annualised rate
- Returned as a percentage. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the late payment fee calculator work?
Late fees are usually stated as a monthly percentage or an annual rate applied daily. Check your contract and local law — some jurisdictions cap what you can charge. The underlying maths is: Fee = Invoice × Rate ÷ Period × Days overdue.
What do I need to enter?
5 values: invoice amount, late fee rate, rate period, days overdue, and flat late fee. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the total now due result mean?
It is returned as a money amount in US dollars and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
Who is this tool for?
It's built for anyone comparing money scenarios before committing — budgeting a payment, sanity-checking a quote, or seeing what a change in rate or term actually costs.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Late Payment Fee Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/invoice-late-fee
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/invoice-late-fee" target="_blank" rel="noopener">Late Payment Fee Calculator — RevenueLab</a> (2026).</p>
Source: [Late Payment Fee Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/invoice-late-fee) (2026).