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
$4,606.58
Interest accrued
$66.58
Interest per day
$1.48
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter invoice amount ($).
- 2Enter days overdue.
- 3Enter annual interest rate (%).
- 4Enter flat late fee ($).
- 5Read your total now due on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
Most contracts allow interest on late invoices. Enter the invoice, days overdue and your contractual rate to see exactly what to add.
Worked example
Using the values the calculator loads with:
Inputs
- Invoice amount: 4500 $
- Days overdue: 45
- Annual interest rate: 12 %
- Flat late fee: 40 $
Results
- Total now due: $4,606.58
- Interest accrued: $66.58
- Interest per day: $1.48
What each field means
Inputs
- Invoice amount ($)
- The invoice amount used in the calculation, measured in $. Starts at 4500 $ so you have a working example on load.
- Days overdue
- The days overdue used in the calculation. Starts at 45 so you have a working example on load.
- Annual interest rate (%)
- The annual interest rate used in the calculation, measured in %. Starts at 12 % so you have a working example on load.
- Flat late fee ($)
- The flat late fee used in the calculation, measured in $. Starts at 40 $ 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.
- Interest accrued
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Interest 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.
FAQ
How does the late payment interest calculator work?
Most contracts allow interest on late invoices. Enter the invoice, days overdue and your contractual rate to see exactly what to add. The underlying maths is: Interest = amount × annual rate × days ÷ 365.
What do I need to enter?
4 values: invoice amount, days overdue, annual interest rate, 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 Interest Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/late-payment-interest
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/late-payment-interest" target="_blank" rel="noopener">Late Payment Interest Calculator — RevenueLab</a> (2026).</p>
Source: [Late Payment Interest Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/late-payment-interest) (2026).