Late Payment Interest Calculator: How to Charge Interest on an Overdue Invoice (Free, No Sign-Up)

If a client hasn't paid your invoice by the due date, you are usually allowed to charge interest on the overdue amount — but almost nobody knows how much that interest actually comes to, or how to say so to the client without sounding aggressive. TidyPapers' Late Payment Interest Calculator works this out for you in the browser, with no sign-up, no account, and no data leaving your device. This guide walks through every field on the page at https://tidypapers.com/late-fee-calculator/ so you know exactly what to enter and what each option means.

What this tool is for

It answers one question: "If my invoice is overdue, how much interest and fees can I add, and what do I say to the client?" It does not send anything anywhere — you fill in the numbers, it calculates, and you copy the result yourself.

Before you start: what you need

• The original invoice amount
• The date the invoice was due
• Whether you want to use the EU's statutory late-payment rate, or set your own rate

Step 1 — Open the calculator and enter the invoice amount

1. Go to https://tidypapers.com/late-fee-calculator/.
2. In the Invoice amount field, type the amount the client still owes (numbers only, no currency symbol).
3. Set the Currency dropdown to match the invoice. The three choices are:
   • € Euro (EUR)
   • $ US Dollar (USD)
   • £ British Pound (GBP)

Step 2 — Enter the dates

1. In Invoice due date, pick the date the payment was originally due (the date on the invoice terms, not today).
2. In Calculation date, leave it as-is if you want interest calculated as of today. This field defaults to today's date — the page itself notes: "Defaults to today. Change it to see interest at a future date." Change it only if you want to preview what the client will owe on some future date (for example, the date you plan to send the reminder).

Step 3 — Add an invoice reference (optional)

There is an Invoice reference (optional, used in the copied reminder text) field. Type your invoice number here (e.g. "INV-0042") if you want it to automatically appear in the reminder text you'll copy at the end. You can leave this blank — it changes nothing about the calculation itself, only the wording of the copied message.

Step 4 — Choose your interest basis

This is the most important setting on the page. There is a toggle labelled Interest basis with two options:

• EU statutory (Late Payment Directive 2011/7/EU) — use this if you or your client is in the EU and you want to charge the legally-defined statutory rate for late commercial payments.
• US / custom — use this for any other country, or if you want to set your own interest rate instead of the EU statutory formula.

If you choose "EU statutory"

Two extra fields appear:

• ECB reference rate — a text field where you type in the current European Central Bank reference rate. The page provides a link, "Check the current ECB reference rate ↗", which takes you to the source so you can look up the current number before entering it — the tool does not fetch this automatically.
• Statutory margin — this is not something you enter; it is shown as a fixed display: "+8 percentage points (fixed)". Under the Directive, the total statutory rate is the ECB reference rate plus this fixed 8-point margin, and the tool adds them for you.

If you choose "US / custom"

Instead of the ECB fields, you get a single field:

• Annual interest rate (%) — type in whatever annual rate you want to charge (check what's allowed in your contract or local law before picking a number).

Step 5 — Add a fixed late fee (optional)

Below the interest settings there's a checkbox for Fixed late fee amount (optional, in addition to interest). Tick it if your contract or local law allows a flat fee on top of interest (some jurisdictions allow a fixed recovery-cost fee alongside statutory interest), then enter the amount. Leave the checkbox unticked if you only want to charge interest.

Step 6 — Calculate

1. Click "🧮 Calculate late fees →".
2. The tool displays five results:
   • Days overdue — how many days have passed since the due date
   • Daily interest — how much interest accrues per day
   • Accrued interest — the total interest built up over those days
   • Flat fees — the fixed fee you entered in Step 5, if any
   • Total owed now — the full amount the client owes today (original invoice + interest + flat fee)

Step 7 — Copy a ready-to-send reminder

1. Click "📋 Copy demand text →".
2. The button shows a "✓" confirmation once the reminder text is copied to your clipboard.
3. Paste it into an email or message to the client. It will already include your invoice reference (if you added one in Step 3) and the calculated amounts.

Related tools on the page

At the bottom of the Late Payment Interest Calculator, TidyPapers links to three related tools you may also want:

• Payment Fees Comparison (/payment-fees-comparison/) — compare how much different payment processors take before you decide how the client should pay you
• Self-Employment Tax Calculator (/self-employment-tax-calculator/)
• How Much to Set Aside for Taxes (/how-much-to-set-aside-for-taxes/)

These three are separate pages you navigate to manually — nothing is passed between them automatically. The invoice generator is different: the "Create reminder invoice" button above (Step 7) queues this calculation's own numbers there automatically — see the dedicated walkthrough, How to Chase an Overdue Invoice, for the full path from this calculator to a sent invoice.

Frequently asked practical questions