Chasing a client for an overdue invoice is awkward enough without also guessing at what to charge them for the delay. This calculator turns your invoice amount, your monthly interest rate, and the number of days it's overdue into a single late fee and a total amount due — so the follow-up email has an exact number instead of a vague "plus interest" and a client who has every incentive to argue about what that means.
How it works
The calculator treats your monthly interest rate as a 30-day rate and scales it in proportion to how many days the invoice has actually been late. Multiply the invoice amount by the monthly rate to get what one full month of lateness would cost, then multiply that by the fraction of a month the payment has actually missed — 15 days late is half a month's interest, 45 days is a month and a half, and 0 days late is correctly zero, not an error.
That proration matters because invoices rarely land on a clean 30-day boundary. A client who pays 6 days late shouldn't owe the same fee as one who pays 29 days late, even though both are technically "late" — straight-line proration keeps the fee proportional to the actual delay instead of jumping in full-month steps. The total due is simply the original invoice amount plus that fee, which is the number to put on your reminder, not the fee in isolation.
Worked example
Say you invoiced a client $2,000, your contract specifies 1.5% monthly interest, and payment is now 30 days overdue.
- One month's interest: $2,000 × 1.5% = $30
- Days-late fraction: 30 ÷ 30 = 1.0 (exactly one month late)
- Late fee: $30 × 1.0 = $30
- Total due: $2,000 + $30 = $2,030
Push that same invoice to 45 days late instead and the fraction becomes 45 ÷ 30 = 1.5, so the fee scales up to $2,000 × 1.5% × 1.5 = $45, and the total due becomes $2,045. Catch it early at 10 days late and the fee drops to $2,000 × 1.5% × (10 ÷ 30) = $10 — the same rate, scaled honestly to how overdue the payment actually is.
How to interpret your result
The late fee is what your contract or invoice terms entitle you to charge for the delay — it isn't a penalty you're inventing after the fact. That only holds up, though, if the rate and the right to charge it were disclosed to the client before the work started, ideally in writing on the original invoice or the contract itself. Springing a surprise interest rate on an already-frustrated client tends to create a payment dispute on top of the payment delay.
Treat the total due as the number you actually ask for, not the fee as a standalone line item you expect the client to add up themselves. And use the fee as leverage carefully: for a client who is a few days late and usually reliable, it can be more valuable to waive it and preserve the relationship than to collect $10 and make the next invoice harder to get paid on time.
Methodology & sources
The formula is lateFee = invoiceAmount × (monthlyInterestPercent / 100) × (daysLate / 30), and totalDue = invoiceAmount + lateFee. Both figures are rounded from unrounded intermediate math, so the fee and the total stay internally consistent with each other at any number of days late, including zero.
What you're actually allowed to charge, and whether a cap applies, varies by jurisdiction and by what your contract says — this calculator does the arithmetic, it isn't a substitute for checking local law or your own contract terms. The UK government's guidance on late commercial payments and statutory interest is a useful example of how explicitly some jurisdictions regulate this: it sets a statutory rate and requires the right to charge it to exist before you can invoke it. Put your own late-fee terms in writing before work begins, and confirm they're enforceable where your client is based.