Free Late Payment Interest Calculator
Find out exactly how much interest is owed on an overdue invoice. No signup required. Enter the invoice amount, due date, and your interest rate, and Encarmo calculates the total amount due instantly.
Overdue Balance Summary
Principal plus accrued late fee
How to calculate late payment interest in 5 steps
Enter the invoice amount
The unpaid principal balance still owed.
Enter the due date
The date payment was originally due, per the invoice terms.
Enter the payment date
The date payment was, or will be, actually received. Use today's date to see what's owed right now.
Enter your interest rate
The annual late payment rate stated in your invoice terms or contract.
Review the total due
Encarmo calculates the number of days late, the interest accrued, and the total amount now owed.
What is late payment interest?
Late payment interest is an additional charge added to an overdue invoice, calculated on the unpaid amount for every day it remains outstanding past the due date. It compensates for the cost of not being paid on time, and it gives a client a concrete reason to pay sooner rather than later.
The three numbers behind a late fee
Principal — What's actually owed
The unpaid invoice amount, before any interest is added.
Days late — How long it's been outstanding
The number of days between the due date and the date payment is actually made.
Annual interest rate — What was agreed
The rate stated in the original invoice terms or contract, applied proportionally for the actual number of days late.
Worked example: a simple late payment interest calculation
$5,000 invoice overdue by 45 days at 12% annual interest
| Invoice amount | $5,000 |
| Due date | March 1 |
| Payment date | April 15 |
| Days late | 45 days |
| Annual interest rate | 12% |
| Interest accrued | $5,000 × 12% × (45 ÷ 365) ≈ $73.97 |
| Total amount due | $5,000 + $73.97 = $5,073.97 |
45 days late at a 12% annual rate adds about $74 to a $5,000 invoice, a modest amount on its own, but the charge grows the longer a client waits, and across several overdue invoices it adds up.
Late fee vs. late interest: what's the difference?
| Late Fee | Late Interest | |
|---|---|---|
| Shows | A fixed, one-time charge for missing the due date | A charge that grows the longer the balance stays unpaid |
| Calculated as | A flat dollar amount or flat percentage | Principal × rate × time elapsed |
| Applied | Once, when payment is missed | Continuously, for every day the invoice stays overdue |
| Best for | Discouraging any lateness at all | Recovering the actual cost of a long delay |
Why charging late payment interest matters
Cash flow protection
Charging late interest offsets some of the cost of not having that cash available, especially when invoices are large or a client is chronically slow to pay.
Encouraging on-time payment
A stated late interest rate on an invoice gives clients a concrete reason to pay by the due date instead of treating it as a suggestion.
Setting clear expectations
Stating the interest rate on every invoice up front avoids an awkward conversation later about whether a late charge is fair.
Knowing what to invoice for
Before following up on an overdue invoice, calculating the exact amount owed, principal plus interest, keeps the request precise instead of a round-number guess.
Why use Encarmo's free late payment interest calculator
Completely free, no limits
Run unlimited calculations with no paywall or watermark.
No signup required
Get the total due instantly as a guest without creating an account.
Works with any interest rate or payment terms
Not tied to one preset rate or arbitrary formula.
Calculates exact days late automatically
From the two dates entered, with zero manual date math.
Clean, downloadable summary
Download the breakdown to attach to a formal collections or follow-up email.
Questions people ask us
Clear, direct answers on methodology, formulas, and usage.
Is this late payment interest calculator really free?
Can I charge any interest rate I want on a late invoice?
What if I didn't state an interest rate on the original invoice?
How is "days late" counted?
Does this compound the interest?
What's a typical late payment interest rate?
Does this work for international invoices?
Should I actually charge the interest, or just calculate it?
Do I need invoicing software to send this to a client?
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