Home / Tools / Late payment calculator
Late payment calculator
Calculate interest on an unpaid invoice. Enter amount, overdue date, payment date, and rate — see accrued interest, optional flat late fee, and the new total due.
Use your contract rate, or a statutory commercial late-payment rate where it applies.
Days overdue—
Interest accrued—
Late fee—
Updated total due—
Simple interest: amount × annual rate × (days ÷ 365). Not legal advice — confirm rates and fees against your contract and local law.
How to calculate late payment interest
Late payment interest compensates you for cash tied up after the due date. The usual simple-interest formula is:
Interest = invoice amount × annual rate × (days overdue ÷ 365)
After you know the number, add it as a clear line item on an updated invoice or statement, then send a professional follow-up. Chasa can draft that email; you send it.
What to do next
- Confirm the rate is in your terms or allowed by local commercial rules.
- Issue an updated invoice that itemizes original amount + interest (+ fee if any).
- Draft an overdue follow-up that explains the new total without burning the relationship.
FAQs
How do you calculate late payment interest on an unpaid invoice?
Multiply the invoice amount by the annual interest rate, then by days overdue divided by 365. Example: $1,000 × 8% × (30 ÷ 365) ≈ $6.58 interest.
What interest rate should I use for late invoices?
Use the rate in your contract or invoice terms. Many freelancers use 1–1.5% per month (about 12–18% annually). Some countries set a statutory rate for commercial late payments — check local rules.
Can I charge late fees if my contract never mentioned them?
Usually no. Late interest and fees are easiest to enforce when they appear in your agreement or invoice terms before work starts. Add them to new contracts going forward.
Does Chasa email clients for me?
No. Chasa drafts follow-up emails; you review and send from your own inbox. That keeps your client relationship in your control.