A late fee only holds up if it was stated on the invoice or contract before the due date passed. This works out what to add to an overdue invoice, based on days late and your stated annual rate.
Your inputs
Invoice amount$3,200
Days overdue21
Annual late fee rate18%
A common range is 12–18% a year (1–1.5% a month) — check your contract's stated terms.
Daily rate applied0.0493%
Late fee accrued$31.05
Total now owed
$3,231.05
Original amount + late fee
Monthly rate equivalent
1.50%
Fee only
$31.05
This applies a simple daily rate across the days overdue rather than compounding monthly, which keeps the fee predictable and easy to explain to a client. Check your local jurisdiction's maximum allowed interest rate on invoices before applying this for real.