Step-by-step walkthrough
Use this companion guide beside the interactive calculator to make sure you understand the “why” behind every click. Each section includes the classroom explanation, student-friendly language, and quick practice prompts.
Collect the data points listed below and double-check their units. Keeping an organized “givens” list is one of the fastest ways to reduce math errors and impress exam graders.
Step 1
Enter the original invoice amount and how many days it's currently overdue.
Step 2
Enter the annual late-payment interest rate stated in your invoice terms or contract.
Step 3
Use the total due figure when following up with the client, and keep a record of the calculation date.
Many jurisdictions cap late-payment interest or require it to be disclosed upfront in the contract. Check local law before charging a rate you haven't already agreed to with the client in writing.
Without a pre-agreed rate, you generally can't unilaterally add one to an existing invoice — add late-fee terms to future contracts instead, and consider a payment plan for the current one.
