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How this calculator works
Late payment terms usually combine pieces: an annual interest rate applied pro-rata for the days overdue, sometimes a monthly percentage, sometimes a flat administration fee. Enter whichever pieces your contract or terms specify and the calculator prices each part separately — interest for the exact number of days, the monthly component, the flat fee — then the total fee and the full amount now owed on the invoice.
Showing the parts separately is deliberate. A debtor querying a fee is owed the working, and “terms say 8% per annum” becomes concrete when it is expressed as a daily amount accumulating on their balance. It also keeps you honest: many jurisdictions cap what can be charged, or only enforce rates agreed before the work began, and itemised arithmetic is easier to check against those rules than a single round figure.
Good practice and limits
The fee is the last resort, not the system. Most late payment is cured earlier and cheaper: clear terms on the invoice, a reminder the day before the due date, another the day after, and a named person to contact. If you do charge, charge exactly what your published terms say — no more, no rounding up — because enforceability usually depends on following your own paperwork.