This free invoice overdue interest calculator calculates daily or monthly interest on an overdue invoice balance at a user-entered rate, with total interest and a new balance. It runs fully in your browser, so nothing you type is sent to a server, and you need no account and no sign-up to use it.
Small business paperwork eats time that should go to serving customers. A freelancer or shop owner who tracks invoices on paper scraps soon loses the thread, and a lost thread means a missed payment, a wrong reminder, or a repeated question. A simple browser tool keeps that one job in one place.
To use the tool, open it on any device and fill in the fields at the top of the page. Press the main button and the tool updates the result at once. If anything changes, update the fields again, and the new result shows right away.
Example: a balance of 1,000 overdue by 30 days at 8 percent a year gives daily interest of about 6.58, so the new balance is about 1,006.58.
Tip: agree the interest rate in your terms before the invoice is late. A rate invented after the delay is hard to enforce.
Tip: check your local rules for late payment interest. Some places set a legal rate, and a higher contract rate may not stand.
Tip: show the interest as its own line on the next invoice. A separate line is clear, while a larger total with no explanation causes disputes.
Common question: Is the interest simple or compound? This calculator uses simple interest, which is the common basis for small late-payment claims.
Common question: What if the rate is monthly, not yearly? Pick the monthly method. It applies the monthly rate for each month overdue, instead of dividing a yearly rate.
Common question: Is this legal advice? No. This is an estimate only, not professional financial or legal advice, so check your terms and local rules.
Please note that every result here is an estimate only, not professional financial advice. Tax rules, late payment rules, and invoice rules differ by country and by trade, so check your own terms and local rules before you rely on a number for a real payment.
Privacy matters for business records, and this tool is built for that. Everything you enter stays in the browser on your own device. There is no account, no upload, and no shared copy, so client names and amounts never travel beyond your own screen.
Keep a short weekly habit around the tool. Check it at the same time each week, clear finished items, and add the new ones for the week ahead. A five-minute weekly check keeps the tool honest, and an honest tool is one a busy owner actually trusts.
A good time to start is at the start of a month or a quarter, when the books are fresh and the habit is easy to form. Set up the tool in one short session, enter the invoices you already have open, and let it carry the routine from there. A calm first setup prevents most later confusion.
If you work with a bookkeeper or an accountant, show them the tool and agree on the same simple routine. Shared routines stop the same invoice from being handled in two different ways, and they stop small errors from reaching the year-end accounts. A short note about how you use the tool is enough.
Clients notice when a small business runs its paperwork well. Clear invoices, polite reminders, and correct balances build trust, and trust brings repeat work. A calm system beats a loud chase every time, and it costs far less energy.
A steady routine helps more than a perfect setup. Use the same device, the same time of week, and the same simple steps, and the paperwork will stop piling up. A business with tidy invoices has more time for the work that earns money.
This tool is free to use as often as you like. Bookmark it, open it each week, and let it carry the small invoicing job while you focus on your customers.