Overdue Invoice Interest Calculator Australia
Got an invoice that has sailed past its due date? Enter the amount, the interest rate from your payment terms, and how many days late it is. We will work out the interest that has built up and the total now owing.
- Invoice outstanding$0
- Daily interest charge$0
- Days overdue0
- Total now owing$0
Enter an invoice amount and the days overdue to see the interest.
Chasing slow payers and patchy cash flow? Our bookkeepers keep your debtors tidy and follow up for you.
Tidy up my debtors →General information only, current for the 2025-26 financial year. Whether you can charge interest, and at what rate, depends on the payment terms in your contract or invoice. This calculator is a guide, not legal or personal tax advice. To set up enforceable terms and tidy debtor follow-up, talk to us.
A Few Things This Calculator Assumes
Can I actually charge interest on a late invoice?
Only if your client agreed to it. The right to charge late payment interest, and the rate, has to be set out in your contract, quote or invoice terms before the work is done. If your terms are silent on it, you generally cannot just add interest after the fact. The fix is simple: put a clear interest clause in your standard terms so it applies every time.
What interest rate should I use?
That is your call, within reason. A common approach for small business is around 10% per year, or a margin above a published benchmark rate. We have pre-filled 10% so you can see how it works, but change it to whatever your own terms say. Avoid setting a rate so high it could be seen as a penalty rather than genuine compensation for being kept out of your money.
Simple or compounding interest?
Most trade invoices use simple interest: the daily rate times the days overdue, applied to the original amount. Compounding adds each day's interest to the balance so future interest is charged on a slightly larger figure. Compounding only applies if your terms specifically say so. We default to simple because it is the safer, more common choice.
Does this include GST?
Interest charged for late payment is generally not subject to GST, because it compensates you for delay rather than being payment for a supply. So you typically calculate interest on the invoice total your client owes (which may itself include GST), but you do not add a further 10% GST on the interest. If you are unsure how to treat it in your accounts, that is exactly the kind of thing our bookkeepers sort out.
Stop Chasing, Start Getting Paid
Let our Perth bookkeepers keep your invoicing tight, your terms enforceable and your debtors followed up, so the money lands when it should. Book a quick call and we will map out the fix.
Free · No lock-in · 2-hour reply guarantee