These offset calculators assume the Australian convention. Offset accounts are standard in Australia and available on a shrinking number of UK mortgages. They barely exist in the United States, so everything in this group assumes the Australian daily-accrual, monthly-debit convention. If your mortgage does not have an offset facility, the universal mortgage tools are the ones you want.
An offset account reduces the balance your interest is calculated on without the money leaving your control. That is the whole mechanic, and it is why an offset balance beats an irreversible extra repayment at the same rate: the interest saving is identical and the money stays accessible.
ASIC found in July 2026 that banks are not always getting this right. Its review of eight lenders covering more than 70% of the Australian home loan market found gaps at all of them. 55% of the failures were an offset account that had been opened but never linked to the loan. Over $55 million has been paid in compensation. The interest charge check exists because of that.
Interest calculation conventions differ by country, which is why these tools are grouped and labelled rather than presented as universal. Australia accrues daily and debits monthly. Canada compounds fixed mortgages semi-annually. The compounding convention calculator sits with the universal mortgage tools because it handles the variation rather than assuming one.