A mortgage is the largest arithmetic problem most people ever take on, and small differences compound into very large ones. These work the real month-by-month numbers rather than a simplified estimate, and the offset tools sit here too, because an offset is a feature of a mortgage rather than a product of its own.
The four tools here answer different questions. The refinance calculator is for deciding whether switching is worth it, and it checks the total-interest question that catches people out when a refinance resets the term. The home loan repayment calculator gives you the repayment weekly, fortnightly or monthly, with the year-by-year split between interest and principal. The interest charge check is for a statement you have already received: it works out what you should have been charged, and the answer depends on which compounding convention your country uses. The rate change calculator is for a rate move that has just been announced.
The thing worth understanding before any of them: in the early years of a mortgage, almost every dollar you pay goes to interest rather than to the debt itself. That is why an extra repayment early is worth so much more than the same repayment later, and why a refinance that resets you to year one can cost more overall even at a lower rate.
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. Offsets are standard in Australia, available on a shrinking number of UK mortgages, and barely exist in the United States, so those tools assume the Australian daily-accrual, monthly-debit convention. Each one carries its own "Applies to" line.
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.