How our calculator works

The engine runs from the entered start date for up to 50 years. Dated transactions and rate changes apply first. From the following day, interest accrues on max(loan balance − offset balance, 0) × annual rate ÷ day basis, before scheduled repayments and regular monthly savings deposits.

The day basis can be 365 throughout, or the actual number of days in each calendar year. Accrued interest is added to the loan at calendar month end, rounded to cents. Interest-only payments also settle accrued interest on payment dates. Regular savings enter on monthly anniversaries of the start date. Shorter months use their final available day.

Sidelo assumes a 100% offset. It does not model partial offsets, account fees, package charges, redraw rules, break costs or lender repayment restrictions. Dates do not adjust for weekends or bank holidays.

Automatic principal-and-interest repayments use the standard monthly amortisation formula, based on the remaining term. Weekly and fortnightly amounts are the monthly result multiplied by 12/52 and 12/26 respectively. These are not the “half a monthly payment every fortnight” convention.

A manual repayment uses your entered amount. Rate-change events change interest but do not automatically reset an existing principal-and-interest payment. A payment too small to cover interest can allow the loan balance to grow.

Interest-only covers the selected period, with additional payments reducing principal. At its end, automatic payments are recalculated using the remaining balance, rate and term; a manual amount is used if selected. The initial interest-only figure is an estimate: actual simulated payments depend on accrued interest and payment dates.

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The monthly saving entered is treated as available after living costs, repayments, extras and other contributions. It is not constrained by the income and expense entries. Differences between the model’s payment budget and actual scheduled payments affect the amount deposited, including during interest-only and after payoff.

Use all budget surplus

Available saving is income less living expenses, actual scheduled repayments and non-super asset contributions. Entered income and expense growth is applied monthly; dated income and expense changes affect subsequent budget periods.

Use budget surplus up to a limit

Offset deposits are capped at the entered monthly limit and the available surplus. The remaining surplus is held separately.

Funding gaps draw on unallocated cash, cash assets, offset and eligible investment balances in the engine’s defined order. Property and super do not fund spending. Any uncovered amount becomes a shortfall without additional borrowing interest. Scheduled loan payments still occur in the simulation. A shortfall means that path would require additional funding.

The “No offset or extras” baseline begins with the same loan and repayment settings, but no offset, extra repayments or offset-saving deposits. Dated interest-rate changes also apply to the baseline; one-off additional repayment events do not.

The savings figure is the baseline’s accumulated interest minus the selected scenario’s accumulated interest, over the model’s simulation period. Where either loan remains unpaid after 50 years, the comparison is not a complete lifetime total. Interest and total repayments are nominal dollar sums, not discounted present values.

The display horizon controls how much of the chart is shown. It does not shorten the engine’s 50-year calculation. A “today’s dollars” view adjusts displayed future balances for the entered inflation assumption; it does not change loan interest.

The full-offset milestone records when offset cash first covers the outstanding loan and accrued interest. It can be temporary if cash is later withdrawn. Payoff records when the model settles the debt, either through repayments or the selected offset-payout option.

Total repayments include modelled loan payments and any offset cash used in a payout. Savings merely transferred into an offset are not loan repayments. An offset payout reduces cash and debt together. The full-offset guide explains the available routing options.

Actual take-home pay can be entered directly. Gross-pay mode uses a simplified Australian resident income-tax calculation for 2026–27, with LITO and an adjustable Medicare percentage. HELP, the Medicare levy surcharge and individual eligibility rules are not inferred. These tax settings remain fixed throughout a scenario; this is not a tax calculation service.

Net worth is a current snapshot of entered assets and debts. Super is a manually entered current balance only; it is not projected or used for spending. Sidelo does not forecast retirement. Cash and investment assumptions can affect mortgage funding and post-offset routing; constant returns are hypothetical, not predictions.

These guides explain arithmetic and product mechanics for Australian owner-occupiers. They do not recommend a lender, loan, investment or financial strategy, and are not financial, credit or tax advice. A disclaimer does not replace accurate calculations or appropriate professional advice about a personal decision.

If a result seems wrong, use Report a problem. Include the relevant settings and expected result, but leave out account numbers and other sensitive information.