Updated April 20, 2026 · Mortgage · Educational use only ·

Offset Mortgage Calculator

Interest saved by parking savings in an offset account

Calculate offset mortgage interest savings. See years saved and total interest reduced by offset balance. Enter mortgage balance to size affordability.

What this tool does

This calculator models how an offset account reduces mortgage interest charges over time. Enter your mortgage principal, interest rate, loan term, and the balance you typically hold in an offset account. The tool simulates month-by-month amortization, calculating interest on the net balance after offsetting, then compares this to a standard mortgage without offset. The result shows the total interest saved in your currency and estimates how many years the offset arrangement shortens your loan repayment period. The calculation assumes a consistent offset balance and standard monthly payments. Results are illustrative only and reflect the mechanics of offset accounts under the stated conditions; actual savings depend on how your offset balance changes over time and on your lender's specific terms.

Enter Values

Currency($)

Mortgage Balance($)

Interest Rate(%)

Term(yrs)

Typical Offset Balance($)

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Formula Used

I_{s a v e d} = I_{s t an d a r d} - I_{o f f se t}

I_{s a v e d}

Interest saved

I_{s t an d a r d}

Interest if no offset

I_{o f f se t}

Interest with offset maintained

Spotted something off?

Calculations or display — let us know.

Disclaimer

Results are estimates for educational purposes only. They do not constitute financial advice. Consult a qualified professional before making financial decisions.

How Offset Accounts Work

An offset account sits alongside the mortgage. Interest is charged on the mortgage balance minus the offset balance. A 500,000 mortgage with 50,000 in offset is charged interest on 450,000 — same monthly payment, but more goes to principal. The offset balance stays available as regular savings.

Offset Accounts vs Extra Principal Payments

Extra principal payment locks money into the mortgage permanently. Offset reduces interest charged while keeping the money accessible. Both produce functionally equivalent interest savings, but offset preserves liquidity. Trade-off: offset mortgage rates are sometimes slightly higher than standard rates.

Run it with sensible defaults

Using mortgage balance of 500,000, interest rate of 6, term of 30, typical offset balance of 50,000, the calculation projects 89,500.00 in interest savings. The defaults serve as a starting point for modelling.

The levers in this calculation

The inputs — Mortgage Balance, Interest Rate, Term, and Typical Offset Balance — do not pull with equal force. Not every input has equal weight. Adjusting one input at a time toward extreme values shows which ones move the result most.

How the math works

Simulates amortization month-by-month with standard monthly payment. Interest each month is charged on max(0, balance minus offset). Compares to the standard amortization without offset to calculate savings. Assumes offset balance held constant. Results are estimates for illustration purposes only.

Testing at different rates

Run the calculation at your current rate, then run it again at a rate 2–3 percentage points higher. That indicates roughly what a product reset could bring at renewal, and provides a check on affordability across different rate scenarios.

What this doesn't capture

The figure excludes arrangement fees, valuation costs, legal fees, insurance, and any early-repayment charges — those can add several thousand to the headline cost. Rate changes at renewal for fixed-term deals will shift the picture further. Use this for the core interest/principal math and add the other costs on top.

Example Scenario

Offset estimate indicates 194,606.16 total interest saved.

Inputs

Mortgage Balance:$500,000

Interest Rate:6%

Term:30 yrs

Typical Offset Balance:$50,000

Expected Result194,606.16

This example uses typical values for illustration. Adjust the inputs above to match a specific situation and see how the result changes.

Sources & Methodology

Methodology

This calculator simulates a month-by-month amortization schedule under two scenarios: a standard mortgage and an offset mortgage. For the standard mortgage, monthly interest is computed on the full outstanding balance. For the offset mortgage, the monthly interest charge applies only to the difference between the mortgage balance and the offset balance held (or zero if offset exceeds mortgage balance). The calculator assumes the offset balance remains constant throughout the term and applies a fixed interest rate. It then compares total interest paid across both scenarios to derive the interest saved. The model does not account for fees, early repayment, changes in interest rates, variations in offset balance over time, or tax treatment of savings. Results are estimates for illustration purposes and reflect the mechanical effect of reducing the interest-bearing balance by the offset amount.

References

MoneyHelper — Buying a Home

Frequently Asked Questions

Is an offset account better than extra payment?

Functionally equivalent on interest saved, but offset preserves access to the cash. Extra payment permanently reduces the balance. Offset wins for emergency fund planning; extra payment wins if the rate on offset mortgages exceeds standard mortgage rates by more than 0.25 percent.

Do offset mortgages cost more?

Often slightly — many and lenders charge 0.1-0.3 percent more for an offset facility than a standard mortgage. The break-even calculation depends on typical offset balance maintained.

Can I have multiple offset accounts?

Some lenders allow multiple offsets, others a single account. Multiple offsets can be useful for separating emergency fund, vacation fund, and tax reserves while still offsetting the mortgage.

What about partial offset?

Some lenders offer partial offset (typically offsets 50 percent of the offset balance). Cheaper fees but half the interest savings. For this calculator, enter half the actual offset balance to model partial offset.

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