Mortgage Rate Delta Impact Calculator

A 200,000 balance at 3.5% with 20 years remaining pays 1,160 monthly. Rate rises to 5%: monthly jumps to 1,320 — a 160 increase, 1,920 a year. Seeing the numbers before a fix expires lets you decide whether to fix early, stay variable, or switch lenders.

When to run this

Fix expiring in 3-6 months — comparing deals as the term ends. Rate cycle changes — the central bank moves rates. Considering overpayments — see if a new rate makes prepayment more attractive. Early repayment decisions — comparing the cost of paying off against keeping cash liquid.

A worked example

With the defaults: outstanding balance of 200,000, current rate of 3.5%, new rate of 5%, years remaining of 20 years. The tool returns 1,319.91. You can adjust any input and the result updates as you type — no submit button, no reload. That's the real power here: seeing how sensitive the output is to one or two assumptions.

What moves the number most

The result responds to Outstanding Balance, Current Rate, New Rate, and Years Remaining. Not every input has equal weight. Adjusting one input at a time toward extreme values shows which ones move the result most.

The formula behind this

Uses the standard mortgage amortisation formula for both current and new rates, computes monthly payment for each, and returns the new monthly and delta. Assumes same remaining term under both rates (no extension). Everything the calculator does is shown in the formula box below, so you can check the math against your own spreadsheet if you want.

Why this matters

A mortgage is usually the biggest single financial commitment a person makes. The difference between a well-chosen product and a hasty one can run into tens of thousands over the life of the loan. Modelling the numbers ahead of a decision shows how sensitive the outcome is to the rate and structure chosen.

What this doesn't capture

The figure shown reflects the core calculation; additional costs such as arrangement fees, valuation, legal fees, insurance, and any early-repayment charges (where applicable) sit on top and can add materially to the total cost of borrowing. Rates and product terms can also change over the life of the loan, which can shift the picture relative to this fixed-snapshot estimate.