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 — time to shop around. Rate cycle changes — the central bank or Fed moves rates. Considering overpayments — see if new rate makes prepayment more attractive. Early repayment decisions — see whether paying off reduces stress vs keeping cash liquid.

A worked example

Try 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 before you sign

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. Running the numbers here before committing is the cheapest form of due diligence available.

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.