Interest Rate Change Impact Calculator

Why Small Rate Changes Compound

A savings rate dropping from 5% to 3% feels like a 2 percentage point change, almost trivial. On 50,000 over 10 years the difference is 13,400 (compound). On 100,000 over 20 years the difference is 64,500. Compounding amplifies small-looking changes into substantial amounts over long horizons.

When Rate Changes Matter Most

Long-term balances (pensions, lifetime savings) see the biggest impact from rate changes. Short-term savings (1-3 years) see much smaller absolute impact even from large percentage-point changes. The calculator takes years as an input so either scenario can be modeled.

What Rate Change to Model

Central bank rate cuts typically translate to savings rate cuts within 2-6 months. A 1pp policy cut often translates to 0.6-0.9pp savings rate cut (the remainder absorbed as bank margin). Model rate changes conservatively — assume 70-90% of policy rate moves flow through.

A worked example

Try the defaults: balance of 50,000, current rate of 5, new rate of 3, years of 10. The tool returns approx -14,274. 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 Balance, Current Rate, New Rate, and Years. 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

Old future value equals balance times (1 plus old rate) to the power of years. New future value substitutes new rate. Difference is the rate-change impact. Results are estimates for illustration purposes only. 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 the number matters

Saving without a target is like driving without a destination — you'll make progress, but you won't know when you've arrived. This tool gives you a concrete figure to work toward, which is the first step in turning a vague intention into an actual plan.

What this doesn't capture

The calculation assumes a steady savings rate and a stable interest rate. Real saving journeys include emergencies, windfalls, and rate changes — especially in easy-access products. The figure is a direction of travel, not a guarantee.