Idle Cash Cost Calculator (UK 2026)

How the Idle Cash Calculator Works

The calculator runs four compounding scenarios in parallel using the inputs you provide. It begins with your gross interest rate and applies your marginal tax band to derive a net rate. That net rate is compounded annually across your time horizon to produce the nominal cash trajectory.

In parallel, your inputted CPI rate is compounded across the same horizon. The two trajectories are combined via the Fisher relation to produce a real yield: the rate at which your purchasing power is actually changing. When net interest sits below inflation, real yield is negative and your balance is shrinking in real terms even though the nominal number is growing.

Finally, the calculator compounds your balance at the user-defined comparator yield to model an alternative deployment scenario. The difference between the two nominal end-states is the headline Forgone Capital Growth figure.

Inputs

• Cash balance held — your current cash deposit position in GBP.
• Gross interest rate (AER) — the headline rate quoted by your savings provider, before tax.
• Marginal tax band — ISA, Basic (20%), Higher (40%), Additional (45%).
• Expected inflation (CPI) — the annual CPI rate you expect over the horizon.
• Time horizon — the number of years you intend to hold the position.
• Comparator deployment yield — the target annualised return you would model against.

Why Idle Cash Erodes Wealth

Cash deposits are the most psychologically comfortable place to park capital and one of the least productive. Three forces conspire against the cash holder simultaneously: income tax on interest, CPI inflation, and the opportunity cost of forgone deployment. Each is small in isolation. Compounded together over five to ten years, they routinely subtract two-figure percentages from real wealth.

Consider a higher-rate taxpayer in 2026 holding £100,000 in an easy-access account paying 4.5% AER. After tax, the effective rate is 2.7%. With UK CPI running near 3%, the real yield is roughly negative 0.3% per year. Over five years, that balance grows nominally to around £114,000 but its purchasing power has fallen below the original £100,000. The same capital deployed at a 7% target yield via UK property income would have compounded to over £140,000 nominally. The gap is the cost of idleness.

ISA wrappers neutralise the tax leg. They do not neutralise inflation or the opportunity cost. A cash ISA paying 4.5% with CPI at 3% produces a real yield of just 1.5%, still well below the return profile of most deployed asset classes — see alternatives to buy-to-let and UK property investment returns benchmarks for comparable yield profiles.

How to Read Your Result

The diagnostic panel surfaces five numbers worth attention. Your net interest rate is what you actually keep on every pound of interest. Real yield tells you whether your cash is winning or losing the inflation race. Cumulative inflation drag shows the gap, in pounds, between your nominal balance and what the same basket of goods costs at the end of the horizon.

The headline Forgone Capital Growth figure is the most actionable. It is the nominal-pound difference between holding cash and deploying at your comparator yield, over your chosen horizon. If this number is meaningful relative to your balance, the question is no longer whether to deploy but how — see how Model One deploys idle capital into UK property income without mortgages or tenant management.

Related Reading

Pages and articles to deepen the analysis once you've run your numbers.

• UK Property Investment — Model One · the flagship deployment route this calculator's comparator yield is calibrated against.
• How to invest £100,000 in UK property income · matched to the calculator's default scenario.
• Passive, hands-off property investment · what deployment looks like operationally.
• Alternatives to buy-to-let · why direct landlording isn't the only deployment option.
• Is property a good investment in the UK? · benchmark on long-run UK property returns.
• Browse all Shaded Canvas tools · more capital and property models.

Frequently Asked Questions