About this tool

How to use the inflation erosion calculator

The tool is most useful when the inputs reflect a specific scenario you want to understand, rather than general curiosity. A realistic starting amount and a considered inflation rate produce more actionable output than round numbers chosen without context.

How inflation erodes purchasing power over time

Inflation is the rate at which the general level of prices rises over time. When prices rise, a given amount of money buys fewer goods and services than it did previously. A sum of money that sits in cash without earning interest loses purchasing power at approximately the inflation rate each year. At 2% annual inflation, £10,000 today has the purchasing power equivalent of around £8,200 in ten years. At 5% inflation, the same £10,000 falls to around £6,140 in real terms over the same period. The effect compounds: each year’s inflation applies to the already-reduced real value from the previous year, so the erosion accelerates in percentage terms as the period lengthens.

This is why the period length matters as much as the inflation rate in the tool’s output. At 3% inflation, the purchasing power loss after five years is approximately 14%. After twenty years, it is approximately 46%. The three-scenario chart in the tool makes this compounding visible: the three lines (2%, user’s rate, 5%) diverge increasingly as the projection extends, illustrating that the difference between a 2% and a 5% inflation environment is modest in the short term but substantial over a decade or more. For a retirement savings pot or a long-term financial plan, understanding the scale of purchasing power erosion at different inflation rates is a more useful frame than the nominal figure alone.

The break-even savings rate: what return preserves real value

To preserve the real value of savings, the interest earned must at least offset the rate of inflation. If inflation is running at 3% per year, a savings account paying 3% AER would preserve real value in nominal terms before tax. However, savings interest is taxable for most savers above the Personal Savings Allowance (£1,000 per year for basic rate taxpayers, £500 for higher rate taxpayers, and nothing for additional rate taxpayers as at 2025/26). Once tax is applied to the nominal interest, the effective return is lower than the headline rate, which means the gross rate needed to break even in real terms is higher than the inflation rate alone.

The break-even panel calculates this specifically. At 3% inflation, a basic rate taxpayer needs to earn a gross rate of approximately 3.75% to receive enough after 20% basic rate tax to offset inflation. A higher rate taxpayer needs approximately 5% gross to receive the same after-tax real return. The difference matters because it determines whether a given savings account rate is genuinely preserving real value or eroding it more slowly than a non-interest-bearing account would. Cash ISAs avoid this tax drag entirely, which is one reason they are widely used as savings vehicles: the full gross rate applies to the balance without tax reducing the effective return. The ISA vs loan cost comparison tool explores some of the wrapper dynamics in more detail.

Inflation and the real cost of fixed-rate debt

The same mechanism that reduces the real value of savings also reduces the real burden of fixed-rate debt. A £20,000 personal loan at a fixed rate means the nominal repayment figure does not change as inflation rises. But the real value of that debt, measured in purchasing power terms, falls each year that inflation is positive. At 3% annual inflation, the real burden of a £20,000 debt balance falls to around £16,400 over ten years and around £13,400 over twenty years, expressed in today’s purchasing power. The debtor is repaying in nominally unchanged monthly instalments, but those instalments represent a smaller and smaller share of real purchasing power as prices rise.

The debt counterpoint panel in the tool shows this calculation directly. It is included as factual context about how inflation interacts with fixed obligations, not as a reason to delay debt repayment. The high nominal interest rates on most unsecured borrowing mean that the interest cost of carrying debt typically exceeds the real-value reduction that inflation produces on the balance. A credit card balance at 24% APR is not made meaningfully more manageable by 3% annual inflation. The counterpoint is most relevant for very long-duration fixed-rate debt such as a mortgage, where the loan term is long enough for inflation to have a material effect on the real value of the outstanding balance, particularly in higher-inflation periods. For a direct comparison of the financial outcome of paying down debt against saving, the pay down debt vs save comparator models both strategies across your chosen period.

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Frequently asked questions

How does the inflation erosion calculation work?

The tool divides the starting amount by (1 + inflation rate) raised to the power of the number of years. This produces the real value of the starting amount in today’s purchasing power terms at the end of the projection period. For example, £10,000 at 3% inflation over ten years: 10,000 divided by (1.03 to the power of 10) gives approximately £7,441. This means £10,000 today has the same purchasing power as £7,441 would have in ten years’ time if prices rise at 3% per year throughout the period.

The three-scenario chart applies the same formula simultaneously at 2%, the user’s chosen rate, and 5% for every year in the projection. The three lines diverge over time because the compounding effect accumulates differently at each rate. The key insight from the chart is not the precise figure at any given year but the shape of the divergence: the gap between the 2% and 5% lines widens continuously, and over long periods the difference in purchasing power outcomes between a low and a high inflation environment is substantial.

What inflation rate should I use in the tool?

The Bank of England’s target rate of 2% per year is the standard planning assumption for the long run in a stable UK monetary environment. UK CPI inflation in the years following the 2021 to 2023 inflation surge returned toward target from a peak of over 11% in late 2022. For a long-term projection such as a 30-year retirement plan, using 2% or 2.5% as the base case and then testing a higher rate such as 4% or 5% as a sensitivity check gives a range rather than a single figure, which is more honest about the uncertainty in long-run inflation.

For a shorter-term projection, the current rate of inflation as measured by the Office for National Statistics’ monthly CPI release is a more relevant starting point than the long-run target, because it reflects the actual rate at which prices are currently rising. The ONS publishes monthly CPI data on its website. For most practical planning purposes, running the tool at both 2% and a higher scenario rate, and looking at the range of real values that produces, is more useful than trying to identify the single correct rate to use.

Why does the break-even savings rate differ for basic and higher rate taxpayers?

Savings interest in the UK is taxable as income above the Personal Savings Allowance for most savers. Basic rate taxpayers pay 20% tax on savings interest above £1,000 per year (2025/26). Higher rate taxpayers pay 40% tax on savings interest above £500 per year. This means a basic rate taxpayer keeps 80p of every £1 of interest earned above the allowance, while a higher rate taxpayer keeps 60p. To receive the same after-tax return, a higher rate taxpayer needs a higher gross rate.

The break-even calculation works backward from the after-tax return needed to offset inflation: at 3% inflation, a basic rate taxpayer needs a gross rate of 3% divided by 0.80, which equals 3.75%. A higher rate taxpayer needs 3% divided by 0.60, which equals 5.00%. If the available savings rate is below the relevant break-even figure, the real purchasing power of the savings balance is declining even if the nominal balance is growing. Cash ISAs, Premium Bonds (up to the prize allowance), and pension contributions all offer ways to receive returns without the same tax drag, which is why they are commonly used to hold savings above the Personal Savings Allowance threshold.

If inflation erodes the value of savings, does that mean cash savings are a poor choice?

Cash savings serve a specific purpose: they are liquid, accessible, and do not carry the risk of falling in nominal value. For an emergency fund, a short-term goal with a known deadline, or any money that may be needed at short notice, the stability of cash is more important than the risk of inflation eroding it modestly over the holding period. The question of whether to hold money in cash is not primarily about inflation; it is about accessibility, time horizon, and risk tolerance.

For money that is not needed for many years, the real value erosion from holding cash at below-inflation rates is a relevant consideration alongside investment alternatives. Stocks and Shares ISAs, pension funds, and investment portfolios have historically produced returns above inflation over long periods, though they carry the risk of falling in value and returns are not guaranteed. The tool shows the purchasing power cost of holding cash at a given inflation rate: what you do with that information depends on your specific situation, time horizon, and approach to risk, and is a decision that may benefit from regulated financial advice for larger amounts.

Does inflation reduce the real cost of all types of debt equally?

The debt counterpoint effect applies to fixed-rate debt where the nominal balance and repayment amount do not change as inflation rises. It applies most meaningfully to long-duration debt where the time period is long enough for the compounding effect of inflation to produce a material reduction in real burden. A 25-year fixed-rate mortgage is the clearest example: the real value of the outstanding balance falls each year that prices rise, even though the nominal balance falls only as capital is repaid.

Variable-rate debt does not benefit from this in the same way because the interest rate, and therefore the repayment, often rises alongside inflation. Much unsecured debt in the UK is variable-rate or has rates that are already high enough to more than offset any inflation erosion of the balance. The practical implication of the debt counterpoint is most relevant for people weighing up whether to overpay a low fixed-rate mortgage or direct those funds elsewhere, not as a general argument for carrying debt. The nominal interest rate on any borrowing is a more important factor in that decision than the inflation reduction in real burden.

This tool is for illustrative purposes only and does not constitute financial or tax advice. Inflation projections use a constant annual rate applied over the chosen period; actual inflation varies year to year and the tool does not model that variability. Purchasing power example figures use illustrative unit costs that do not reflect actual prices for any individual. Break-even savings rate calculations use simplified tax band assumptions based on 2025/26 rates and do not account for the Personal Savings Allowance, cash ISA wrappers, or other tax reliefs that may apply to individual circumstances. Tax rates and allowances are subject to change. Actual outcomes will depend on your individual circumstances.