Hundred SummersInflation calculator: what your money will buy, and what it needs to earn
What a sum buys in five, ten, twenty or thirty years at a chosen inflation rate, the amount needed then to match it, how fast money halves in value, and the real return on savings earning interest, done properly. Your figures never leave your browser.
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Worked example
Computed when this page was built by the same engine that runs the calculator above, so the figures below and the live tool can never disagree. Change any input above to see your own.
| Inputs | |
|---|---|
| Amount today | £10,000 |
| Years | 20 |
| Inflation a year | 2.5% |
| Interest on the money | 4.0% |
| Results | |
|---|---|
| What it buys in 20 years, in today's money | £6,103 |
| Purchasing power lost | 39% |
| Needed then to match today | £16,386 |
| Money halves in value every | 28 years |
| Real return at 4% interest | 1.46% a year |
The next question
What will regular saving grow to after inflation? →Savings growth calculator
Or answer it with everything else: the full Hundred Summers plan connects your pay, pensions, savings, property, tax and spending year by year, and stress-tests them across 500 simulated market histories.
Read the guides
- Premium Bonds, cash and tax: where the money you might need should sit →The personal savings allowance and why higher-rate savers cross it at ordinary balances, the after-tax comparison that decides Premium Bonds against a savings account, cash ISAs, and what inflation does to money that sits still.
- Inflation: the assumption everything rests on →What 2.5% a year does to money over a retirement, what UK prices actually did since 1988 on the ONS index, why every figure in a Hundred Summers plan is in today's money, and how the plan is stress-tested against inflation that does not behave.
Small rate, big effect
Inflation is the assumption every projection rests on and the one most often left implicit. At the Bank of England’s 2% target a pound loses half its value in about 35 years; at the 2.5% Hundred Summers uses as a planning default, about 28; at the 10%-plus seen in 2022, seven. A retirement that starts at 60 and runs to 95 spans all of that, which is why a “£30,000 a year” that is not indexed is a plan to get steadily poorer.
Real returns, done properly
Subtracting inflation from an interest rate is close but wrong; the Fisher equation divides instead, and the gap grows with the numbers. The Hundred Summers plan works entirely in today’s money using this deflation, models inflation as a persistent process rather than a constant in its 500-scenario stress test, and replays the 1966–81 grind through your own figures in the historical backtest. The drawdown calculator shows what indexed spending does to a pot.
Sources
- Bank of England, Inflation and the 2% target.
- ONS, Inflation and price indices (CPI history, including the 2022 peak above 10%).
- The 2.5% planning default and the deflation method: Hundred Summers methodology.