Rule of 72 & Investment Doubling Time Calculator
Use the Rule of 72 to instantly calculate how long it takes any investment to double — or what rate you need to reach your doubling target.
Rule of 72 Guide
The Rule of 72
Divide 72 by the annual percentage return to approximate the number of years to double your money. At 6%: 72/6 = 12 years. At 9%: 72/9 = 8 years. At 12%: 72/12 = 6 years. The rule is an approximation (exact calculation uses natural logarithm: time = ln(2)/ln(1+r) ≈ 0.693/r). It is most accurate for rates between 3-12% and becomes less precise at extremes. The Rule of 72 works for any exponential growth: population, inflation eroding purchasing power (72 ÷ inflation rate = years for prices to dou
Compounding and the Doubling Effect
A £10,000 investment doubling every 10 years (7.2% annual return): year 0: £10,000; year 10: £20,000; year 20: £40,000; year 30: £80,000; year 40: £160,000. Starting at 25 and investing until 65 with 7.2% returns: your money doubles four times in 40 years. This is why starting early matters so dramatically — the last doubling (from £80k to £160k) produces as much absolute money as all the previous doublings combined. Each doubling adds more in absolute terms than all previous doublings — the cor
Inflation and the Negative Rule of 72
The Rule of 72 also applies to how quickly inflation halves your purchasing power. At 3.6% inflation: 72/3.6 = 20 years to halve purchasing power. At 7.2% inflation (seen in 2022-23 UK): 72/7.2 = 10 years for prices to double. This is why holding cash long-term destroys real wealth — even in a savings account paying 4%, if inflation is 5%, your real return is negative. The real return = nominal rate − inflation rate. Protecting purchasing power requires returns that at minimum match inflation, a
Practical Applications for Investors
The Rule of 72 makes investment decisions intuitive without a calculator. A pension fund promising 5% annual returns: 72/5 = 14.4 years to double. Starting at 30 with £50,000, retiring at 72: three doublings = £50k → £100k → £200k → £400k. A savings account at 4%: 72/4 = 18 years to double. A Stocks and Shares ISA historically averaging 7%: 72/7 = ~10 years to double. Cryptocurrency or speculative assets: promises of 20% annual returns imply doubling every 3.6 years — which should prompt questio
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