The Rule of 72: divide 72 by your interest rate to get the years needed to double your investment.
At 6% return, money doubles in 12 years. At 10%, in 7.2 years. At 3%, in 24 years.
Reach for this tool whenever a financial decision hinges on this type of calculation. Small differences in rate or term become large differences in total cost or return over multi-year horizons — differences that only become visible when you run the actual numbers.
A frequent error is using annual rates where monthly rates are required (or vice versa). Simply dividing an annual rate by 12 is only an approximation — the correct conversion for compound calculations uses the (1 + r)^(1/12) − 1 formula.
An employee receives a counter-offer from another employer: a £4,000 salary increase but no pension contribution versus the current role's lower salary with 8% employer pension. Running both through the finance calculator shows the true net financial value of each offer.
Divide 72 by the annual interest rate to estimate years to double money. Example: 72 / 8% = 9 years.
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