Rule of 72
The Rule of 72 is a simple mental shortcut to estimate the number of years required for an investment to double in value by dividing 72 by the annual rate of return.
The Rule of 72 was one of the most practical heuristics in personal finance precisely because it required no calculator. By dividing 72 by an investment's expected annual return, an investor could instantly estimate the doubling time. At 8 percent, money doubled in roughly nine years; at 12 percent, in six years; at 6 percent, in twelve years. The rule worked with reasonable accuracy for interest rates between 6 and 10 percent and became slightly less precise at extremes.
In the Indian context, the rule offered vivid comparisons. A savings account earning 3.5 percent per annum took about 20.6 years to double. A PPF at 7.1 percent (the rate applicable for the quarter ending March 2024) doubled in approximately 10.1 years. An equity mutual fund that historically delivered 12 percent compounded annual returns doubled every six years. These comparisons made the opportunity cost of parking long-term money in low-yield instruments immediately tangible without requiring spreadsheets.
The rule was equally useful for understanding the erosion caused by inflation. Indian CPI inflation averaged around 5–6 percent over the decade to 2024. At 6 percent inflation, the purchasing power of a rupee halved in twelve years, illustrating why idle cash lost real value over time. An investor who understood this relationship was more motivated to deploy savings into instruments that at least matched or exceeded the inflation rate.
A lesser-known variant was the Rule of 70, preferred by economists when working with continuously compounded rates, and the Rule of 69.3, which was mathematically exact for continuous compounding. For everyday financial planning, however, 72 was favoured because it had more integer divisors (2, 3, 4, 6, 8, 9, 12, 24, 36), making mental arithmetic easier across a wider range of interest rates.
The Rule of 72 also worked in reverse: an investor who knew they needed to double their money in a specific number of years could divide 72 by those years to determine the required annual return. Needing to double in eight years meant requiring a 9 percent annualised return, which helped set realistic expectations about instrument selection.