The Rule of 72: Estimate How Fast Money Doubles
The Rule of 72 is a quick way to estimate how many years it takes an investment to double at a given interest rate.
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Frequently asked questions
What is the Rule of 72?
It estimates how many years an investment takes to double: divide 72 by the annual percentage return. At 6%, that is 12 years.
How accurate is the Rule of 72?
Very close for rates between about 6% and 10%. At extreme rates it drifts slightly from the exact compound-interest result, but it remains a handy approximation.
How do I find the rate needed to double in a set time?
Divide 72 by the number of years. To double in 8 years you need about a 9% annual return.
What are the Rules of 114 and 144?
They estimate tripling (114 ÷ rate) and quadrupling (144 ÷ rate) time, using the same divide-by-a-constant idea as the Rule of 72.
Does it work for inflation too?
Yes. Dividing 72 by an inflation rate estimates how long until prices double — at 3% inflation, about 24 years.
Why 72 specifically?
72 has many small divisors and closely matches the exact doubling maths for typical rates, which makes the mental arithmetic easy and accurate.
What the Rule of 72 tells you
The Rule of 72 is a fast mental shortcut for how long an investment takes to double at a fixed annual rate: simply divide 72 by the percentage rate. It works because of the maths of compound growth and is accurate enough for quick estimates without a calculator.
Examples
At 6% a year, money doubles in about 72 ÷ 6 = 12 years; at 8%, about 9 years; at 9%, about 8 years. The same logic runs in reverse: if you want your money to double in 10 years, you need roughly a 7.2% return.
Related rules
Use the Rule of 114 to estimate tripling time (114 ÷ rate) and the Rule of 144 to estimate quadrupling time (144 ÷ rate). For example, at 8% money triples in about 14 years and quadruples in about 18.
How accurate is it?
The Rule of 72 is an approximation; it is most accurate for rates between about 6% and 10%. At very low or very high rates the precise compound formula diverges a little, but for everyday estimates the rule is close enough and far quicker than working out logarithms.
Results are estimates for general guidance in South Africa and may not reflect the latest local rates, fees or rules. Check official sources before making decisions.