The 'Rule of 72' is a simplified way to determine how long an investment will take to double, given a fixed annual rate of interest. By dividing 72 by the annual rate of return, investors can get a rough estimate of how many years it will take for the initial investment to duplicate itself.
For example, the rule of 72 states that $1 invested at 10% would take 7.2 years ((72/10) = 7.2) to turn into $2. In reality, a 10% investment will take 7.3 years to double ((1.10^7.3 = 2).
When dealing with low rates of return, the Rule of 72 is fairly accurate. This chart compares the numbers given by the rule of 72 and the actual number of years it takes an investment to double.
Rate of Return  Rule of 72  Actual # of Years  Difference (#) of Years 
2%  36.0  35  1.0 
3%  24.0  23.45  0.6 
5%  14.4  14.21  0.2 
7%  10.3  10.24  0.0 
9%  8.0  8.04  0.0 
12%  6.0  6.12  0.1 
25%  2.9  3.11  0.2 
50%  1.4  1.71  0.3 
72%  1.0  1.28  0.3 
100%  0.7  1  0.3 
Notice that, although it gives a quick rough estimate, the rule of 72 gets less precise as rates of return become higher. Therefore, when dealing with higher rates, it's best to calculate the precise number of years algebraically by means of the future value formula.
(To learn more, see Understanding the Time Value of Money.)

What is the difference between the rule of 70 and the rule of 72?
Find out more about the rule of 70 and the rule of 72, what the two rules measure and the main difference between them. Read Answer >> 
What can I use the Rule of 70 for?
Discover how the rule of 70 works, and learn about some of the different ways it can be applied to future forecasting and ... Read Answer >> 
How is the rule of 70 related to the growth rate of a variable?
Find out more about the rule of 70, what it is used for and how it is related to the growth rate of a variable. Read Answer >> 
What does the rule of 70 indicate about a country's future economic growth?
Find out more about the rule of 70, what it measures and what it indicates about a country's future economic growth rate. Read Answer >> 
How do I adjust the rule of 72 for higher accuracy?
Read about the Rule of 72, why it is only an approximation, and how the Rule of 69.3 can be substituted in for more accurate ... Read Answer >>

Investing
How To Double Your Money Every 6 Years
Investing according to the rule of 72 is a good starting point for achieving your saving goals. 
Investing
What is the Rule of 70?
The rule of 70 is an easy way to calculate how many years it will take for an investment to double in size. 
Financial Advisor
Ready for the Fiduciary Rule? You Should Be
Despite the opposition it faces, advisors should still plan to comply with the fiduciary rule. Here's why. 
Investing
Rule Of 72
Learn more about this quick approximation that can determine roughly the number of years it'll take your money to double. 
Personal Finance
Financial Rules of Thumb: Harmful or Helpful?
While financial rules of thumb can be helpful at times, they can also be dangerously wrong. 
Investing
Look Smart: Financial Calculations You Can Do In Your Head
Ditch your financial calculator and use these handy tips for estimating complex financial calculations on the fly. 
Investing
5 Top Ways to Double Your Investment
From risky maneuvers to slowandsteady strategies, we look at five methods to double your money. 
Financial Advisor
Indie BDs: Trump Should Drop the Fiduciary Rule
A majority of independent brokerdealers want Trump to repeal the fiduciary rule, a recent survey reveals. 
Financial Advisor
How Trump Could Repeal, Soften Fiduciary Rule
Donald Trump has threatened to repeal the new fiduciary rule. Here’s what he could do if he wants to kill the rule in its present form. 
Insights
DOL Fiduciary Rule: Everything You Need to Know
The Department of Labor (DOL) Fiduciary Rule expands the “investment advice fiduciary” definition under the Employee Retirement Income Security Act of 1974 (ERISA) and began to go into effect ...

Rule Of 72
A shortcut to estimate the number of years required to double ... 
Rule Of 70
A way to estimate the number of years it takes for a certain ... 
Yearly Rate Of Return Method
More commonly referred to as annual percentage rate. It is the ... 
Uptick Rule
A former rule established by the SEC that requires that every ... 
Filter Rule
A trading strategy where technical analysts set rules for when ... 
Five Percent Rule
A regulation that requires a broker to use fair practices and ...