Calculate compound growth in your head in 3 seconds. Works for savings, stocks, inflation, and debt. No calculator needed.
⏱ 5 min read🧠 5 mental math quizzes🧮 Calculator verification
What is the Rule of 72?
Your friend says: "My stock portfolio is up 12% annually." In your head: 72 ÷ 12 = 6 years. "So it'll double in 6 years!" That's the Rule of 72.
72 ÷ Return Rate (%) = Years to Double
Mental calculation: How long until $1M becomes $2M at a given return rate?
Real-World Quick Reference Table
Annual Return
Years to Double
Common Example
2%
36 years
Savings Account
4%
18 years
High-Yield CD
6%
12 years
Bonds/Dividend Stocks
8%
9 years
Balanced Portfolio
10%
7.2 years
S&P 500 Average
12%
6 years
Growth Funds
18%
4 years
High-Risk Investments
Real-Life Applications
Real Use5 Ways Rule of 72 Helps Daily Decisions
1. Savings Account Comparison
"This CD pays 3.6% annually." 72 ÷ 3.6 = 20 years. "So my $100K becomes $200K in 20 years? That's way too slow. I need better returns."
2. Stock Market Expectations
"S&P 500 historically averages 10%." 72 ÷ 10 = 7.2 years. "$1M becomes $2M in about 7 years of average returns."
3. Inflation's Silent Theft
"Inflation is 3%." 72 ÷ 3 = 24 years. "In 24 years, a coffee that costs $5 today will cost $10. My money's purchasing power halves!"
4. Loan Interest's Danger
"Credit card debt at 18%." 72 ÷ 18 = 4 years. "If I don't pay it off, my $10K debt becomes $20K in 4 years!"
5. Reverse Calculation: Required Return
"I want to double my $50K in 5 years." 72 ÷ 5 = 14.4% annual return needed. "That's aggressive. I need growth stocks or high-risk investments."
💡 The Rule of 72 helps you quickly assess whether opportunities are worthwhile before pulling out a calculator.
Accuracy Check: How Reliable is Rule of 72?
Return Rate
Rule of 72
Actual Years
Error
2%
36.0
35.0
+1.0 year
4%
18.0
17.7
+0.3 years
6%
12.0
11.9
+0.1 years
8%
9.0
9.0
0.0 years
10%
7.2
7.3
-0.1 years
12%
6.0
6.1
-0.1 years
Accuracy Notes
The Rule of 72 is extremely accurate in the 6-10% range. For very low rates (1-4%), use 69.3 instead. For very high rates (15%+), use 78. But for quick mental math, 72 works great.
Quiz: Test Your Mental Math
⚡ Quiz 1
Your investment grows at 9% annually. How many years to double?
8 years (72 ÷ 9 = 8)
⚡ Quiz 2
You want to quadruple $1M at 6% return. How many years?
24 years (4x = 2x twice. Double once: 12 years. Double again: 12 years. Total: 24 years)
⚡ Quiz 3
You want to double your money in 6 years. What return rate do you need?
Inflation is running 4% annually. When will that $5 coffee become $10?
18 years (72 ÷ 4 = 18). Inflation is a hidden tax that silently erodes purchasing power.
⚡ Quiz 5
Credit card revolving interest: 18% annual rate. Unrepaid $5K debt becomes how much in 4 years?
$10K (72 ÷ 18 = 4 years to double). High-interest debt doubles in 4 years—a classic case where compound interest works AGAINST you.
Advanced Trick: 2x, 4x, 8x Multiples
One "72 cycle" = double. Two cycles = 4x. Three cycles = 8x.
Example: 8% return, 9 years to double, 18 years to 4x, 27 years to 8x.
$1M → $2M in 9 years → $4M in 18 years → $8M in 27 years. This is why long-term investing creates real wealth.