Investing · Video Lesson
The Rule of 72
The Rule of 72 is a quick mental shortcut for estimating how long it takes money to double at a given interest rate: divide 72 by the rate. This lesson teaches the rule, connects it to compound interest, and shows how higher rates — and starting early — dramatically speed up growth.
For Teachers
Lesson at a glance
- Topic
- Investing & Money Math
- Grade Level
- Grades 9–12 + adult
- Resource Type
- Video Lesson + Worksheet
- Estimated Time
- 45–60 minutes
- Format
- Direct instruction + calculation practice
- Materials
- Video, worksheet, calculators, whiteboard
What Students Learn
Learning objectives
- State the Rule of 72 and use it to estimate doubling time
- Explain how the rule connects to compound interest
- Calculate doubling time for several interest rates
- Describe how a higher rate shortens doubling time
- Explain why starting to invest early maximizes growth
Video Lesson
Watch: The Rule of 72
Materials
What you’ll need
- Internet access for the video
- Printed copies of the worksheet quiz (one per student)
- Calculators
- Whiteboard or projector for worked examples
Key Terms
Vocabulary
- Rule of 72
- Divide 72 by the annual interest rate to estimate the years for money to double.
- Compound interest
- Earning returns on both your money and prior gains.
- Doubling time
- How long it takes an investment to grow to twice its value.
- Rate of return
- The annual percentage an investment earns.
- Principal
- The starting amount invested.
For Teachers
Lesson plan
Estimated time: one 45–60 minute class period.
Lesson sequence
- Introduction (10 min). Ask: if you earned 6% a year, how long until your money doubles? Introduce the Rule of 72 as a fast estimate.
- Watch the video (15 min). Play the lesson video. Ask students to note the formula and why 72 is used.
- Discussion (10 min). Connect the rule to compound interest, why a higher rate shortens doubling time, and why starting early matters.
- Activity — calculate (15 min). Students apply 72 ÷ rate for several rates (e.g., 2%, 4%, 8%, 12%) and compare the doubling times.
- Quiz (8 min). Students complete the printable quiz; the answer key is included for teacher use.
Assessment
Assess the calculations and the printable quiz.
This lesson is for educational purposes only and is not investment advice.
Discussion
Discussion questions
- What does the Rule of 72 estimate, and how do you use it?
- How does the Rule of 72 relate to compound interest?
- How does a higher interest rate change the doubling time?
- Why does starting to invest early make such a big difference?
- Where might the Rule of 72 be only a rough estimate?
Printable Quiz
The Rule of 72 — Quiz & Answer Key
Multiple-choice quiz based on the video, with an answer key for teacher use.
Unlock the full Money Instructor library
Members get unlimited access to worksheets, lesson plans, and teacher resources across every financial literacy topic — budgeting, taxes, credit, banking, and more.
More Investing resources
Browse all Investing resources →
What is a 401(k)? Lesson
Students learn what a 401(k) is, how employer matching and tax advantages work, and why starting to contribute early maximizes compound growth.
Video Lesson · Grades 9–12What is a Bond? Lesson
Students learn what a bond is, why entities issue them, the main types, how investors earn from them, and how bonds add stability to a portfolio.
Video Lesson · Grades 7–12What is a Mutual Fund? Lesson
Students learn what a mutual fund is, how diversification lowers risk, the role of the fund manager, how fees affect returns, and the main fund types.
Video Lesson · Grades 7–12Printables & worksheet tools