Rule of 72 race: a 7th grade compound interest estimate game
What it is: a 7th grade math game where teams use the Rule of 72 to estimate how long money takes to double at different interest rates, then check with a year-by-year compound interest table. It builds number sense with percents and shows why time matters so much when money earns interest.
Time: about 35 minutes. Grade: 7th grade.
Materials: rate cards (2, 3, 4, 6, 8, 9 and 12 percent), calculators, a table template with the columns Year and Balance, and a starting amount of $100.
- Teach the rule: years to double is about 72 divided by the yearly interest rate. At 9 percent, about 8 years.
- Each team draws a rate card and estimates the doubling time in under 30 seconds.
- Teams check with the table: multiply the balance by 1 plus the rate each year until it passes $200.
- Compare the estimate with the table. The rule is an estimate, so it can be off by a little.
- Race round: teams answer quick questions, such as "At 4 percent, how long to double? How long to grow to 4 times as much?" (About 18 years; about 36 years.)
- Discuss: the rule works for debt too. Unpaid debt at 18 percent doubles in about 4 years.
- Close: this is a math tool for understanding interest, not advice about where to put money.
Differentiation: students who need support use only the 6, 8 and 12 percent cards. Students who are ready explore why 72 is a handy number (it has many factors). English learners can use the words double, rate and estimate.
Quick check: ask: at 3 percent, about how many years does it take $500 to double?
Save this activity to your library so it is ready when you need it.
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About this resource
- Shared by
- @UpOnlyLearning · Curriculum team
- Posted
- Oct 31, 2026
- Type
- Lesson plan
- Grade
- Grades 6–8
- Subject
- Math, Life skills & money
- Language
- English
- License
- Free for classroom, co-op and tutoring use. Please don't resell.