How accurate is the Rule of 72?

It's most accurate for rates between 6-10%; outside that range the true doubling time (based on compound interest math) diverges slightly.

Rule of 72 Calculator

The Rule of 72 is a quick mental-math shortcut for estimating how long an investment takes to double at a fixed annual rate of return.

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Years to Double ≈ 72 / Interest Rate

The Beginner's Guide to Financial Calculators: Loans, Interest, and Investment Math

How mortgage payments, compound interest, and the Rule of 72 are actually calculated, with real worked examples.

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