How accurate is the Rule of 72?

It is very accurate for rates between 2% and 30%. At a 8% rate, for example, the exact answer is 9.006 years while the Rule of 72 gives 9 years. At very high or very low rates the error grows, so treat it as a quick estimate rather than a precise calculation.

Why 72 and not 70 or 69?

72 has many divisors (2, 3, 4, 6, 8, 9, 12) making mental arithmetic easy. 69.3 is the mathematically exact constant (ln 2 × 100) but 72 is more convenient and only slightly less accurate for common rates.

Does this account for taxes and fees?

No. The Rule of 72 uses a nominal gross rate. To get a realistic picture, subtract your effective tax rate and annual fees from the stated rate before entering it.

Can I use this for inflation?

Yes. Enter the inflation rate to see how long it takes for prices to double (i.e., for your purchasing power to be cut in half). For example, at 3% inflation, purchasing power halves in about 24 years.

What is the Rule of 114 and 144?

The Rule of 114 estimates how long to triple an investment (114 / rate), and the Rule of 144 estimates quadrupling (144 / rate). These are extensions of the same concept.

Rule of 72 Calculator

Name: Rule of 72 Calculator
Availability: InStock
Author: Nham Vu

Enter an annual interest rate to find how long your investment takes to double, or enter a target number of years to find the required rate.

Rule of 72 Calculator

Annual Interest Rate (%)

Enter any rate between 0.1% and 500%.

Enter a value and click Calculate.

Doubling Time at Common Rates

Annual Rate	Rule of 72 (years)	Exact (years)

Summary

Enter an annual interest rate to find how long your investment takes to double, or enter a target number of years to find the required rate.

How it works

Choose a calculation mode: find doubling time from a rate, or find required rate from years.
Enter the annual interest rate (%) or the target number of years.
The calculator applies the Rule of 72 formula: Years = 72 / Rate, or Rate = 72 / Years.
A comparison table shows doubling times for common rates so you can see the context.
Results update instantly as you type.

Use cases

Quickly estimate how long a savings account or CD takes to double.
Compare compounding rates side by side without a spreadsheet.
Understand the impact of inflation on purchasing power over time.
Evaluate whether an investment rate meets your financial goals.
Teach the concept of compound interest in a classroom setting.
Sanity-check compound interest projections from a financial advisor.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu