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Rule of 72 Calculator | How Long to Double Your Money

Updated

Rule of 72 Calculator

The Rule of 72 is a simple way to estimate how long it takes for your money to double at a given interest rate. This page has the reference table, worked examples, the inflation version, the reverse formula for the rate you need, and how accurate the shortcut is. It is one of the calculators on our investing basics hub; for exact projections with contributions, use the compound interest calculator or the investment calculator.

Formula: Years to Double = 72 ÷ Interest Rate

Quick Reference Table

Interest RateYears to Double
2%36 years
3%24 years
4%18 years
5%14.4 years
6%12 years
7%10.3 years
8%9 years
9%8 years
10%7.2 years
12%6 years
15%4.8 years

Examples

Example 1: Stock Market Returns

Suppose a stock portfolio returns 10% a year (long-run market averages are covered in average stock market returns).

  • 72 ÷ 10 = 7.2 years to double your investment

$10,000 invested today becomes, by the Rule of 72:

  • $20,000 in about 7.2 years
  • $40,000 in about 14.4 years
  • $80,000 in about 21.6 years
  • $160,000 in about 28.8 years

With exact annual compounding at 10%, the four milestones arrive at about 7.3, 14.5, 21.8 and 29.1 years, so the shortcut is close.

Example 2: Savings Account

Suppose a savings account pays 3% interest (current rates are tracked in best HISA rates).

  • 72 ÷ 3 = 24 years to double

Example 3: GIC Investment

Suppose a 5-year GIC pays 4.5% interest.

  • 72 ÷ 4.5 = 16 years to double

The Rule of 72 for Inflation

You can also use the Rule of 72 to see how inflation erodes purchasing power:

Inflation RateYears to Lose Half Value
2%36 years
3%24 years
4%18 years
5%14.4 years
7%10.3 years

At 3% inflation, $100 today will buy only about $50 worth of goods in 24 years.

Finding the Required Rate

You can flip the formula to find what rate you need:

Formula: Rate Needed = 72 ÷ Years to Double

GoalRate Needed
Double in 5 years14.4%
Double in 7 years10.3%
Double in 10 years7.2%
Double in 15 years4.8%
Double in 20 years3.6%

Why 72?

The number 72 is convenient because it has many divisors (1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72), making mental math easy. The exact number depends on the rate. With continuous compounding it is 69.3 (the natural log of 2, times 100). With annual compounding it rises with the rate: about 70 at 2%, about 72 near 8%, and about 76 at 20%. That is why 72 works well for the rates most savers and investors see, and why it is easier to divide than 69.3.

Limitations

The Rule of 72 assumes:

  • Compound interest (interest on interest)
  • Constant rate of return
  • No additional contributions or withdrawals

For precise calculations including regular contributions, use our compound interest calculator.

Using the Rule of 72 in a Canadian Context

TFSA Growth Estimates

Because TFSA growth is tax-free, the Rule of 72 applies directly to your after-tax returns: there is no drag from annual tax on gains or interest.

At 8% annual return inside a TFSA:

  • 72 ÷ 8 = 9 years to double
  • A $30,000 TFSA doubles to $60,000 in 9 years with no tax payable on withdrawal

RRSP Growth Estimates

RRSP investments also grow tax-deferred, so the Rule of 72 works the same way inside the account. The tax hit comes on withdrawal, but the compounding is uninterrupted in the meantime, a major advantage over a taxable account.

GIC Rates vs. Equities

GIC rates vary by institution and term and change often. As one example, EQ Bank posted 3.70% on a 1-year and 4.30% on a 5-year registered GIC as of October 5, 2026; at the 5-year rate, the Rule of 72 gives about 16.7 years to double. Confirm current rates with the issuer or in the best GIC rates guide. Using the Rule of 72 at a range of example rates:

GIC Rate (example)Years to Double
3.5%~20.6 years
4.0%18 years
4.5%~16 years
4.75%~15.2 years

By contrast, the S&P 500 compounded about 10% a year from 1928 to 2025 in US dollars with dividends reinvested, which the Rule of 72 turns into a doubling about every 7.2 years, with large declines along the way. Canadian index returns are covered in average stock market returns.

Sources

The figures and rules on this page come from these sources, last checked against them between September 14, 2026 and September 15, 2026. How we check facts.