WiseCalcs

Rule of 72 calculator

Enter a positive annual growth or interest rate as a percentage. The calculator estimates the years to double, triple, and quadruple an amount with the rules of 72, 114, and 144.

Written by Kenneth Schrøder, Founder of WiseCalcs

Updated ·Sources verified

For a quick doubling estimate, divide 72 by the annual rate written as a percentage. The calculator also uses 114 and 144 with the same rate to estimate tripling and quadrupling.

%

Years to double

9 yr

Years to triple
14.3 yr
Years to quadruple
18 yr

Years to double is the main result. The figures below show estimates for tripling and quadrupling at the same rate. Values are shown with up to one decimal; a positive time below 0.1 year is shown as under 0.1 year.

How does it work?

These are quick estimates, not promises or exact investment results. They assume one fixed positive rate and do not include fees, taxes, or extra payments. You enter the rate yourself; the calculator does not look up a current rate.

Rules of 72, 114, and 144

t2≈72rt3≈114rt4≈144r\begin{aligned} t_2 &\approx \frac{72}{r} \\ t_3 &\approx \frac{114}{r} \\ t_4 &\approx \frac{144}{r} \end{aligned}
t₂
Approximate years for an amount to double.
t₃
Approximate years for an amount to triple.
t₄
Approximate years for an amount to quadruple.
r
The positive annual growth or interest rate you enter, in percent.

At 8%, 72 ÷ 8 estimates 9 years to double. 114 ÷ 8 estimates 14.25 years to triple, and 144 ÷ 8 estimates 18 years to quadruple.

Method & sources

This is a quick shortcut for a single amount growing at one fixed rate. It is an estimate, not a prediction of an investment result.

Sources

Where this method comes from — use these references to understand the formula, assumptions, and limits.

How we calculate

  • The rule of 72 is an estimate, not an exact formula. The exact method uses a logarithm, a calculation that works backwards from compound growth.
  • At very low or high rates, the estimate can be further from the exact calculation.
  • It assumes one fixed positive rate over the whole period, with interest earned on earlier interest.
  • You supply the rate yourself; the calculator does not look up market rates.
  • Fees, taxes, and additional contributions are not included.

Rounding

Years are shown with up to one decimal. A positive time below 0.1 year is shown as under 0.1 year instead of zero. The calculation uses full precision.

What this calculator does

With compound growth, interest is added to an amount and later earns interest too. The rule of 72 is a simple shortcut for estimating that growth. Enter the annual rate as a percentage: the calculator divides 72 by that number to estimate the years to double. It also divides 114 and 144 by the same rate to estimate tripling and quadrupling. Results are shown with up to one decimal; positive times below 0.1 year are shown as under 0.1 year.

How to use it

  1. Enter the annual growth or interest rate as a percentage.
  2. Read the approximate years to double below.
  3. Check the years to triple and quadruple for the same rate.

Formula and an 8% example

At 8%, 72 ÷ 8 = 9, so the estimate for doubling is 9 years. For tripling, 114 ÷ 8 = 14.25, which the calculator displays as 14.3 years. For quadrupling, 144 ÷ 8 = 18.

Why 72?

At 1%, the rule of 72 gives 72 years. With a fixed rate compounded once a year, the calculated doubling time is about 69.7 years. That gap shows why the rule is a guide, not a promise. The exact calculation uses a logarithm, a calculation that works backwards from compound growth. You do not need it here. You can enter a positive rate, but the shortcut assumes the rate stays fixed and does not include fees, tax, or later payments.

Common mistakes

  • Entering the rate as a decimal (0.08) instead of a percent (8).
  • Treating the result as exact. It is a quick approximation, not a precise figure.
  • Adding fees, taxes, or later contributions to the result. The calculator uses only the rate you enter.

When it's useful

Use it for a quick check of an interest rate, a growth assumption, or an inflation rate before you make a fuller calculation.

FAQ

How does the rule of 72 work?
Divide 72 by the annual rate, written as a percent. The result is the approximate number of years for the amount to double at that rate.
What are the rules of 114 and 144?
They are the same idea for other multiples. Divide 114 by the rate to estimate the years to triple, and 144 by the rate to estimate the years to quadruple.
How accurate is the rule of 72?
It is an estimate, not an exact figure. At very low or high rates, it can be further from the exact calculation. For a full projection, use a compound-interest calculator.
Does it work for inflation?
Yes, as a quick estimate. Divide 72 by the inflation rate to estimate how many years it takes for prices to double, or for purchasing power to halve.
Should I enter the rate as a percent or a decimal?
Enter it as a percent. For 8%, type 8, not 0.08. The rule divides 72 by that percent figure directly.
Can I share a calculation?
Yes. Select Share. If your device offers a share panel, it opens; otherwise the calculator copies a link that restores the same rate.

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