How long does it take to double your money with compound growth?

How long does it take for compound growth to double your starting amount? The answer depends on the return and how often it compounds. If the same annual return repeats with no deposits or withdrawals, the size of the starting amount does not change the doubling time.

The Rule of 72 gives a quick estimate. We will start with that shortcut, then calculate how $1,000 grows to $2,000 at an assumed annual return of 6%.

Estimate the time with the Rule of 72

Divide 72 by the annual return expressed as a percentage to estimate the number of years needed to double. The shortcut assumes a constant positive annual return with compounding.

Estimated years to double ≈ 72 ÷ annual return (%)

At 6%, 72 ÷ 6 = 12, so the estimate is about 12 years. At 3%, it is about 24 years; at 9%, about eight. This is an approximation, so it can differ from the actual compound calculation.

Example: doubling $1,000 at 6% annually

Suppose you start with $1,000, apply a 6% return at each year-end and reinvest all the gains. There are no additional deposits or withdrawals, and fees and taxes are excluded.

At the end of year one, you have $1,060. The next year’s 6% applies to that full balance, giving $1,123.60. Earning a return on earlier gains is what makes the growth compound.

Balance after n years = $1,000 × 1.06ⁿ

At the end of year 11, the balance is $1,898.30, still below $2,000. At the end of year 12, it is $2,012.20. In this example, the Rule of 72 estimate matches the first full year at which the target is reached.

Why does the equation give 11.90 years rather than 12?

Solving the compound growth equation for exactly twice the starting amount gives ln(2) ÷ ln(1.06) ≈ 11.90 years. That mathematical answer allows fractional years.

This example, however, credits the return once at each year-end. The balance is below the target after year 11 and above it after year 12, so the calculator reports 12 complete periods.

What changes at a different return?

Keep the starting amount and annual compounding unchanged, then vary only the return. Compare the estimate with the first year-end balance that passes twice the starting amount.

At 9%, the Rule of 72 suggests eight years. But $1,000 grows to about $1,992.56 after eight full years, just short of $2,000. It first passes that target at the end of year nine.

The shortcut is useful for a quick estimate. For the balance at a particular time or the first period that reaches a target, calculate the amount at each period using your assumed return.

Try your own starting amount and return

The button below opens the calculator with a starting amount of 1,000, a return of 6% per period and a target of 2,000. Change the starting amount and return to suit your scenario, and set the target to twice the starting amount.

Open the 6% example in the calculator →

You choose what a period means. In this example, the return is annual, so each period is one year. Treating a period as one month while entering 6 would apply 6% every month, not 6% per year.

With the same return and compounding frequency and no additional contributions, doubling $1,000 takes just as long as doubling $10,000. The rate and compounding frequency determine the time.

Using the result in a real situation

The returns in this article are assumptions. Changing returns, deposits or withdrawals change the outcome. Fees and taxes reduce what you keep, while inflation changes what the resulting amount can buy.

The calculator used in this example models a single starting amount. If you add money each month, use our compound interest calculator with monthly contributions to include those deposits and choose when they are made.

The result is a mathematical scenario based on your inputs. It does not promise an investment return or a date when your money will double.

Reference: Investor.gov’s explanation of compound interest