Estimate how long money takes to double
Estimate how many years it takes an investment to double at a given rate (or the rate needed to double it in a given time), using the Rule of 72 alongside the exact math.
The Rule of 72 is a quick mental-math approximation (72 divided by the rate, or by the years). The exact figure above uses the real compounding formula, and the two drift further apart away from typical mid-single-digit to low-double-digit rates.
Years to double an investment, or the rate needed to double it in a given time.
A growth rate, or a number of years, depending on what you picked.
The quick Rule of 72 approximation, alongside the exact figure from the real compounding formula.
Frequently asked
What is the Rule of 72?
A mental-math shortcut for estimating how long it takes an investment to double at a given annual rate: divide 72 by the rate. It also works in reverse, dividing 72 by the years to estimate the rate needed.
How accurate is it?
Reasonably close for typical rates, roughly 6 to 10%. It drifts further from the exact answer at very low or very high rates, which is why the exact figure (from the real compounding formula) is shown alongside it.
Why not just always use the exact figure?
The Rule of 72 is useful specifically because it's fast to do in your head without a calculator. This tool gives you both: the quick estimate for intuition, and the exact number for precision.
Is this financial or tax advice?
No. This is a general estimate for planning purposes only, not financial, investment, or tax advice. Rates and figures can change. Confirm current numbers with a qualified professional before making decisions.