What Is the Rule of 72?

The Rule of 72 is a simple shortcut investors use to estimate how long it takes for an investment to double in value. You divide the number 72 by your expected annual rate of return, and the answer is the approximate number of years it'll take to double.

It's not a precise formula — it's a quick mental tool. But it's accurate enough to give you a powerful sense of how time and return rates work together, especially over the long time horizons involved in investing for a child.

The formula is intentionally simple:

• Years to double ≈ 72 ÷ annual return rate

So if you expect an average annual return of 8%, you'd divide 72 by 8 and get 9 — meaning the investment would roughly double every 9 years. At 6%, money would double about every 12 years. At 4%, about every 18 years.

You can flip the formula too. If you want money to double in a certain number of years, divide 72 by that number of years to find the return rate you'd need.

Here's the Rule of 72 in action at different average annual return rates:

• 4% return: doubles about every 18 years
• 6% return: doubles about every 12 years
• 7% return: doubles about every 10 years
• 8% return: doubles about every 9 years
• 10% return: doubles about every 7 years

Imagine a $1,000 birthday gift invested at an average 8% annual return. By the Rule of 72, that money would roughly double every 9 years — so $1,000 could become about $2,000 in 9 years, $4,000 in 18 years, and $8,000 in 27 years (illustrative, not guaranteed).

The reason the Rule of 72 is so eye-opening is that doublings stack up. Each doubling isn't an addition — it's a multiplication. After one doubling, you have 2x. After two, 4x. After three, 8x. After four, 16x.

Consider a $500 gift invested at birth at an average 8% return:

• At age 9: ~$1,000 (1st doubling)
• At age 18: ~$2,000 (2nd doubling)
• At age 27: ~$4,000 (3rd doubling)
• At age 36: ~$8,000 (4th doubling)

That's the magic of compounding seen through the Rule of 72: the same $500 quietly multiplying itself in the background while a child grows up.

The Rule of 72 makes the trade-off between time and return rate easy to see. A higher return rate shortens the time to double — but more time available also lets more doublings happen.

For adults investing for retirement, there's often only enough time for two or three doublings. For a child invested from birth, there can be time for three or four doublings before they even reach adulthood — and many more across their lifetime.

That's why a small, early gift can outperform a much larger gift made later. It has more doublings ahead of it. You can see this play out with the Newborn Investment Growth Calculator or the What Could $1,000 Become? calculator.

The Rule of 72 is a shortcut, not a guarantee. A few things worth knowing:

• It's an approximation. The math is most accurate for return rates between roughly 6% and 10%. Outside that range, the estimate drifts a bit.
• It assumes a steady return. Markets don't deliver the same return every year — some years are up, some are down. The Rule of 72 uses an average.
• It ignores fees and taxes. Investment fees and taxes on gains can slow down real-world doubling.
• It assumes no withdrawals. The money has to stay invested for the doublings to occur.

Used as a planning tool, though, the Rule of 72 is hard to beat for quickly understanding the power of long time horizons.

Children have the longest investment horizons of anyone. That makes the Rule of 72 especially powerful when planning for them. A gift invested at birth has roughly two decades for doublings to happen before adulthood — and potentially five or six decades before retirement.

The same principle applies to birthday money and holiday gifts. A small amount redirected from a toy into a long-term investment account can quietly double, then double again, while the child grows up. That's also why compound growth is the engine behind investing early — the Rule of 72 just makes it easy to picture.

A KinderShares registry gives family one place to contribute toward a child's future, so those long-term doublings can actually get started.

Examples on this page use illustrative rates of return for educational purposes only and are not guarantees. KinderShares does not provide tax, legal, or investment advice.