If you invest for long enough it's possible that you'll end up with twice as much money as you started with. Achieving this feat is incredible, and accomplishing it without dolling out additional money is even better!

It can be done, though, and it's simpler than you probably think. Here's how long it would take, depending on how your accounts are allocated between stocks and bonds.

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## The rule of 72

The rule of 72 is a popular way of estimating how long it will take for your investment to double, and it can be calculated fairly easily using an average rate of return from an asset allocation model. You simply take the number 72 and divide by it the rate of return you think you'll receive. So if you think you'll earn 10% over the life of your investments, then it would take approximately 72/10 = 7.2 years for \$100,000 to turn into \$200,000.

## A conservative portfolio

A portfolio that consists of 100% bonds would be considered conservative, and this allocation would've earned an average rate of return of 6.1% between the years of 1926 and 2020. Using the rule of 72, it would take 11.8 years to double your investment at this rate.

## A moderately conservative portfolio

If you added 30% stocks to your accounts over this same time period, you would've seen your average rate of return increase to 7.7%. According to the rule of 72, you would have twice as much money in 9.4 years.

## A balanced portfolio

If you owned a balanced portfolio, you would hold 50% stocks and 50% bonds, and your rate of return would've increased to 8.2% on average. At this rate return, you could double what you started with after about 8.8 years.

## A growth portfolio

With a growth portfolio, you would've owned about 70% stocks and 30% bonds, and your rate of return over this time period would have been 9.4%. At this rate of return, it would take 7.7 years for your money to double.

## An aggressive growth portfolio

If you owned all stocks, your accounts would be considered aggressive and would've grown by 10.3% on average every year. And according to the rule of 72, your accounts would've doubled in value after 7 years.

Asset Allocation Rate of Return Time to Double
Conservative 6.1% 12 years
Moderately Conservative 7.7% 9.4 years
Balanced 8.2% 8.8 years
Growth 9.4% 7.7 years
Aggressive Growth 10.3% 7 years

Calculations by author.

## Risk tolerances

Picking a rate of return with the lowest number of doubling years may be tempting, but doing this may make meeting your goals even harder. These projections of when your money could double are based on consistency, and missing just a few of the best stock market days could bring your rate of return down by a lot.

For example, if you invested in the S&P 500 from Jan 2, 2000, through Dec. 31, 2020, you'd have earned an average rate of return of 7.5%, and your money would've doubled every 9.6 years. But missing out on the 10 best days over this 20 year period would've brought your return down to 3.35% -- and with this rate of return, it would take 21 years before your investments doubled!

It's also possible that as you get older your appetite for risk will decrease. If this is the case, you may find that you have an aggressive portfolio in your early working years, a growth portfolio in your middle years, and more of a conservative portfolio in your latter years. The time it takes for your accounts to double uses an average of these different asset allocation models instead of just one.

## Past performance doesn't guarantee future performance

Over the last 94 years, you would've received these rates of return -- but over shorter periods of time, the rates of return that you experience could vary quite a bit. If you invested \$10,000 into large-cap stocks on Jan. 2, 2000, you would've gotten an average rate of return of 1.59% over the next seven years and an ending account balance of \$11,300. This is mostly because the decade started off with 3 negative years of returns due to the dot-com bubble bursting. If, on the other hand, you started investing in January 2010, you would've just missed the Great Recession and ended up with an average rate of return of 13.89% and about \$28,300 seven years later -- more than double your initial investment.

This shows that this rule isn't an exact science, and is subject to the whims of different market cycles. That's why time in the market is so important. The longer your money can stay invested, the better your chances of seeing your investments grow twofold.

Doubling your money may seem impossible or extremely difficult, but it's an attainable goal that you can reach. And getting there doesn't require that you take on uncomfortable amounts of risk or volatility. Just that you allow your accounts enough time for growth.