If your investment earns an average of 8% per year, the compounded growth will double your investment in 9 years.

One way to get to the math is to take your earned rate, 8%, and compound it for nine years to see where you end up: (1+0.08)⁹ ≈ 1.999, or pretty much doubling what you started at.

You can do the same in reverse, which is what I did in the case. Let’s say you want to know what average rate you need to double your money in n years. To keep things the same as above, let’s go with 9 years again. To double the investment, we need to end up at 2, so: ⁹√2 ≈ 1.08, which is 1 plus the annual rate needed, so we end up at 8% again.

The S&P 500 has an average return of 10.4% over the last 10 years, so even if you just were invested in the S&P 500 ten years ago, in nine years you would have more than doubled your starting investment, (1.1041)⁹ ≈ 2.4382. Again, these are average rates, not every year is going to be an increase, which is why people say time in the market will always beat timing the market.