Saving $1 million for retirement may seem like a lofty goal, but it's more attainable than you might think.

If you have $100,000 stashed away in your retirement fund, you're already making fantastic progress. Growing it into $1 million just requires time and consistency.

Exactly how much you'll need to save each month depends on your timeline. Compound earnings will help your savings grow more over time, but it takes several decades to see substantial returns. The more time you have to save, the less you'll need to invest each month.

Here's exactly what it might take to turn $100,000 into $1 million or more.

1. Invest ,000 per month for 20 years

If you only have 20 years to invest, it's still possible to build a million-dollar portfolio -- but you will need to invest more per month.

Exactly how much you'll need to save will depend on what type of returns you're earning on your investments. Historically, the stock market itself has earned an average rate of return of around 10% per year. In other words, the market's annual highs and lows have averaged out to roughly 10% per year over decades.

To play it safe, let's assume your investments are earning a modest 8% average annual return. If you're starting with $100,000, investing $1,000 per month would result in around $1.015 million after 20 years.

2. Invest 0 per month for 25 years

If you have even a few more years to grow your savings, it becomes significantly easier to accumulate $1 million or more.

With just five more years, you'll only need to invest $400 per month if you're still earning an 8% average annual return. After 25 years, your initial $100,000 investment will amount to around $1.036 million.

3. Invest

3. Invest $0 per month for 30 years

per month for 30 years

Perhaps the easiest way to build a million-dollar nest egg, however, is to give your portfolio three decades to grow. With enough time, you won't need to invest any money each month to reach your goal.

If you have at least 30 years to build your savings, your initial $100,000 will grow into around $1.006 million with $0 in monthly contributions, assuming you're still earning an 8% average annual return.

In this scenario, compound earnings are doing all the heavy lifting for you -- and it's a testament to just how powerful time can be. If at all possible, then, it's wise to start investing as early in life as you can. You don't need to have a lot of money to invest, but when time is on your side, it will be exponentially easier to save $1 million or more.

Making the most of your money

There's no single right way to grow $100,000 into $1 million or more, as it will depend on your timeline and how much you can afford to invest each month.

That said, the earlier you can begin investing, the easier it will be to reach your goal. Regardless of how many years you have left to save for retirement, by getting started now, you can accumulate more than you might think over time.