Building a million-dollar retirement fund isn't easy, but it's also not impossible. You don't need to be wealthy or a stock market expert to save $1 million or more, but you will need the right strategy.

Your approach will depend primarily on how many years you have until retirement and how much you can afford to invest each month. Time is your most valuable resource, so the longer you have to save, the easier it will be to reach your goal.

If you already have $100,000 saved for retirement, here's what it will take to turn it into $1 million or more.

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1. Invest $375 per month for 25 years

Thanks to compound earnings, your savings will accumulate exponentially faster the longer they have to grow. The shorter your time frame, then, the more you'll need to invest per month to reach your goal.

Say, for example, you're investing in a retirement account such as a 401(k) or IRA, and your investments are earning a modest 8% average annual return (which is just below the market's historic average).

If you already have $100,000 saved, investing $375 per month would amount to roughly $1.014 million after 25 years.

2. Invest $50 per month for 30 years

If you have just a few more years to save, it's far easier to accumulate a significant amount of money. Even just five extra years can cut your monthly saving goal down by hundreds of dollars, making it more attainable.

For example, say you're still earning an 8% average annual return, but rather than giving your money only 25 years to grow, you invest for 30 years. In this scenario, you'd only need to invest $50 per month to go from $100,000 to around $1.075 million.

3. Invest $0 per month for 32 years

The simplest option, however, is to let your money sit without making any additional contributions at all. You'd still earn returns on your initial $100,000 balance, and that alone can take you to $1 million.

Again, the more time you have, the easier this will be. If you're earning an 8% average annual return, your initial $100,000 can grow into around $1.174 million within 32 years -- even if you don't invest a dime between now and then.

What if you're falling behind?

Compound earnings are incredibly powerful, and the best way to take advantage of them is to start investing as early as possible.

If you're off to a late start, though, that's OK. Whether you're already nearing retirement age or feel you can't invest enough to make a difference, it's still better to start now rather than put it off. Even a small amount can add up over time, and that's always better than nothing.

For example, say you're still starting with $100,000, but you only have 10 years to save. Even if you only invest $10 per month at an 8% average annual return, you'd still end up with around $218,000 after a decade. 

Saving for retirement is tough, but small steps can make a big difference over time. By getting started early and saving as much as you can afford each month (regardless of how much or little that may be), you can earn more than you might think.