When it comes to retirement savings, theory often differs from reality.

On average, Americans believe they need $1.7 million to retire, according to a recent survey from Charles Schwab, which looked at 1,000 participants in 401(k) plans nationwide.

In fact, "that's a pretty good number if you average out age and median salary across the U.S.," said Nathan Voris, a managing director at Schwab Retirement Plan Services.

However, "the bulk of folks do not get there," he said.

More than half of those polled are contributing 10% or less of their salary to their 401(k) — their largest source of retirement savings — with an average annual contribution of $8,788, Schwab found.

That's is a good start but may not suffice for most, Voris said, especially if you start saving for retirement later in life.

For example, if you start in your 20s, stashing 10% to 15% of your salary each year could be enough to retire comfortably, according to Schwab. But if you don't start until age 45 or older, you would need to set aside as much as 35% of your salary annually — a goal few workers achieve.

In addition to those basic guidelines, experts recommend using a retirement calculator to get a more accurate picture of your retirement number.