Fees are one of the most important factors of successful retirement investing — they determine how much ends up in your pocket after mutual funds and 401(k) plan providers take their cut. But most people are oblivious. If they take note of costs at all, fees of one, two or even three percent may not sound all that high. 

But things look different when you run the numbers over time as costs compound. And fees usually are much higher in plans sponsored by small businesses. Often, they don’t offer low-cost passive index fund choices, and expense ratios are high. 

My latest “Retiring” column for The New York Times offers the example of a hypothetical young saver to illustrate the career-long effect of plans with a variety of fee levels. The results are staggering. 

We considered a 28-year-old worker with a starting salary of $75,000 who saves diligently in her 401(k) account throughout her career. She contributes 6 percent of her salary annually and receives a 3 percent matching contribution from her employer. The scenario shows the effect of what she will have at three possible retirement ages. At 65, her portfolio is nearly 66 percent smaller in a high-cost plan compared with the lowest.

If you’re in a mediocre or bad 401(k) plan, what should you do? 

If your employer’s plan offers an annual matching contribution, save enough to capture it — doing otherwise leaves money on the table. Also, pay careful attention to your investment choices, and look for the least expensive options — if possible, a low-cost index fund that tracks the entire stock market. 

After you’ve captured the match, consider low-cost options outside your 401(k) for additional saving. Learn more in my “Retiring” column for The New York Times.