The average balance for investors in Fidelity 401(k) and IRA plans combined is $212,600, compared with $71,700 for someone with only a Fidelity IRA, the investment group said Wednesday. For workers with a just a Fidelity 401(k), the balance is even lower: $69,100.

"This unique and integrated analysis covering more than 18 million accounts clearly underscores the importance of using both an IRA and 401(k) to help maximize long-term, tax-deferred growth potential, and as essential complements to other retirement income sources such as Social Security, a pension, annuities and brokerage accounts," Kathleen Murphy, president of personal investing for Fidelity Investments, said in a statement.

Investors with both retirement plans make much larger annual contributions than investors who have just one type of plan, the study found. The average annual contribution for investors with both an IRA and a 401(k) is $10,300, but the average annual contribution for someone with only an IRA is just over $4,000. Investors with 401(k)s contribute slightly more, $5,750, every year.