Need the ROI on your 401(k)? Use this calculator to find the effective annual rate of return on your 401(k) contributions, when factoring in the tax deduction, inflation, your employer’s contribution rate and other components. Why should you participate in the 401(k) Plan? Because, regardless of your retirement account's investment performance, the combination of tax savings and company matching contributions will generate a "real return" to you. This "real return" will vary depending on your rate of contribution, tax bracket, and assumed return on invested contributions.