If the plan knows the amount of after-tax employee contributions to the plan, the plan should show the correct taxable amount in box 2a. If the Form 1099-R does not show the taxable amount in box 2a and has box 2b Taxable amount not determined marked, you would need to use the Simplified Method to calculate that taxable amount. When entering the Form 1099-R into TurboTax and indicating that periodic ("regular") payments were received, TurboTax will offer the opportunity to figure out how much of the distribution is taxable using the Simplified Method. In doing so, "Plan cost" is the amount of the employee's after-tax contributions to the plan. Once he necessary information is entered, TurboTax will use the calculated amount as the taxable amount and will carry the Simplified Method details forward when you transfer in this year's tax file to begin next year's tax return with TurboTax.
