Also called: IRA basis, Form 8606
If any of your traditional IRA money came from non-deductible contributions, every withdrawal or conversion is treated as part taxable and part tax-free in proportion to your total IRA balances.
All of your traditional, SEP and SIMPLE IRAs are treated as one account, valued at December 31 of the year of the distribution. You cannot convert only the after-tax dollars. Basis is tracked on IRS Form 8606.
Balances in employer plans such as a 401(k) are not included in the calculation.