How do i calculate the basis for my retirement distribution for PA

"Basis" in a retirement plan is the amount of after-tax dollars that the taxpayer has contributed to the plan in after-tax dollars.

This is also called "cost" or "contribution".

Since contributions to a 401(k) plan in Pennsylvania are not excluded from income, by definition, all your contributions to the 401(k) plan are after-tax and part of your basis.

See the Pennsylvania website and another PA webpage here.

Basis for PA retirement purposes is the sum of all your contributions (do not include employer contributions or earnings) less the sum of all previous distributions from the retirement plan.
Most often needed for PA purposes when you take an early distribution (box 7 coded 1 or2) from a retirement plan.