Post-Tax Contribution

A retirement contribution made from income that has already been taxed, with qualifying withdrawals typically tax-free.

Example

At a 30% tax rate, 1,000 of salary becomes 700 in the account, and qualifying withdrawals are not taxed.

With equal tax rates now and later, it gives the same result as a pre-tax contribution; the difference is only which rate applies.

Pre-Tax Contribution — A retirement contribution made before income tax is applied, with tax paid instead when money is withdrawn.
Marginal Tax Rate — The rate of tax applied to your next unit of income.

The account is a wrapper; what you hold inside it is a separate decision — A retirement account is a legal container with tax rules and access limits. It is not an investment itself, and the choice of wrapper and the choice of holdings are two different decisions. (9 min)
Pre-tax or post-tax is really a bet on your tax rate now versus later — The two main account structures give the same result if your tax rate is the same when you contribute and when you withdraw. The choice turns on whether you expect your rate to be higher or lower in retirement. (12 min)