Information on the tax rate for a 401(k) withdrawal made during retirement can be found on the website of U.S. News and World Report under the site Money section. The tax rate depends upon the holder's tax bracket rate at the time of withdrawal from the account.
Taxes for money withdrawn from a 401(k) plan include both the principal contributions and gains on those contributions over the life of the plan, according to U.S. News and World Report. Since taxes for a Roth IRA are made at the time of the initial contribution, retirement withdrawals are not taxed. Wells Fargo provides a calculator for 401(k) contributors to estimate owed taxes for withdrawals during retirement.