It's easy. See here:

1) Before tax 401k contribution: All withdraw from your account is will be your income for that year.2

2) after tax 401k contribution: the original contribution will not be taxed. The earning/return from your contribution will be taxed as part of your income.

Your broker know how much earning/original contibution from your accout.

所有跟帖： 

• does not sound right for roth 401k. Withdraw from roth 401k will -ye2000- ♂ (0 bytes) () 01/24/2022 postreply 16:26:34

• 这个解释是错的。Roth 401K 提现时免税。 -faststep- ♂ (0 bytes) () 01/24/2022 postreply 16:48:18

• I didn't mention roth 401k. nothing wrong! -孔方兄94- ♂ (0 bytes) () 01/24/2022 postreply 16:54:12

• 即使不是ROTH IRA，你的说法也完全错了。WITHDRAW多少能算为成本由所有IRA总值决定，而非你想象的那样简单。 -DSc- ♂ (0 bytes) () 01/24/2022 postreply 17:21:30

• 你说法不对。如不是ROTH，税后的IRA要和其它所有IRA/401K户头一起平均算，最后 拿出的钱能算成成本而免税的非常少。 -DSc- ♂ (396 bytes) () 01/24/2022 postreply 17:19:26