Early withdrawal from a 401(k) before age 59½ typically triggers a 10% penalty on the amount withdrawn, in addition to ordinary income taxes owed on that distribution. So if you withdraw $10,000 early, you'd owe $1,000 in penalty plus income taxes based on your tax bracket.
However, several exceptions exist where you can withdraw without the 10% penalty, including: substantially equal periodic payments (SEPP) calculated using IRS methods, disability, death (beneficiaries), medical expenses exceeding 7.5% of adjusted gross income, health insurance premiums while unemployed, qualified domestic relations orders (QDRO), and certain employer plan distributions after age 55 if you separated from service. Some plans also allow loans instead of withdrawals, which avoid penalties if repaid properly.
You'll still owe income taxes on early withdrawals even with an exception to the penalty—only Roth 401(k) contributions (not earnings) can be withdrawn tax and penalty-free. The specific rules depend on your plan's provisions, your age, and the reason for withdrawal. Consulting a tax professional can help you understand the exact consequences for your situation, especially if you qualify for an exception.