With a traditional 401k, which is pre-tax money, when you increase the % that you contribute you can increase the number of withholdings on your W-4 to keep your take-home pay the same or close to what it is now. Keep in mind that the amount that you contribute comes right off of the top of your gross income for tax purposes so you don't need to have as much withheld throughout the year. With a Roth 401k, which is after-tax money, there is no tax benefit (now) so you would not change your W-4. The tax benefit comes in when you start withdrawing from the Roth as withdrawals are tax-free (for now---don't be surprised if the govt doesn't change this)