Just a quick question my HR department couldn't answer...

I want max out my Roth 401k so I want to contribute $18,000 total in 2017.
My Salary is $75,000
Tax bracket is 25%
Roth 401k contributions are calculated with after tax dollars correct?

So $75,000 * 0.75 = $54,000 dollars left after taxes.

$54,000 * X% = $18,000

So 33% is the magic number?

Edit: I that $18,000 limit on pre-tax or post tax dollars?

They can't do it by a dollar amount?

A couple of thoughts.
Most benefit plans take the percentage of your gross pay to calculate retirement investments even though you won't get the tax deduction for your investment. Check that first with them.
If they really calculate based on net, remember that 25% is your marginal tax bracket, but not all your income is taxed at that rate. They should also be able to do a flat amount.

Kudos for living frugally enough to max it out.

The other (better IMO) way to look at it is how often do you get paid?  If it's twice a month you would have to contribute $692.31/check to reach $18,000.  If you aren't contributing yet, you've lost January and maybe one check out of February.  If so, you'll have to contribute $782/check.

Here's what I recommend:  Plans typically have a limit to how much (%) you can contribute/check.  If your plan allows you to contribute that much (33% as you state) or more, contribute as much as you can afford for a paycheck or two and look at the numbers to see what they actually took out and how much you would have to contribute/check to max it out by the end of the year.

I find it's better to contribute a little heavy up front early in the year and then taper off slightly at the end of the year.  First, as stated above, it gives you a chance to get ahead, look at what you have to do to max and keeps you from getting caught short (not being able to contribute enough to max) at the end of the year.  Second, time value of money:  generally speaking, the longer your money is invested, the greater your return, at least in the long run and since you can't withdraw your money (without penalty) until you're at least 59.5, I'm going to assume that's long term for you.  Finally, you may be able to reduce your contributions a bit near the end of the year to put a little more money in your check...for things like Christmas.

The other question is, does your company match?  If so you want to tweak your per check contributions to match as closely as possible exactly what it takes to max out your contributions with your last paycheck of the year, that will maximize your company match as well.

Are you married or single?

If married, using the Roth option makes more sense.  If single then it seems you would save more by first maxing a pre-tax 401k then putting the extra in a Roth IRA.

It will cost you $22,500 after taxes to put $18,000 in a Roth 401k (as a single guy).  That same $22,500 could put $18,000 in a pre-tax 401k and an additional $3375 in a Roth IRA.

If your single and plan to get married sometime before retirement, the tax advantages of maxing your pre-tax 401k are even higher.

Isn't the maximum Roth contribution limit for 2017 $5500 is you're under 50, and $6500 is you're over 50?

That's for a Roth IRA.  The Roth 401k is either the same as the traditional 401k limit or very close.  Wiki states $17,500 but I'm thinking it may just not be updated to the current 401k limits.

That's how my employer does it. It's based on a percentage of gross pay. It's not affected by state and local taxes, healt/dental/life insurance, SS and Medicare, etc. I would ask your HR if this is how they do it. In that case, at $75k gross, you would need 24%.

Also, depending on your fund options through your 401k, you may want to consider an IRA. My 401k fund options suck, so I started by contributing just enough to my 401k for the employer match. Then I maxed out an IRA. Now my raise from my 2016 year-end review is going back to my 401k even though I don't much care for the options. Since I already max my IRA, my 401k is the only choice.

Congrats on being able to max out the 401k on that salary. That's a large percentage of your pay. I'm only at 18.6% of my gross pay into retirement savings at this point.

2016 & 2017 401(k) (Traditional & Roth) are both $18k with $6k catch-up.