Tax Question

My wife made $45K of taxable income this year but only paid ~$2,880 in Federal Tax. From my figuring (we're filing jointly so it's not so cut & dry), she should've paid closer to $6K. We're in the 22% tax bracket.
Any tax gurus out there that can tell me:
a) should she have paid closer to $6K
b) WTH happened to only pay that small amount in taxes (I paid almost 4 times as much as her but didn't make almost 4 times as much as her)
Supposedly she's claiming 0 dependents on her W-4 but I have a feeling it's way higher than that & that's why this happened but I know very little about taxes.

***ETA***
I should mention when I did mine, before adding her, it showed we were getting $3,499 back (I add a little extra each paycheck). After adding hers it says we owe $2,880. By adding her, we stayed in the same tax bracket as it was w/just mine.

 

Lots of variables here and whether the employer is withholding correctly. First, find out what she actually claimed on the W4 the employer is using. The amount withheld varies based on pay and frequency, so without knowing all the info, it's hard to say if the correct amount was held out or not.

She definitely needs to adjust regardless of the reason. Without knowing all your specific deductions, she should probably be having around 12% withheld.

Sometimes claiming 0 still isn't enough. My wife and I both claim 0, pay extra each pay period, plus my wife has hers withheld at the single rate.

If you look at the tables here for her pay frequency, it will give you an idea of what the correct amount should be from the employer standpoint, not necessarily from yours.
https://www.irs.gov/publications/p15t
The new W4 has the step 2 box to check if married filing jointly and it withholds at a higher rate assuming you will both make about the same pay. My guess is she either claimed more than 0 or it is being held out based on the older W4 rate. Have her find out what she was claiming and then have her fill out a new W4 at the correct rate with the step 2 box checked. Then, look at what's actually being withheld and make sure it's enough. If not, have her fill out a new one and add whatever additional per paycheck she needs. Sounds like a lot of work, but it's easier than coming up with a bunch of cash come next April.

As an example, based on the above tables, if she made $45K/yr and was paid weekly, the rate based on the new W4 is $39/wk ($2028/yr) or $71/wk ($3692/yr) with step 2 checked, or using the new W4 single rate and step 2 it would be $124/wk ($6488). As you can see, that's quite a difference at the end of the year, and all of those may still be less than what's needed.

I am able to make all those changes online so it's easy to fine tune my withholding without bothering someone in payroll multiple times. I adjust mine every year based on how well I did the previous year. My goal is to come within 1% +/- of tax liability. This year I came in right at 2% of paying in what I owed.