I'm confused with how many allowances I should be claiming on the new w4 for 2018. I am married, filing jointly, with one child, I work and my wife doesn't, and total income will be less than $101,401, the way I read it I am suppose to fill out allowances for corresponding letters is: A:1, B:1, D:1, E:4, F:1 - to give me a total of 8 allowances, that seems high?
Yeah, for many I can see how that can lead to someone claiming more allowances than they should. I wonder if it's "by design" so that you can be assessed an under withholding penalty if enough is not withheld.
Understand that there is no more self-exemption, or dependent exemption. All you get married filing joint is the $24,000 standard deduction, which is completely different from the old exemption.
Below, I'm referring to the worksheet on page 3 of the instructions which if you don't have them available to you with your worksheet, you can see/print them at https://www.irs.gov/pub/irs-pdf/fw4.pdf.
You are claiming the child tax credit (which has nothing to do with the now defunct dependent exemption) on line E. This is for any dependent that qualifies as "your child" (like stepson for example) that will be under the age of 17 on Dec 31 of 2018. (If they will be 18 on Dec 31, 2018, you don't get the child tax credit. So you would NOT enter a number in section E for a child that will be 17 or older on Dec 31 of 2018.)
Section F is for "other" dependents that do not qualify as "your child". Now in section F it tells you to enter a number for each "eligible" dependent. If you read the instructions on page 2 it clearly identifies this as a child that will be age 17 or older on Dec 31 of that tax year. That means if your child qualifies for the child tax credit for line E, then that same child does NOT meet the qualifications for line F.
Overall, my bet is that your "correct" number of allowances should be 4 or lower.