I highly recommend this standard rule of thumb when you are married with no kids/deductions ... the OLD W-4 return had the option "married but withhold at the higher SINGLE rate" but on the new W-4 you both need to be smart enough to check the SINGLE/MFS box for both of you (yes you can legally even if you file jointly on the return) . Then after a couple of paychecks under the new withholding do a bit of a what if to see if that is going to be enough. If not then add a flat amount to line 4C of the W-4 to make up the difference if needed. Say you will be short $1000 of withholding and you are paid monthly and there are 4 months left to the year you would simply add $250 to line 4C on one of your W-4 forms or split that amount between both of your W-4 forms as desired.
The online tools and W-4 worksheets can be confusing when you change jobs instead of having additional jobs. So if you use those tools think of the old job and new job as one job for the year and not a second job you work at the same time as the first job.
