How do I claim zero withholdings on the new W4. My wife and I earn enough that if we are not careful we will get dinged for underpayment.

[Critter-3]

If you BOTH check the SINGLE option (YES single) and leave the rest of the lines BLANK then you will have the most possible withheld.   If after a couple of paychecks you see that is still not going to be enough then one or both of you can add an amount per paycheck (line 4C)   to be withheld IN ADDITION to the reg withholding. 

[Opus 17]

The new form doesn't work like the old form.  It has a total of 4 pages including instructions and worksheets, and you will need to file a new W-4 at all your jobs.  If you want your withholding to be correct without giving the IRS an interest-free loan for the year, read and follow all the instructions and worksheets, using either the 2-earner worksheet or checking the box at 2c.

The IRS has a web site where you can check your withholding, but it's down for "maintenance" (probably to reprogram for 2022).  After getting a couple new paychecks under the new W-4, try using the IRS site to double-check.

https://www.irs.gov/individuals/tax-withholding-estimator

[SteamTrain]

Zero allowances, not zero withholding.

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The new W-4 for the Feds doesn't have allowances anymore  (State equivalent forms might, but that's a different issue).

Go ahead and fill out the new W-4 form exactly as it asks for in it's instructions:

Forms & Instructions | Internal Revenue Service (irs.gov)

BUT

...... on line 4c put in an extra amount that you want withheld for every paycheck...to eventually cover what you expect to owe next year.  Yeah, it could be a wild guess, but you may get an idea of what is needed after you've fully completed this year's tax return, and see the amount you owe (if you owe)