Should I 0 or 1 on a Form W4 for Tax Withholding Allowance being a dependent?

If you put "0" then more will be withheld from your pay for taxes than if you put "1"--so that is correct. The more "allowances" you claim on your W-4 the more you get in your take-home pay.  Just do not have so little withheld that you owe at tax time.

https://turbotax.intuit.com/tax-tools/calculators/w4/

 But are you sure you will be a dependent on your parents' return?  You are 21?  Are you still a full-time student?  If you are a full-time student then your parents can still claim you as a dependent.

Hello,

Thanks for your answer. I just graduated from college so I am not longer a full-time student. In addition, as 2017 tax, I was claimed as dependent and the W4 form I need to fill is from 2017.

What do you mean you are just now filling out a 2017 W-4?  Why?  If you are starting a job in 2018 you should be filling out a 2018 W-4.  
It should look like this:  <a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-pdf/fw4.pdf">https://www.irs.gov/pub/irs-pdf/fw4.pdf</a>
If you were a full-time student for the first 5 months of 2018 then your parents will still be able to claim you as a dependent for 2018.

For some reasons (I asked them why (waiting for response)) they are still using the 2017 version. However, I was a full-time student for the first 5 months of 2018.

So when you file your own return for 2018, you will say on it that you can be claimed as a dependent on someone else's tax return.

So, I should state 0 in this case

I am 21 years old. On my W-4 I am currently as Single with 0 allowances.

I am currently living with my significant other, who is not working. Could I put myself as Head of Household with 0 allowances on my 2020 W-4?

Any advice? 

@aruiz37

My advice would be to keep the Single with zero allowances on your W-4.  

Unless you have a 'qualifying person' on your return, you would not be able to use the Head of Household filing status when the time comes to file your 2020 tax return.  As a result, using that filing status on your W-4 may lead to not having enough taxes withheld.

Take a look at the following TurboTax article for more information about the Head of Household filing status:  What is a "qualifying person" for Head of Household?

@stevewnbrg Do you have a scenario you would like to elaborate on for assistance? The IRS has updated the W-4 form for 2020. Here are some frequently asked questions as resolved by the IRS to assist in completing the form.

W-4 FAQs

Is the income tax separated from the earn child income or is it all together

You can claim either 0 or 1 on your W-4. It won't create problems with the IRS, it will just determine how much you'll get back on your tax return next year. If you claim 0, you will get less back on paychecks and more back on your tax refund. If you claim 1, you will get more back on your paychecks and less back on your tax refund when you file next year. However, you will want to check and make sure that, if you do claim 1 on your W-4, you are still having enough taken out that you will not end up owing anything in taxes when you file next year. 

*** All information was obtained from personal experience. No information was obtained from an outside source or anything/anyone in connection with the IRS or Tax Forms/Companies***

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As part of the Tax Cuts and Jobs Act, the IRS and Treasury adjusted the tax withholding tables, which affects the amount of income taxes withheld from your pay.  For 2020, W4's have been completely redesigned.  Exemptions or Allowances are no longer applicable. 

My suggestion is to use the IRS Tax Withholding Estimator here. 

FAQs on the 2020 Form W4.