What is the effect of filling in zeros in the personal allowance worksheet of the form w-4?

The worksheet is a personal aside. The processors don't so much as look at your worksheet. I am pretty sure you are asking about what to put on line 5, the number you are asking about is the number of exemptions. The number of exemptions is used in calculating your federal wtihholding from your paychecks. The simple answer is that the higher the number of exemptions, the less they withhold each check. If you put 0, they withhold the most based on exemptions. If you put over 9, you will probably have to explain why to the Internal Revenue Service at some point. If you are single and put 5, they will withhold less and make your paychecks more but it will also increase the likelihood that you will owe them money when you file your federal taxes. If you look on your last year's tax return, the second section is Exemptions. That is the number you should probably use. Things that would not make that the number you should use, are if you got married/divorced, had a child, had a child move out and there are other things that could make the number you should use different. Also, if the exemptions number is correct but you had to pay income tax with your return, you may want to put a lower number on line 5 of the W-4 so that you probably won't have to pay next year.