W-4 allowances reduce taxable income, lower tax withholding and increase take-home pay. However, if you claim allowances you’re not entitled to, you’ll likely owe the Internal Revenue Service some cash. Allowances are based on each employee’s personal and financial situation; there are no set rules for how many you can claim. The key is to look at your own situation truthfully, and never make a claim you can't support.
Read through lines A through G of the Personal Allowances Worksheet section of the W-4 and choose only the allowances that apply to you. Each allowance has its own requirements. For example, on line A, enter "1" to claim an allowance for yourself if no one else can claim you as a dependent. You may claim an allowance for your spouse on line C or enter "0" to avoid having too little tax withheld. If you have child dependents, you can claim an allowance for each of them on line D if your spouse doesn't claim them on his W-4.
You might need to fill out page 2 of the W-4 if you both work, intend to itemize your deductions, or claim income adjustments. For example, as of 2012, if your combined income exceeded $10,000 you had to fill out the Two-Earners/Multiple Jobs Worksheet on page 2 to avoid having too little tax withheld from your paychecks. Also, if you plan to claim certain deductions on your tax return, such as medical expenses, charitable contributions and home mortgage interest, complete the Deductions and Adjustments Worksheet on page 2.
Withholding Certificate Completion
Fill out the Employee's Withholding Allowance Certificate on page 1 of the W-4. Include your filing status on line 3. If you plan to file a separate tax return, check the "Married, but withhold at higher Single rate" box. If you intend to file jointly, check the "Married" box. Transfer the total allowances from line H of the personal allowances section to line 5 of the withholding certificate. If you completed the Two-Earners/Multiple Jobs Worksheet on page 2, put the extra amount that should be withheld from each of your paychecks on line 6. Sign and date the certificate and give it to your employer.
Although the IRS doesn't cap W-4 allowances, the form may be subject to review. The agency once required employers submit W-4s for employees who claimed more than 10 allowances, but it wasn't necessary at the time of publication. The IRS can ask employers to resubmit a W-4 for someone who appears to be claiming the incorrect number of allowances. The IRS might send the employer a lock-in letter to change those allowances.
You may change your allowance count whenever you undergo a change in your personal or financial situation. Complete a new W-4 and submit it to your employer in time for the pay period you want the change to occur. For example, if you adopt a baby, you need a new form to claim the adopted child as an additional dependent.