Generally, it's not a good idea to allow the federal government to borrow your money interest-free. However, filling out a W-4 form with the objective of maximizing your refund when you file your federal income taxes can be advantageous. With your larger refund, you can pay off debt or place it into savings. Generally, you fill out the W-4 Form when you complete the I-9, the form documenting your employment eligibility. In fact, you don't need the I-9 to complete the W-4 if you're already working; you can obtain a W-4 Form from your employer or the IRS website.
Look at the "Personal Allowances Worksheet" of the W-4 form. Calculate all of your allowances as you normally would, entering "1" on line "A" for yourself and "1" on line "B" if you meet the conditions for the allowance (e.g., single or married and have only one job). Enter "1" for your spouse and for each of your dependents. Enter "1" for each of the other allowances if they apply, such as the head of household and child and dependent care allowances.
Add the number of allowances you've entered and place the total on line "H" of the worksheet.
Subtract from line "H" of the "Personal Allowances Worksheet" the number of allowances you wish to "give up" through the year in return for a larger refund at tax time. Claiming fewer allowances makes a larger part of your income subject to the federal withholding tax. If you're single, for example, you can claim "0" to have all of your income subject to the tax. Place the resulting number on line 5 of the W-4 Form.
Enter your filing status in Box 3 of the W-4 form. Check the "Single" box if you're single. Check the "Married, but withhold at higher Single rate" box if are married, but would like to be taxed at the higher rate. In some cases, for example, if you're married and you're spouse also works, you may not have enough federal taxes withheld at the "Married" rate. If you typically do have a refund at tax time, though, you can check the "Married, but withhold at higher Single rate" option to maximize your refund.
Figure any additional amount you want withheld from each paycheck. Adding more to your withholding gives you a larger refund when you file your taxes. Divide the total amount you want withheld for the year by the number of paychecks you receive. For example, divide $500 by 26 if you receive biweekly pay. Enter this amount on line 6 of the W-4.
You can use Worksheets 1, 4 and 7 of IRS Publication 919 to project the amount of federal tax you'll owe for the year versus the amount withheld from your paycheck, and use this as a basis to figure out how many -- or how few -- allowances you want to claim on the W-4.
Having too little tax withheld from your paycheck subjects you to IRS penalties; no penalties exist for over-paying.