If you owed tax last year or received a large refund you may want to adjust your tax withholding. Owing tax at the end of the year could result in penalties being assessed. On the other end, if you had a large refund you lost out on having the money in your pocket throughout the year. Changing jobs, getting married or divorced, buying a home or having children can all result in changes in your tax calculations.
The IRS withholding calculator on IRS.gov can help compute the proper tax withholding. The worksheets in Publication 505, Tax Withholding and Estimated Tax can also be used to do the calculation. If the result suggests an adjustment is necessary, you can submit a new W-4, Withholding Allowance Certificate, to your employer.