The IRS released the revised W-4 Withholding Calculator on their website on February 28 along with the revised 2018 version of Form W-4. The revision arrived slightly later than promised by Treasury Secretary Steven Mnuchin who testified on February 15 that the calculator would be out within a week.
The revised calculator takes into account changes made by the Tax Cuts and Jobs Act. The calculator works for most individuals, but the IRS warns that it is not meant for taxpayers with more complex situations.
As the website warns:
IMPORTANT NOTE: This Withholding Calculator works for most taxpayers. People with more complex tax situations should use the instructions in Publication 505, Tax Withholding and Estimated Tax, expected to be updated in early spring. This includes taxpayers who owe self-employment tax, alternative minimum tax, the tax on unearned income of dependents or certain other taxes, and people with long-term capital gains or qualified dividends.
Since spring begins on March 20 this year, the publication is likely a few weeks away from being published.