The IRS is providing more and more tools for the taxpayers to access their information online, calculate Child tax credit, or estimate their w2 withholdings (employees).
The withholding estimator is a series of questions (it is a long process as it is really detailed), but it might help you estimate your w2 withholdings better. If you need to change your withholding, you will have to submit a w4 form to your payroll processor, HR department or whoever is involved with your payroll.
IRS Link – https://www.irs.gov/individuals/tax-withholding-estimator
