Individuals: Make a payment of your estimated tax for 2018 if you did not pay your income tax for the year through withholding (or did not pay enough in tax that way). Use IRS Form 1040-ES. This is the final installment date for 2018 estimated tax. However, you do not have to make this payment if you file your 2018 return and pay any tax due by January 31, 2019.
Employers: For Social Security, Medicare, withheld income tax, and non-payroll withholding, deposit the tax for payments in December 2018 if the monthly rule applies.