As a general rule, we’re required to pay our federal income tax over the course of the year through withholding, estimated tax payments, or a combination of the two. Unless an exception applies, you pay a penalty if these payments total less than 90% of your tax liability. The tax law that took effect in 2018 changed so many rules that we can expect a larger than usual number of taxpayers to fall short of 90% that year. The IRS says it will waive the penalty if you paid at least 85%. The waiver isn’t automatic, though: you have to file Form 2210.