IRS Publication 15 (Circular E)
The annually updated employer's tax guide that contains federal withholding tables and deposit rules.
IRS Publication 15 (Circular E), Employer's Tax Guide, is the annually updated reference for U.S. employers covering federal income tax withholding, FICA, FUTA, deposit rules, deposit schedules, penalty calculations, and recordkeeping requirements. The publication is reissued each January and is the authoritative source for employer-side federal payroll tax administration.
The companion publications Pub 15-A (Employer's Supplemental Tax Guide) and Pub 15-T (Federal Income Tax Withholding Methods) cover specialized topics — independent contractor vs. employee classification, fringe benefits, supplemental wage withholding, and the wage bracket and percentage method withholding tables. All three are free downloads from irs.gov.
For small-business payroll, Publication 15 is the document to consult any time you have a question about a federal payroll mechanic that your software's UI doesn't fully explain. The most consulted sections are usually the deposit rules (Section 11), the deposit schedule chart (Section 11), the lookback period rules (Section 11), and the FICA wage base table (Section 9).