Schemes: federal income tax by the Pub 15-T percentage method — one scheme per W-4 filing status (Single/MFS, Married Filing Jointly, Head of Household; register each employee in theirs) honoring the W-4 steps (dependents_credit, other_income, deductions, extra_withholding on the registration); Social Security (6.2% each, annual wage base); Medicare (1.45% each) plus Additional Medicare (0.9% over $200k, per-period approximation); FUTA (0.6% employer accrual on the first $7,000).
What each employee provides
- Their W-4: filing status picks the scheme; steps 3–4 go on the registration as parameters.
Worked example — $10,000 monthly, Single, $50 extra withholding
Annualized $120k − $15k standard deduction → $18,047/yr federal tax → $1,553.92/mo with the extra. Social Security $620 each side; Medicare $145 each; FUTA $42 accrued in January until the $7,000 base is used up.
Limitations
State income tax and SUTA are data-only additions per state (none seeded yet); the Additional Medicare per-period approximation reconciles on the 941/W-2; multiple-jobs W-4 (step 2) uses the higher-withholding tables, not seeded. Direct EFTPS deposits and return submission to the IRS are a later phase.