US payroll tax data is fragmented across IRS, 50 state agencies, and thousands of local jurisdictions. Payroll Tax API covers 4,246 jurisdictions: federal income brackets, FICA, FUTA, all 50 states + DC, 200+ counties, 3,278 cities, and 713 school districts. One API call returns all applicable rates stacked. Effective-date versioned. Free tier: 1,000 req/month, no credit card.