Certaa · v1 · US Payroll Tax API

The payroll tax engine built for everyone building payroll.

Real-time gross-to-net calculation across all 4,700+ US tax jurisdictions. A modern REST API with transparent pricing, full local tax coverage, and no lock-in.

Get early access See capabilities

POST/v1/calculate

200 OK · 18ms

{
  "calculationId": "calc_01j9x2mzk4…",
  "grossPay":       "5000.00",
  "netPay":         "3521.48",
  "totalTaxWithheld": "1478.52",
  "employeeTaxes": [
    { "taxType": "federal_income",   "amount": "568.42",
      "jurisdiction": "US" },
    { "taxType": "state_income",     "amount": "218.76",
      "jurisdiction": "US-CA" },
    { "taxType": "social_security",  "amount": "310.00" },
    { "taxType": "medicare",         "amount": "72.50" },
    { "taxType": "state_disability", "amount": "55.00",
      "jurisdiction": "US-CA-SDI" }
  ],
  "appliedJurisdictions": ["US","US-CA","US-CA-SDI"],
  "metadata": {
    "processingMs": 18,
    "ratesAsOf": "2026-01-01",
    "ruleSnapshot": "snap_20260101"
  }
}

4,700+Tax jurisdictions

<50msAvg. calculation time

99.99%Uptime SLA

51States + DC

Capabilities

Everything you need to run payroll taxes.

The full federal + state + local tax stack, delivered as a clean REST API you actually want to integrate.

Core

Gross-to-Net Engine

Real-time federal, state, and local tax calculation for any employee type — W-2, NRA, FICA-exempt, supplemental wages, and gross-up.

Jurisdictions

Full Local Tax Coverage

PA Act 32 PSD codes, OH and KY municipal, AL occupational taxes, NY MCTMT, OR Metro SHS — every withholding jurisdiction in the US.

Compliance

Automated Rule Updates

Tax rules monitored and updated as governments publish changes. Pin a rule snapshot with X-Rule-Snapshot for deterministic reproducibility.

Developer

REST API + OpenAPI Spec

Clean JSON in, clean JSON out. Full OpenAPI 3.1 specification, Postman collection, and client libraries. No SOAP, no XML.

Infrastructure

Audit Trail

Every calculation logged with the exact rule version, jurisdiction set, and inputs used. Full audit trail for compliance and dispute resolution.

Advanced

Simulation & Gross-Up

Calculate net from gross or gross from a desired net. Run jurisdiction simulations before processing payroll. Batch up to 1,000 employees per call.

How it works

One API call. Complete payroll taxes.

Send one HTTP request

Pass the employee pay data, W-4 elections, work address, and pay period to /v1/calculate.

Engine resolves jurisdictions

We geocode the work address and automatically apply every applicable federal, state, and local tax rule.

Receive a complete paycheck

Get back net pay, itemized withholdings, jurisdiction list, source citations, and processing metadata in under 50ms.

Comparison

How we stack up.

A modern, independent payroll tax API with transparent pricing and no lock-in — see how Certaa compares.

	Certaa	Symmetry (Gusto)	Vertex
Independent ownership	Not owned by a payroll company	Owned by Gusto since 2021	Independent
API architecture	REST / JSON + OpenAPI 3.1	REST, partial coverage	SOAP / XML (legacy)
Developer-first	✓	Partial	✗
Transparent pricing	Contact us	Sales-negotiated only	Enterprise contract
Self-hosted option	Docker available	✗	Enterprise only
Source refs per response	✓ Every call	✗	✗
NRA / visa worker support	✓	✓	Limited
Pay stub generation	JSON + printable HTML	✗	✗

Early access

We're building in the open. Get on the list.

Certaa is in early access. Leave your email and we'll reach out when we're ready to onboard new API customers.