Tax

Paycheck-withholding (W-4) checkup

Are you on track with federal withholding? Projects the year's federal tax liability (reusing the tax engine) from the embedded tax situation, compares it against what will be withheld at the current pace (year-to-date plus the per-paycheck amount over the remaining pay periods), and reports the expected refund or balance due, the status, and the per-paycheck W-4 adjustment (line 4c) to land on target. Supply the prior year's tax to also check the §6654 safe harbor and flag underpayment-penalty risk. Deterministic; not tax advice.

post/v1/tax/w4-checkup

Request

  • Base URL: https://api.numeratica.com
  • URL: https://api.numeratica.com/v1/tax/w4-checkup
  • Auth: one of:
    • HTTP bearer
    • API key in header X-API-Key

Request body

ytd_withheldnumber required

Federal income tax withheld so far this year.

per_paycheck_withholdingnumber required

Federal withholding per remaining paycheck.

pay_periods_remaininginteger required

Paychecks left in the year.

prior_year_taxnumber

Prior year's total tax (enables the §6654 safe-harbor check).

prior_year_aginumber

Prior year's AGI (picks the 100% vs 110% safe-harbor threshold).

Example request

{
  "tax": {
    "tax_year": 2026,
    "state": "CA"
  }
}

Response

The withholding checkup

result_idstring
engine_versionstring
disclaimerstring

Example response

{
  "result_id": "w4_1a2b3c4d5e6f7a8b",
  "engine_version": "w4-checkup-1.0.0",
  "inputs": {
    "tax": {
      "tax_year": 2026,
      "state": "CA"
    }
  }
}

Changes

No recorded changes to this operation across all 1 revision of this API.