For developers
Arc & Ledger Tax Data API
U.S. tax data with its homework attached: annual federal tax parameters, filing deadlines with legal date rolls, and the IRS quarterly interest-rate table, served as JSON where every value carries the authority citation that sets it and the tax year it belongs to. Built and reviewed by a tax firm led by an Enrolled Agent, from the same versioned data that powers our own calculators.
Why this exists
The official sources publish this data as PDF revenue procedures, prose notices, and per-quarter revenue rulings: there is no official API for the numbers every tax calculator, payroll tool, and fintech app needs. This API packages those facts as structured data without dropping the part that matters most, the citation trail back to the primary source.
Every value carries its authority
Each number ships with the citation that sets it (a Revenue Procedure, a Code section, a Revenue Ruling) and a link to the official source, so you can verify instead of trusting.
Vintaged, never silently rewritten
Values are keyed to their tax year and effective window. A corrected value is a new release with a changelog entry, and every response reports the serving version.
Deadline math is law, not arithmetic
Due dates observe IRC 7503: weekends, federal legal holidays, and DC Emancipation Day roll dates forward. Responses show the statutory and observed dates separately.
Data, never advice
The API accepts no taxpayer information and computes no taxpayer-specific answers. Every response carries a general-information disclaimer; an Enrolled Agent reviews tax-sensitive content before release.
Endpoints
Federal tax parameters for a vintage: SE tax, standard deduction, bracket tables, mileage (including the 2026 mid-year split), retirement limits, OBBBA provisions, penalties, FBAR/FATCA thresholds, and a California section. Every value carries its authority citation, official URL, and effective window.
curl https://api.arcandledger.com/v1/tax-parameters?year=2026
One parameter by stable id, with every effective window (useful for mid-year changes).
curl https://api.arcandledger.com/v1/tax-parameters/mileage.business
Federal filing deadlines for six form families (Form 1120 + 5472, 1040-NR, 1065, 1120-S, FBAR): the statutory date, the observed date after the IRC 7503 weekend and holiday roll, and the extension.
curl https://api.arcandledger.com/v1/deadlines?year=2025
The four quarterly estimated-payment due dates for a tax year, rolled to business days.
curl https://api.arcandledger.com/v1/deadlines/estimated-payments?year=2026
The IRC 6621 quarterly interest-rate table from 2020-Q1: underpayment, overpayment, corporate overpayment, large corporate underpayment, and the over-$10,000 corporate rate, each quarter cited to its Revenue Ruling or the IRS rate page.
curl https://api.arcandledger.com/v1/interest-rates?from=2024-Q1
Reference profiles for 26 common IRS notices (CP2000, CP14, LT11...): meaning, deadline clock, options, common errors, urgency.
curl https://api.arcandledger.com/v1/notices/CP2000
Reference profiles for 28 tax documents (W-2, the 1099 family, K-1s, 1042-S...): who sends it, key boxes, where it goes on a return.
curl https://api.arcandledger.com/v1/tax-documents/1099-K
Which tax years the parameter catalog covers, and the date the interest-rate table is verified through.
curl https://api.arcandledger.com/v1/vintages
The machine-readable spec lives at https://api.arcandledger.com/v1/openapi.json. Responses use a stable envelope: data, meta (serving version and tax year), and a general-information disclaimer.
Access and plans
No key required
Every endpoint, full data, 30 requests per minute per IP. Enough for development, side projects, and most internal tools. Please cache responses (they change on a quarterly and annual cadence, and say so).
API keys and higher limits
Keyed plans with higher rate limits and commercial-use terms are rolling out. Tell us what you are building through the contact page and we will set you up.
Updates follow the law's own calendar: parameters annually with the IRS inflation adjustments, interest rates quarterly with each Revenue Ruling, and an automated monitor plus a review gate stand behind the cadence. The API stores no taxpayer data and accepts none. Also for AI assistants: our free MCP server answers tax questions with the same underlying data.