Data Sources & Methodology
Every number on EquityDue should be traceable: to a formula printed on the page, and to a primary source with a date. This page explains how that works in practice.
Where the data comes from
Federal figures — supplemental withholding rates, the $1 million supplemental wage threshold, Social Security wage base, Medicare rates, safe-harbor percentages, and bracket thresholds — are taken from IRS publications and instructions, principally Publication 15 (Employer's Tax Guide), Publication 505 (Tax Withholding and Estimated Tax), and the annual inflation-adjustment revenue procedures.
State figures — supplemental withholding rates and related payroll rules — are taken from each state's department of revenue or taxation: withholding guides, employer circulars, and administrative rules. Where a state has no wage income tax, that is recorded explicitly rather than shown as a zero without context.
Two rules apply throughout: no figure is sourced from another calculator or aggregator site, and every table links the documents it was compiled from next to a verified-as-of date.
How calculators are built
Calculators run entirely in your browser as plain JavaScript with their rate constants stored in readable JSON files. Each tool page publishes three things alongside the inputs:
- The formula in plain text — the same arithmetic the code performs, written out so it can be checked by hand.
- Variable definitions — every input and intermediate value defined, with units.
- A worked example — the formula applied step by step to concrete numbers, so you can verify the tool reproduces it.
Calculators intentionally model the general rules. They do not model every edge case — multi-state vesting allocations, local taxes, AMT interactions, or employer-specific plan quirks — and each tool page notes the simplifications it makes.
Update policy
- Annual review (January): after the IRS and states publish figures for the new tax year, every table and calculator constant is re-verified against current primary sources and its verified-as-of date is refreshed.
- Off-cycle updates: when a mid-year law change alters a rate or threshold, affected pages are corrected as soon as practical.
- Corrections: verified reader-reported errors are fixed promptly. Report one via the contact page — a primary-source link speeds things up.
What this site does not do
EquityDue does not prepare returns, does not model individual filings end to end, and does not give advice. It is a reference layer between the primary sources and your own professional adviser.
Content on this page and across EquityDue is informational only and is not tax, legal, or investment advice.
Common questions
- Where does EquityDue's tax data come from?
- From primary sources only: IRS publications and instructions (such as Publication 15 and Publication 505) for federal figures, and each state's revenue department documentation for state supplemental withholding rates. EquityDue does not copy figures from other calculator or aggregator sites.
- How often is the data updated?
- Every table is reviewed at least once a year, in January, after federal and state figures for the new tax year are published. Off-cycle updates happen when a law change takes effect mid-year. Each table displays the date it was last verified.
- Is EquityDue tax advice?
- No. Everything on this site is general information for educational purposes. Calculators produce estimates from the inputs you give them and cannot account for your full situation. Decisions with tax consequences should be reviewed with a qualified tax professional.