Calculation Methodology and Corrections
FreeKalc calculators are estimates generated from versioned tax rules. Each financial page should state its tax year, assumptions, primary sources and important exclusions.
Current rules versions
- US federal and state salary model: 2026.10.14; standard single W-2 employee, no dependants, no local tax and no itemized deductions. Married federal calculations are supported, but state tax is disabled in married mode until joint state schedules are implemented and tested.
- Netherlands salary model: NL-2026.10.14; Box 1 rates plus general and employment credits. Employer-paid WW/WIA and the employer Zvw levy are not treated as blanket employee deductions.
- Other calculators: use the tax year and source notes displayed on their pages; they remain estimates subject to the limitations shown there.
QA gates
- Official-source fixtures for material rates and thresholds.
- Band-boundary and cap tests.
- Arithmetic invariants: gross equals net plus modeled deductions.
- Cross-render tests: shared engine, state pages, ranking hub, prose and SVG charts must agree.
- Full internal-link and JSON-LD validation before deployment.
Rounding and pay periods
Annual tax is calculated first. Displayed annual, monthly and biweekly values are rounded independently to the nearest whole currency unit, so displayed columns can differ by one unit when multiplied back. Weekly and hourly pages may show decimals.
Limitations
Results do not model every credit, benefit, local tax, pension, household circumstance, residency rule or mid-year change. They are general planning guidance. Confirm decision-critical figures with the relevant tax authority, payroll department or a qualified adviser.
Correction history
| Date | Rules | Material correction |
|---|---|---|
| 2026-10-14 | NL-2026.10.14 | Removed incorrect blanket employee deductions for WW/WIA and employer Zvw from 12 Netherlands calculator/result pages; regenerated tables, prose and charts. |
| 2026-10-14 | 2026.10.14 | Moved US state calculations into one generated engine; updated official California 2026 inputs, corrected material state-rule drift, disabled unsupported married state mode and regenerated state outputs. |
| 2026-10-14 | Site templates | Removed unrelated lender disclaimers, malformed Dutch links, off-topic structured data and unsupported “reviewed” claims. |
Primary-source policy
Material rates should link to the responsible tax authority or government publication, not merely a secondary summary. Where a final 2026 amount is not yet published, the page must identify the provisional basis rather than present it as final.
Report a correction
Send the affected URL, inputs, expected output and supporting source through the contact page. Material changes are added to this history after tests pass.