Methodology

Data last reviewed: September 25, 2026 · Tax year 2026

This page explains where our numbers come from, how the calculations work, what they assume, where they stop being reliable, and what we do when we get something wrong. If a figure here disagrees with an official source, the official source is right and we want to hear about it.

Sources and when each was last checked

Every externally sourced rate on this site is registered with its source, coverage and verification date. This table is generated from that register, so it reflects what the calculators actually use rather than a description of it.

Verified means someone compared the data field by field against the source named, on the date shown. Provisional means we believe it is correct and safe to publish, but it has not been checked that way. We publish the distinction rather than implying everything has been audited. Currently 21 of 24 datasets are verified; the remaining 3 are state income tax regimes, state and local sales tax rates, city and county income taxes on the city paycheck pages.

We prefer controlling or original material over commentary. Where a secondary source disagrees with the governing document, the governing document wins. That is not hypothetical: the 2026 head-of-household 24% bracket is widely published as ending at $201,775, but IRS Revenue Procedure 2025-32 sets it at $201,750, and we follow the Revenue Procedure.

The same figures are published for reuse. The data and studies page has reports you can download and cite, and the free tax data API serves the 2026 federal brackets and state tax data these calculators read, as JSON.

How the calculations work

Income tax is computed marginally, not with a single flat rate. Each slice of income is taxed at the rate for its own bracket, so moving into a higher bracket never reduces take-home pay. In outline, a paycheck estimate runs:

1. Start from gross pay and convert it to an annual figure.
2. Subtract pre-tax deductions, then the standard deduction for the filing status (single: $16,100 for 2026).
3. Apply the federal brackets to what remains, one band at a time.
4. Add Social Security at 6.2% up to the $184,500 wage base, and Medicare at 1.45% with no cap, plus the 0.9% additional Medicare tax above the filing-status threshold.
5. Apply state income tax using that state's own rules: no tax, a flat rate, or its own brackets, after any state-specific exemption or deduction.
6. Convert back to the chosen pay frequency.

Every calculator draws these federal figures from one shared file. There are no per-page copies of the bracket table, and a build check fails if one is added.

What the results assume

• You take the standard deduction. Itemising changes the result, sometimes substantially.
• Income is even across the year. Bonuses, mid-year raises, or partial years shift withholding.
• State figures use single-filer brackets unless the calculator says otherwise.
• Results are for a full tax year at the rates shown, with no credits applied unless a calculator names them.
• Rounding is to whole dollars at the final step, so a figure may differ by a dollar from a payroll system that rounds at each stage.

Limitations, and what these tools are not

These calculators are estimates for planning. They are not tax advice, not a substitute for a return, and not a payroll system. If money or a filing decision turns on the answer, check it against the official source or a qualified professional before acting.

Specifically, they do not model:

• Tax credits, including the child tax credit and earned income credit, unless a calculator names them.
• Itemised deductions, the alternative minimum tax, or the net investment income tax except where stated.
• Local or municipal income taxes beyond the notes shown on a state's page.
• Multi-state income, part-year residency, or reciprocity agreements between neighbouring states.
• Non-resident, foreign, or military-specific state exemptions.
• Mid-year rate changes: results use the rates in effect for the stated year, and some states phase rates in over several years.

Sales tax rates are shown for a city's main jurisdiction. Special districts can push the rate higher at a specific street address, so treat a city rate as a close approximation rather than an exact figure for one shop.

Who maintains this, and how often

Calcinum is built and maintained by Rakesh Choudhary, Founder & Editor, who is accountable for the formulas, the source data, and any defect you report.

Every dataset carries its own re-verification deadline in the provenance register. The build fails once a deadline passes, so a review is forced rather than remembered. Beyond those, the full site is reviewed at least annually, in January, when federal and state figures change for the new tax year.

If an update is late: A dataset past its re-verification deadline fails the build, so nothing ships on stale data. If a source has not published replacement figures by then, the dataset is marked provisional and readers see that on the page rather than the site silently continuing to present old figures as current.

When we get something materially wrong, this is what happens:

1. Fix at source so every page using the figure is corrected at once, rather than patching pages individually.
2. Add a check or test that would have caught it, since a lesson that is not enforceable does not hold.
3. Publish the error, its size, and the new safeguard in the correction log at /corrections.
4. If the figure could have changed a filing or a medical decision, say so plainly in that entry rather than describing it as a minor update.

This site also targets WCAG 2.2 Level AA; our accessibility statement records what has been tested and what has not.

How we handle corrections

When we find a figure that was wrong, we fix it at source so every page using it is corrected at once, and we publish what happened. Our correction log records each error, who it affected, how wrong it was, how we found it, and the check that now stops it recurring — because a lesson that is not enforceable tends not to hold. The most recent entry is from September 25, 2026.

We do not quietly edit a wrong number and move on, and we do not remove old entries once they are fixed. If you have found something wrong, tell us — it is the fastest way this gets better.