Methodology

Source hierarchy

Figures are accepted from primary government publications only, in this order of preference:

1. The revenue procedure or statute itself, as published in the Internal Revenue Bulletin.
2. Official IRS forms, instructions, and publications.
3. Official agency releases, such as the Social Security Administration's contribution and benefit base.
4. State revenue and taxation department publications, for state-level figures.

Secondary sources — news coverage, tax software blogs, other calculator sites — are never used as the basis for a published figure. They are occasionally useful for finding out that a change happened; they are not evidence of what the change was.

Transcription

Rates and thresholds are transcribed by hand from the source document into a dedicated data file, one file per tax year and topic. No figure is carried over from a previous year, and none is interpolated or extrapolated.

Every data file records where its contents came from: the source name, a link, the effective date, the review date, and a status of official, estimated, or draft. The calculator reads that metadata and displays it, so the provenance strip you see on each page is generated from the same record the calculation uses. They cannot drift apart.

When a figure is withheld

If a figure cannot be confirmed against a primary source, it is not published. Concretely, that means one of three things happens:

• The field is omitted from the calculation and the results panel explains what was left out and which direction it would move your answer. The refundable child tax credit is handled this way.
• The input becomes user-supplied. Sales tax rates and state income tax rates work like this — you enter a rate you can verify, and the output is labelled as based on your figure.
• The whole calculator stays unpublished and is listed as in development. Several are, for exactly this reason.

What does not happen is a plausible-looking number appearing without a source behind it.

Testing

The calculation engine is tested independently of the interface. The suite covers zero and negative income, every filing status, each bracket edge and the values immediately either side of it, very large incomes, capital loss limits, the Social Security wage base, Additional Medicare Tax thresholds, and the reverse sales tax calculation.

The most valuable tests are the bracket fixtures. Because Rev. Proc. 2025-32 publishes each band as a cumulative amount plus a marginal rate, the expected values could be taken straight from the IRS tables rather than from our own output. An error in any bracket boundary, or rounding drift anywhere in the chain, fails the suite immediately.

A second suite checks the pages themselves: that every asset path resolves, every label points at a real control, every ARIA reference exists, structured data parses, no element id is duplicated, every field a calculator reads exists in its form, and no page carries a duplicated title, description, or canonical URL.

Rounding and precision

All money is handled as integer cents internally. Values are converted once on the way in and once on the way out, so intermediate results never accumulate floating-point error across bracket boundaries. Rates are stored as decimals rather than percentages so no calculation divides by 100 midway through.

Where a published worksheet rounds at a particular step, we round at that step. Where it does not, we carry full precision to the end.

Review cycle

Each page shows the date its figures were last reviewed. Reviews happen when the IRS publishes annual inflation adjustments, when legislation changes a rate or threshold, when the SSA announces a new wage base, and whenever a reader reports a discrepancy.

If you believe a figure here is wrong, please tell us and include the official source you are working from. Corrections backed by a primary source are made promptly.