Primary sources first
Federal parameters come from the IRS or Social Security Administration. State links point to tax authorities. VAT guidance links to the European Commission.
Calculation methodology01 02 03 04
Transparent enough to audit. Simple enough to use.
TaxMathKit is independently maintained by the Saigetsu233 project account. It turns published schedules into planning calculators and records the formula, source trail, rate year, review date, and important exclusions.
Version every rate set
Tax-year inputs are kept separate. Material changes are added to the public version log instead of silently replacing an old method.
Expose and test the formula
We describe the tax base before the rate. Progressive schedules run band by band and core functions have automated examples.
Name what is missing
Credits, surtaxes, phaseouts, local sourcing rules, and special categories are excluded unless a calculator explicitly includes them.
Source review ledger
Federal brackets & deductionsIRS 2025 and 2026 inflation-adjustment releases
Estimated paymentsIRS Publication 505 (2026)
Lottery withholdingIRS Instructions for Forms W-2G and 5754 (2026)
Self-employment taxIRS Topic 554, Schedule SE instructions, SSA wage base
State sales-tax planning presetsTax Foundation 2026 state and population-weighted average local rates, compiled from Sales Tax Clearinghouse and state revenue departments
Planning presets versus exact local rates
State selection can fill a 2026 population-weighted average combined sales-tax rate so a visitor can complete a planning estimate without researching a rate first. The interface labels this as an average: address-level districts, product taxability, tax holidays and vehicle rules can differ, so exact receipt or filing work still needs an official local lookup.
Rounding, privacy, and local saving
Calculations use full numeric precision in the browser and round displayed currency for readability. Calculator inputs are not sent to TaxMathKit analytics. If you choose “Save inputs locally,” the values remain in that browser’s local storage and can be deleted with browser site data.
Editorial process
A tool starts with a real planning question, is mapped to primary sources, implemented as a separate formula, tested with worked examples, and reviewed for exclusions. Guides must answer a distinct question; changing only the keyword is not a new article.
How to use an estimate
Use it to explore scenarios, create a rough reserve, or understand a formula. For a return, payment deadline, transaction quote, or legal decision, verify current official instructions and consult a qualified professional where appropriate.