Methodology and Sources: How Our Tax Calculators Work

This page explains where every number on the site comes from, how the calculators combine them, and where they deliberately simplify. Last reviewed October 30, 2026.

What the calculators do

All calculators run in your browser using one shared calculation engine. Given your pay, filing status and deductions, the engine works in the same order a payroll system and a tax return do: pre-tax deductions reduce wages, the standard deduction reduces taxable income, the federal brackets are applied, credits are subtracted, FICA taxes are calculated on wages, and state income tax and state payroll programs are added. The result is expressed per year and divided by the number of pay periods.

Federal figures for 2026

Brackets and standard deduction. IRS Revenue Procedure 2025-32. Standard deduction: $16,100 single, $32,200 married filing jointly, $24,150 head of household. Rates run from 10% to 37%; the 37% bracket starts at $640,600 for single filers.
Social Security. 6.2% on wages up to the 2026 wage base of $184,500 (Social Security Administration).
Medicare. 1.45% on all wages, plus the 0.9% Additional Medicare Tax above $200,000 (single/head of household) or $250,000 (married filing jointly), per IRS Topic 560. The employer withholds the additional tax above $200,000 regardless of filing status; the calculator reflects the amount owed for your status.
Child tax credit. $2,200 per qualifying child under the 2026 rules, applied to the estimated annual liability.
401(k) and HSA limits. $24,500 elective deferral, $8,000 age-50 catch-up, HSA $4,400 self-only and $8,750 family coverage for 2026.
Self-employment tax. 12.4% Social Security on 92.35% of net earnings up to the wage base and 2.9% Medicare on all of it, with half deducted from income (IRS Topic 413).
Qualified overtime deduction. The One, Big, Beautiful Bill Act deduction applies to the premium portion of overtime pay, up to $12,500 ($25,000 joint), phasing out above $150,000 ($300,000 joint) of modified AGI, for tax years 2025 through 2028.
Estimated tax. 2026 due dates are April 15, June 15 and September 15, 2026 and January 15, 2027. The 2027 schedule is April 15, June 15, September 15, 2027 and January 18, 2028.

State figures

State rate tables, standard deductions, personal exemptions and credits for all 50 states and DC are taken from the Tax Foundation’s 2026 compilation and checked against state revenue department publications where the sources disagreed or a state had changed its law (for example, Maine, West Virginia, Connecticut and Wisconsin). State payroll programs such as California SDI, New York SDI and Paid Family Leave, Washington’s PFML and WA Cares, Massachusetts PFML, New Jersey and others are modeled with their current rates and caps. Sales tax rates are from the Tax Foundation’s midyear 2026 report.

Known simplifications

A calculator that tried to reproduce every employer’s payroll software would never be finished. These are the places where our estimates differ from a real paycheck or return:

Withholding versus liability. The tools estimate the tax you actually owe for the year on annualized pay, not the exact amount your employer withholds using IRS Publication 15-T tables and your Form W-4. Your withholding may be higher or lower.
Local income taxes are not included unless you enter a local rate. Maryland counties default to 3.0% as an average.
State dependent exemptions and credits are not modeled. Head-of-household status is treated as single at the state level except where a state publishes separate head-of-household schedules or follows the federal standard deduction (including Maine, Minnesota, Oklahoma and Connecticut).
Specific state items not modeled: South Carolina’s age-based deduction, Ohio’s income-tiered exemption, Oregon’s federal tax subtraction, Connecticut’s tax-rate add-back, and state deductions for overtime or tips such as those in Alabama and Georgia.
Approximations. California brackets are the 2025 brackets indexed forward because the 2026 table was not available when we reviewed. North Dakota’s married-filing-jointly brackets, Utah’s credit phase-out thresholds and Wisconsin’s standard-deduction phase-down are approximated. State pages that rely on an approximation say so.
Bonuses. Federal supplemental withholding (22% flat or aggregate) is modeled. State supplemental withholding rates, which differ by state, are not.
Self-employed workers. The estimated tax tools do not include state payroll programs for the self-employed or state estimated-tax rules. The QBI deduction is phased out above the IRS income thresholds as if the business were a specified service business, and the W-2 wage and property limits that can allow a deduction for other businesses are not modeled, so the estimate may be slightly high for some high earners.
2027. The IRS has not published 2027 inflation adjustments, so the 2027 estimated-tax schedule uses 2026 parameters and is labeled as such.

Testing

The calculation engine has an automated test suite that checks bracket math, Social Security and Medicare caps, the Additional Medicare Tax, self-employment tax, safe-harbor thresholds, hourly conversions and state calculations against published examples and independently computed values. The tests are rerun every time the site is rebuilt.

Corrections

If you find a figure that is wrong or out of date, please tell us at contact@dollarfinch.com with a link to the official source. Corrections are reviewed and the site is updated when the source supports the change.

Methodology and sources