Every data source, computation step, and edge case behind the calculator — documented so any number it produces can be traced back to the exact rule that produced it.
Tax year 2026
8 official sources
9 edge cases modeled
Reviewed Oct 2026
This page is the engine's public rulebook. It explains where every rate comes from, the exact order of computation, the tricky cases most calculators skip, and how the data stays current. If you ever want to audit a number, start here — then open the glossary for any term.
01
Every calculation traces to a published official source. The engine never embeds undocumented rates, and no figure is estimated where a statutory value exists.
IRS Publication 15-T
v2026.01
Federal income tax withholding methods: percentage-method tables, Worksheet 1A logic, standard schedules
IRS Publication 15 (Circular E)
2026
Employer tax rules: supplemental wage rates, fringe treatment, deposit context
Social Security Administration
$176,100 · 2026
Social Security wage base and OASDI rate
OBBBA — P.L. 119-21
2025 enactment
Qualified tips and overtime deductions, revised standard deduction, SALT changes
SECURE 2.0 Act
2026 phase-in
Roth catch-up mandate for high earners; super catch-up for ages 60–63
State DOR publications
2026 Q1
Each state's brackets, flat rates, credits, SDI/SUI employee rates
PA DCED PSD registry
05/2024
Pennsylvania Act 32 PSD codes, resident and non-resident EIT rates
FLSA — 29 U.S.C. §207, §778.115
statute
Overtime regular-rate computation and blended-rate withholding
02
The engine is a pure function: the same inputs always produce the same result, in the same order, on every device. Intermediate values are shown in the result breakdown so every line traces back to a rule.
01
Annualize & normalize inputs
Gross pay is converted to an annual figure from the selected frequency; pay dates anchor FICA cap tracking and month-boundary effects.
02
Apply pre-tax deductions
Traditional 401(k), Section 125 health premiums, HSA/FSA reduce income-taxable wages per their FICA treatment — 401(k) stays FICA-taxable; §125 premiums do not.
03
Federal income tax
Publication 15-T percentage method on annualized taxable wages, honoring W-4 Step 2(c) doubled tables, Step 3 credits, and Step 4(a)/(b) adjustments.
04
OBBBA adjustments
Qualified tips and qualified overtime deductions applied within statutory caps and income phase-outs.
05
FICA
6.2% Social Security up to the $176,100 wage base (tracked per paycheck, so the exact convergence check is flagged), 1.45% Medicare uncapped, plus 0.9% Additional Medicare above thresholds.
06
State income tax
The selected state's 2026 rules: graduated brackets or flat rate, state-specific standard deduction or allowances, and state credits.
07
Local taxes
Where the state levies them — PA Act 32 EIT (resident vs. workplace PSD maximum-rate rule), NYC resident tax on NY AGI, Maryland county rates, Ohio school districts, and others.
08
Net pay & frequency translation
Annual results divide back to the selected frequency, with per-paycheck, monthly, and annual views of every line.
03
These are the situations where generic calculators quietly return wrong numbers. Each is modeled explicitly.
W-4 Step 2(c) Multiple Jobs
Withholding tables doubled rather than ignored, exactly as the IRS checkbox requires.
FLSA §778.115 blended overtime
Regular rate recomputed when a bonus is paid alongside overtime, so withholding matches the statute.
Supplemental wages
Flat 22% federal rate applied when bonuses are paid separately from regular wages.
PA Act 32
Resident vs. workplace PSD comparison with the higher-rate rule and reciprocal exemptions.
NYC AGI divergence
City tax computed on New York AGI, not federal AGI.
FICA convergence
The exact paycheck where Social Security withholding stops is identified and shown.
SECURE 2.0 Roth catch-up
Contributions reclassified to Roth above the wage threshold.
Super catch-up
The enhanced limit for ages 60–63 applied when it exceeds the standard catch-up.
Multi-state reciprocity
Covered state pairs exempt nonresident wages from the work state's income tax.
04
The engine honors every box on the current W-4. Here is what each step actually does to your paycheck.
Step 1 — Personal info
Name, address, Social Security number, and filing status. Filing status selects the withholding tables; it is the single biggest driver of per-paycheck withholding.
Step 2 — Multiple jobs or spouse works
Fill this only if you have two or more jobs in total, or you file jointly and both spouses work. The options: use the IRS estimator for the most precise result, check the box to roughly double withholding, or leave it blank and expect to owe at filing.
Step 3 — Claim dependents
Qualifying children under 17 credit up to $2,200 each in 2026, plus dependent credits. These reduce withholding dollar-for-dollar — but only if your income qualifies for the full credit.
Step 4 — Other adjustments
4(a): other income you'll have without withholding (interest, dividends, retirement). 4(b): deductions beyond the standard (itemizing? put the excess here). 4(c): extra tax you want withheld each pay period.
Step 5 — Sign & date
Unsigned forms are invalid; the employer must withhold at the highest rate for a single filer with no adjustments until a valid W-4 is on file.
05
Tax logic lives in versioned data files — one federal file and one file per state — fully decoupled from computation code. When the IRS or a state publishes new rules, only the data file changes.
Update policy: federal and state data files are re-verified against official publications each quarter, and immediately upon mid-year changes. Each file carries its own version stamp shown on every page it affects.
06
The engine downloads to your browser and computes locally. Salary inputs never leave your device — there is no account, no email capture, and no server-side computation. Saved scenarios (where supported) use your device's local storage only.
07
Educational estimate — not tax advice. Results are computed from officially published rates but cannot capture every personal circumstance (itemizing, unusual credits, nonresident alien status, and similar). Verify decisions with a qualified tax professional.
Questions about the engine
Is this calculator accurate?
It computes from officially published 2026 rates — IRS Pub 15-T, SSA figures, and each state's own rules — and models the edge cases most calculators skip. It is still an educational estimate: unusual circumstances (itemizing, unusual credits) can shift the real number.
Where do the tax rates come from?
Eight official sources, listed in section 01 above: IRS Publications 15-T and 15, SSA data, the OBBBA law (P.L. 119-21), SECURE 2.0, state departments of revenue, Pennsylvania's DCED PSD registry, and the FLSA statute itself.
How often is the data updated?
The engine runs versioned data files — one federal, one per state. They are re-verified quarterly and updated immediately when officials publish changes. Every calculator page shows the versions in use.
Why does my result differ from other calculators?
Most tools skip W-4 Step 2(c), apply flat supplemental rates to overtime, and ignore local taxes like PA Act 32 or NYC's AGI basis. Those differences are exactly where generic calculators are wrong — see section 03.
Do you store my salary?
No. The engine downloads to your browser and computes locally. There is no account, no email capture, and no server-side computation.
Audit the numbers on your own paycheck.