Federal Withholding — How the IRS rules turn your salary into withholding — and the 2026 law changes that matter.
The IRS form you give your employer that sets federal income tax withholding. Since 2020 it uses steps instead of allowances: filing status, multiple-jobs adjustment, dependents, and other income. Change it and your new paycheck is visible instantly.
Single, Married Filing Jointly, Married Filing Separately, or Head of Household. It selects the withholding tables and standard deduction applied to your pay. Choosing the wrong one is the #1 cause of a surprise tax bill.
The fixed dollar amount subtracted from annual income before tax is computed. For 2026 the OBBBA law raised it to $32,200 (married joint), $24,150 (head of household), and $16,100 (single). The calculator applies it automatically.
Your marginal rate is the tax on your next dollar (your bracket). Your effective rate is total tax divided by total income — always lower. Confusing the two is the most common paycheck myth.
Bonuses, commissions, and overtime above the regular rate. When paid separately, employers may withhold federal tax at a flat 22% supplemental rate instead of regular tables.
Public Law 119-21 (2025) added temporary above-the-line deductions for qualified tips and qualified overtime, plus a higher standard deduction. The engine applies both where they apply to you.
W-4 Step 2(c). Checking it doubles the standard withholding tables so two-earner or two-job households don't under-withhold. Many calculators ignore it; this engine does not.
W-4 Step 3. Each qualifying child reduces withholding by up to $2,200 under the 2026 child tax credit. It reduces withholding, not necessarily your final tax.
FICA & Retirement — The payroll taxes that come out of almost every paycheck, and the retirement rules that change them.
The combined Social Security (6.2%) and Medicare (1.45%) payroll taxes — 7.65% total for employees. FICA applies to gross wages with no standard-deduction offset.
The maximum wage subject to the 6.2% Social Security tax: $176,100 for 2026. Above it, no further SS tax is withheld — the engine flags the exact paycheck where this happens.
1.45% of every dollar of wages, with no cap. Employers match it, for a combined 2.9%.
An extra 0.9% on wages above $200,000 (single) or $250,000 (married joint). Only the employee pays it — there is no employer match.
401(k), traditional health premiums, HSA/FSA contributions withheld before income tax is computed. They reduce taxable wages but generally not FICA (401(k) is FICA-taxable; Section 125 health premiums are not).
Contributions withheld after tax. They don't reduce today's withholding, but qualified withdrawals are tax-free.
From 2026, employees earning over $145,000 (indexed) must make their 401(k) catch-up contributions as Roth. The calculator enforces the mandate automatically.
SECURE 2.0 raised the catch-up limit for workers aged 60–63 to 150% of the standard catch-up amount for 2025 onward. Provide your age and the engine applies the higher limit.
Pay & Frequency — From gross to net: the pay terms that decide how often and how much lands in your account.
Total earnings before any deductions: salary ÷ pay periods, plus wages, overtime, bonuses, and tips.
What actually reaches your bank account after taxes and deductions. The calculator's hero number — shown per paycheck, monthly, and annually.
How often you're paid: weekly (52), bi-weekly (26), semi-monthly (24), or monthly (12). Bi-weekly years occasionally produce three paychecks in one month.
Semi-monthly = 24 fixed dates (e.g. 1st and 15th). Bi-weekly = every 14 days = 26 checks. Same annual salary produces different per-check amounts.
Under the FLSA, non-exempt employees earn 1.5× their regular rate beyond 40 hours/week. Federal withholding on it follows supplemental-wage rules.
FLSA §778.115: when a bonus is paid with overtime, the regular rate must be recomputed and OT withheld on the blended amount — most calculators overstate the tax. This engine uses the statutory method.
The portion of overtime pay eligible for the new above-the-line deduction under P.L. 119-21, within annual caps and phase-outs. Applied automatically when overtime is entered.
Reported tips in qualified occupations earn a new deduction up to $25,000 (with income phase-outs) for tax years 2025–2028. The calculator applies it within statutory limits.
State & Local — Where you live can matter as much as what you earn — the state rules most calculators get wrong.
The state's tax on your wages: graduated (most states), flat (e.g. Illinois 4.95%, Pennsylvania 3.07%), or none (nine states). Each state's brackets, credits, and deduction treatment differ.
Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, and Wyoming levy no wage income tax. New Hampshire taxes only interest/dividends; Washington only high capital gains.
City or county wage taxes — from Pennsylvania's Act 32 EIT to NYC's resident tax and Maryland's county rates. The calculator applies locals where the state has them.
Pennsylvania's local Earned Income Tax is collected per Political Subdivision (PSD) — the rate of your resident municipality follows you, unless your work PSD's rate is higher. See the Act 32 methodology study in Research.
New York City computes resident tax on New York AGI, which differs from federal AGI. Calculators that shortcut this get NYC wrong; the engine models it.
Neighboring-state agreements (e.g. IL–WI, PA–NJ, MD–DC) that exempt nonresidents from the work state's income tax. The engine honors them for covered pairs.
Mostly employer-paid, but employees contribute in a few states (e.g. PA 0.07%, NJ, AL). Appears on stubs there as a separate line.
Employee-funded disability programs in states like California (1.2% up to an SDI wage cap) and New York (0.5% to a cap). Withheld automatically where they apply.
On Your Stub — Decode the actual lines printed on your earnings statement.
The record of gross pay, each tax, each deduction, and net pay for the period and year to date. Requirements vary by state — some mandate detailed stubs, others none.
Cumulative amounts since January 1. YTD figures drive Social Security cap detection and explain why late-year checks can be slightly larger.
The pre-2020 W-4 system (2019 and earlier forms). No longer valid for new forms — if a calculator still asks for 'allowances,' it is using outdated logic.
Income tax and the employee half of FICA come from your pay. The employer's 7.65% FICA match, FUTA, and most SUI are on top of gross — never printed as employee deductions.
Non-cash benefits taxed by adding to wages (e.g. life insurance over $50,000 coverage). It raises taxable wages without changing net pay.
Quarterly payments for income with no withholding — freelance income, interest, tips. If your paycheck under-withholds, Form 2210 penalties can apply; safe-harbor rules protect you.
