Data source & methodology
AuthoritativeDataSource: IRS Publication 15-T (2025)—Percentage Method tables and W-4 logic for federal income tax withholding; IRS Topic No. 751 (Social Security & Medicare tax rates); SSA contribution & benefit base (2025 wage cap); IRS Topic No. 560 (Additional Medicare Tax); IRS Newsroom (standard deduction amounts for tax year 2025 under OBBB). Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
- IRS Publication 15-T (2025), percentage method tables. :contentReference[oaicite:0]{index=0}
- Social Security & Medicare: IRS Topic No. 751; SSA 2025 wage base. :contentReference[oaicite:1]{index=1}
- Additional Medicare Tax thresholds (0.9%). :contentReference[oaicite:2]{index=2}
- 2025 standard deduction amounts under OBBB (IRS Newsroom summary). :contentReference[oaicite:3]{index=3}
The formula explained
Let \(P\) be gross per pay, \(N\) the number of pays/year, \(r_s=6.2\%\) up to OASDI cap, \(r_m=1.45\%\) + \(0.9\%\) above threshold \(T\) (by filing status), \(r_{st}\) the state/local rate, \(d_{pre}\) pre-tax per pay, \(d_{post}\) post-tax per pay.
Annualization for FIT (Pub. 15-T):
\[ \text{Annual Wages } W_a = \max\left(0, (P-d_{pre})\cdot N - SD - AJ\right) \]
Where \(SD\) is the standard deduction (2025) by status; \(AJ\) is the Step 2(b) adjustment if “multiple jobs” is checked (per 15-T), and the Step 3 dependents credit \(C\) is applied after computing tentative tax:
\[ \text{FIT}_a = \text{TaxBrackets}(W_a) - C \quad;\quad \text{FIT}_p = \max(0,\ \text{FIT}_a / N) + \text{ExtraWithholding} \]
FICA/Medicare per pay:
\[ \text{SS}_a = 0.062 \cdot \min(W_{fica,a},\ \text{OASDI\_cap}) \quad;\quad \text{Med}_a = 0.0145 \cdot W_{fica,a} + 0.009 \cdot \max(0, W_{fica,a}-T) \]
with \(W_{fica,a} = (P - d_{pre})\cdot N\). State/local (flat for modeling):
\[ \text{State}_p = r_{st} \cdot (P - d_{pre}) \]
Net pay per period:
\[ \text{Net}_p = P - d_{pre} - \text{FIT}_p - \text{SS}_p - \text{Med}_p - \text{State}_p - d_{post} \]
Glossary of variables
- Pay type / frequency: Salary (annual) or Hourly × hours × weeks; frequency determines periods/year.
- Filing status: Single, Married filing jointly, Married filing separately, Head of household.
- Multiple jobs: W-4 Step 2(b) adjustment that increases annualized taxable wages per 15-T to reduce under-withholding.
- Dependents credit: W-4 Step 3 credit amount (e.g., $2,000 per qualifying child <17, $500 other dependents) converted to per-pay credit.
- Pre-tax deductions: 401(k)/403(b), HSA, Section 125 (health premiums, FSA). Reduce FIT and FICA wages as applicable.
- Post-tax deductions: After-tax items taken from net pay.
- State/local rate: Flat modeling % for combined state/local withholding (varies by jurisdiction).
How it works: a step-by-step example
Scenario. Single filer paid biweekly (26), salary $60,000; multiple jobs unchecked; dependents credit $0; 5% 401(k) and $0 other pre-tax; state/local 3%; extra withholding $0.
- Per-pay gross \(P = 60000/26 = \$2{,}307.69\).
- Pre-tax per pay \(d_{pre} = 5\%\cdot P = \$115.38\). Taxable wages per pay for FIT = \(P-d_{pre}=\$2{,}192.31\).
- Annualized FIT wages \(W_a = \$2{,}192.31\cdot 26 - SD_{single}\). Using 2025 \(SD= \$15{,}750\): \(W_a \approx \$41{,}250\).
- Apply 2025 brackets to \(W_a\) → compute annual FIT, divide by 26 → \(\text{FIT}_p\).
- FICA: OASDI \(6.2\%\) on wages up to the 2025 cap; Medicare \(1.45\%\) (no cap). Additional 0.9% does not apply at this income.
- State/local \(3\%\) of \((P-d_{pre})\).
- Net per pay \(=\) gross − pretax − FIT − OASDI − Medicare − state/local.
FAQs
How accurate is the federal withholding?
It follows the IRS 2025 percentage method from Pub. 15-T with standard deduction and W-4 options (Step 2(b), Step 3, Step 4(c)). Employers may use wage-bracket tables, custom rounding, or different pretax benefit handling, so small differences are normal. Sources: Pub. 15-T. :contentReference[oaicite:4]{index=4}
What Social Security wage base is used?
The 6.2% OASDI applies up to the 2025 wage base ($176,100). Source: SSA/IRS. :contentReference[oaicite:5]{index=5}
When does the 0.9% Additional Medicare Tax apply?
Above $200k (single/other), $250k (MFJ), $125k (MFS) annual Medicare wages. Source: IRS Topic 560. :contentReference[oaicite:6]{index=6}
Does it handle bonuses?
Yes—temporarily increase the pay period gross and recalc; supplemental flat rates vary by employer/state and are not modeled separately here.
State taxes vary—how should I model them?
Use the state/local % input. If your state has tiered brackets or credits, set an approximate effective rate or run multiple scenarios.
Tool developed by Ugo Candido. Content reviewed by the CalcDomain Editorial Board.
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