Calculation Methodology
Every formula, every source, every exclusion — documented in full. This page explains exactly how the calculator works and where its numbers come from.
Data current as of: March 2026
Step-by-Step Calculation Order
The calculator follows the exact deduction sequence used by federal payroll systems. Getting this order right is critical because different deductions affect different tax bases.
Step 1: Determine Gross Biweekly Pay
The calculator looks up the annual salary from OPM's locality pay table for the selected grade, step, and locality area. If the annual salary exceeds the Executive Level IV pay cap ($197,200 for 2026), the cap is applied. Gross biweekly pay is calculated as:
Gross Biweekly = min(Annual Salary, $197,200) ÷ 26
Step 2: Calculate Pre-Tax Deductions
The following deductions are subtracted from gross pay, but they have different effects on the two tax bases used for subsequent calculations:
| Deduction | Reduces Federal Taxable Income? | Reduces FICA Wages? |
|---|---|---|
| FERS Contribution | Yes | No |
| Traditional TSP | Yes | No |
| Roth TSP | No (post-tax) | No |
| FEHB Premium (Premium Conversion) | Yes | Yes |
| FEGLI Premium | Yes | No |
FEHB premiums reduce both federal taxable income AND FICA wages (under Premium Conversion, 26 U.S.C. §125). FERS contributions and TSP contributions reduce federal taxable income but do not reduce FICA wages. Getting this wrong would produce incorrect Social Security and Medicare tax calculations.
The two resulting tax bases are:
Federal Taxable Wages = Gross − FERS − Traditional TSP − FEHB − FEGLI
FICA Taxable Wages = Gross − FEHB only
Step 3: Calculate FICA Taxes
Applied to FICA Taxable Wages (not Federal Taxable Wages):
- Social Security: 6.2% of FICA Taxable Wages, stopping when year-to-date FICA wages reach $184,500. Authority: 26 U.S.C. §3101(a); wage base per SSA.gov.
- Medicare: 1.45% of all FICA Taxable Wages (no cap). Authority: 26 U.S.C. §3101(b).
- Additional Medicare: 0.9% on FICA wages exceeding $200,000 (single) or $250,000 (married filing jointly). Authority: 26 U.S.C. §3101(b)(2); IRS Topic 751.
Step 4: Calculate Federal Income Tax
The calculator uses an annualized approach:
- Start with biweekly Federal Taxable Wages (from Step 2).
- Annualize: multiply by 26 pay periods.
- Subtract the standard deduction for the selected filing status ($16,100 single / $32,200 MFJ / $24,150 HoH — per IRS Rev. Proc. 2025-32).
- Apply the 2026 progressive tax brackets (post-OBBBA, P.L. 119-21).
- De-annualize: divide annual tax by 26 to get biweekly withholding estimate.
Actual payroll systems use the IRS Publication 15-T percentage method for biweekly withholding, which can produce slightly different per-paycheck amounts than the annualized method. Typical difference: $0–40 per month. The annualized method produces accurate annual tax liability estimates.
Step 5: Calculate State Income Tax
State income tax is calculated using the state-specific brackets or flat rate for the selected state. Taxable income for state purposes is approximated using the federal taxable income adjusted for the state's own standard deduction where applicable. States with no income tax (AK, FL, NV, NH, SD, TN, TX, WA, WY) return $0.
Step 6: Calculate Net Pay
Net Biweekly = Gross − FERS − TSP (Traditional + Roth) − FEHB − FEGLI − Social Security Tax − Medicare Tax − Federal Tax − State Tax
FERS Contribution Rates
Federal employees under FERS contribute a percentage of basic pay to the FERS pension fund. The rate depends on hire date:
| Tier | Rate | Hire Date | Authority |
|---|---|---|---|
| FERS (Original) | 0.8% | Before January 1, 2013 | 5 U.S.C. §8422(a) |
| FERS-RAE | 3.1% | Jan 1, 2013 – Dec 31, 2013 | P.L. 112-96 |
| FERS-FRAE | 4.4% | On or after January 1, 2014 | P.L. 113-67 |
Rehired employees with 5 or more years of prior FERS-creditable civilian service may be covered under an earlier FERS tier regardless of their most recent hire date. This is governed by 5 CFR Part 841 and OPM Benefits Administration Letters 13-102, 14-102, and 14-107. If you are a rehired employee, check with your agency HR office to confirm which tier applies to you.
The House Committee on Oversight originally proposed increasing FERS contributions to 4.4% for all tiers (Section 90001 of the HOGR committee print). This provision was removed by the House Rules Committee before passage on May 22, 2025. It was never enacted. Rates remain as listed above. Source: CRS IF13020, NARFE analysis July 2025.
TSP Government Match Formula
The calculator uses the tiered TSP match formula (not a simple 5% match). The formula per 5 U.S.C. §8432(c):
| Employee Contributes | Agency Auto (1%) | Agency Match | Total Agency |
|---|---|---|---|
| 0% | 1% | 0% | 1% |
| 1% | 1% | 1% | 2% |
| 2% | 1% | 2% | 3% |
| 3% | 1% | 3% | 4% |
| 4% | 1% | 3.5% | 4.5% |
| 5%+ | 1% | 4% | 5% |
Even if an employee contributes 0%, the agency contributes 1% automatically. The match structure means the first 3% of employee contributions are matched dollar-for-dollar, while contributions from 3–5% are matched at 50 cents on the dollar. Contributions above 5% receive no additional match.
Traditional vs. Roth TSP
Traditional TSP contributions are pre-tax — they reduce your current federal taxable income and therefore your current federal income tax. Roth TSP contributions are after-tax — they do not reduce your current taxable income. Neither type reduces FICA wages.
The calculator correctly applies this distinction: Traditional TSP reduces the Federal Taxable Wages used to calculate federal income tax; Roth TSP does not.
2026 TSP Contribution Limits
Per IRS Notice 2025-67 and TSP Bulletin 25-3:
- Elective deferral limit: $24,500 (IRC §402(g))
- Standard catch-up (age 50+): $8,000
- Super catch-up (ages 60–63): $11,250 (SECURE 2.0 Act §109)
- Ages 64+: Reverts to standard catch-up limit ($8,000)
FEHB Premium Conversion
Under FEHB Premium Conversion (the default for most employees), FEHB premiums are deducted pre-tax under a Section 125 cafeteria plan (26 U.S.C. §125). This reduces both federal taxable income and FICA wages — making FEHB premiums uniquely important in the deduction order.
The calculator accepts user-entered biweekly FEHB premium amounts because premiums vary significantly by plan. Default values use OPM's weighted average enrollee share for reference. The OPM Plan Comparison Tool (opm.gov) provides exact premiums by plan.
Federal Income Tax Brackets (2026)
The calculator uses the 2026 federal income tax brackets as published by the IRS in Revenue Procedure 2025-32, reflecting the One Big Beautiful Bill Act (P.L. 119-21, signed July 4, 2025) which permanently extended the Tax Cuts and Jobs Act rate structure. Authority: IRC §1(j).
| Rate | Single | Married Filing Jointly | Head of Household |
|---|---|---|---|
| 10% | $0 – $12,400 | $0 – $24,800 | $0 – $17,700 |
| 12% | $12,401 – $50,400 | $24,801 – $100,800 | $17,701 – $67,200 |
| 22% | $50,401 – $105,700 | $100,801 – $211,400 | $67,201 – $105,700 |
| 24% | $105,701 – $201,775 | $211,401 – $403,550 | $105,701 – $201,775 |
| 32% | $201,776 – $256,225 | $403,551 – $512,450 | $201,776 – $256,225 |
| 35% | $256,226 – $640,600 | $512,451 – $768,700 | $256,226 – $640,600 |
| 37% | Over $640,600 | Over $768,700 | Over $640,600 |
Standard Deductions (2026)
| Filing Status | Standard Deduction |
|---|---|
| Single | $16,100 |
| Married Filing Jointly | $32,200 |
| Head of Household | $24,150 |
Source: IRS Revenue Procedure 2025-32. Standard deductions confirmed from IRS.gov announcement of 2026 inflation adjustments.
State Income Tax
The calculator includes state income tax calculations for the following states and jurisdictions, using rates verified against each state's department of revenue and cross-referenced with the Tax Foundation 2026 State Income Tax Rates report:
Graduated tax states: DC, VA, MD, CA, NY, OH (flat above exemption amount).
Flat tax states: GA (5.09%), CO (4.4%), PA (3.07%), IL (4.95%), NC (3.99%), AZ (2.5%).
No income tax states: AK, FL, NV, NH, SD, TN, TX, WA, WY.
The calculator uses simplified state standard deductions and does not include local or county taxes. Notable omissions: Maryland county income taxes (typically 2.25%–3.20%), New York City income tax (3.078%–3.876%), and states not listed above (which return $0 state tax). See the full exclusion list below.
FEGLI Basic Life Insurance
When FEGLI Basic is selected, the calculator uses a simplified premium rate. Actual FEGLI Basic premiums vary by age band. The Basic Insurance Amount (BIA) is the employee's annual salary rounded up to the next $1,000, plus $2,000. The calculator uses the biweekly rate of $0.15 per $1,000 of BIA (under-35 rate per OPM). Actual rates increase with age.
FEGLI premiums reduce federal taxable income but do not reduce FICA wages. Authority: 5 U.S.C. Chapter 87.
Source Authority Table
Every number used by the calculator must trace to one of these sources:
| Data | Primary Source | Legal Authority | Update Cycle |
|---|---|---|---|
| GS Base Pay | OPM Pay Tables | 5 U.S.C. Chapter 53 | January |
| Locality Rates | OPM Locality Tables | 5 U.S.C. §5304 | January |
| Pay Cap (Exec. Level IV) | OPM Executive Schedule | 5 U.S.C. §5304(g)(1) | January |
| FERS Original (0.8%) | OPM FERS Info | 5 U.S.C. §8422(a) | Monitor legislation |
| FERS-RAE (3.1%) | OPM | P.L. 112-96 | Monitor legislation |
| FERS-FRAE (4.4%) | OPM | P.L. 113-67 | Monitor legislation |
| TSP Match Formula | TSP.gov | 5 U.S.C. §8432(c) | Monitor legislation |
| TSP Limits (2026) | TSP Bulletin 25-3 | IRC §402(g); SECURE 2.0 §109 | November |
| SS Rate + Wage Base | SSA.gov | 26 U.S.C. §3101(a), §3121(a)(1) | October |
| Medicare Rate | IRS Pub 15 | 26 U.S.C. §3101(b) | Annual |
| Additional Medicare | IRS Topic 751 | 26 U.S.C. §3101(b)(2) | Annual |
| Federal Tax Brackets | IRS Rev. Proc. 2025-32 | P.L. 119-21 (OBBBA), IRC §1(j) | January |
| State Tax Rates | Each state Dept. of Revenue | Varies by state | January |
| FEHB Premiums | OPM Premiums Page | 5 U.S.C. Chapter 89 | October |
| OBBBA Deductions | IRS.gov FS-2025-03 | P.L. 119-21 | Monitor (expire 2028) |
| SECURE 2.0 TSP Rules | TSP Bulletin 25-3 | SECURE 2.0 §§109, 603 | Monitor |
What This Calculator Does Not Include
The following factors are not included in the calculator's estimates. This is the complete exclusion list.
Pay Adjustments Not Included
- Overtime pay, holiday pay, night differential, Sunday premium pay
- Hazard pay, hardship duty pay
- Special Salary Rate (SSR) tables — the calculator uses standard GS pay tables only
- Salary offset for overpayments
- Mid-year changes to grade, step, locality, or tax rates
Deductions Not Included
- FEDVIP dental and vision insurance premiums
- FSA (Flexible Spending Account) contributions
- HSA (Health Savings Account) contributions
- Union dues
- Garnishments, child support, alimony
- Military deposit service credit payments
- TSA/tax-sheltered annuity (403(b)) contributions
- Charitable contributions via CFC (Combined Federal Campaign)
- Allotments (savings bonds, etc.)
OBBBA Tax Provisions Not Implemented (P.L. 119-21, 2025–2028)
- No-tax-on-overtime deduction — up to $12,500 single / $25,000 MFJ, phases out above $150,000 / $300,000 MFJ
- No-tax-on-tips deduction — up to $25,000 for qualifying occupations
- Additional senior deduction — $6,000 for age 65+, phases out above $75,000 single / $150,000 MFJ
- Enhanced child tax credit — $2,200 per qualifying child (2025–2028)
SECURE 2.0 Act Provisions Not Implemented
- Super catch-up TSP contributions for ages 60–63 ($11,250 vs. standard $8,000 catch-up). SECURE 2.0 §109.
- Mandatory Roth catch-up for employees whose prior-year FICA wages exceeded $150,000 (effective January 1, 2026). SECURE 2.0 §603.
- TSP Roth in-plan conversion
Tax Calculation Simplifications
- W-4 additional withholding and adjustments not modeled
- Itemized deductions not supported (standard deduction only)
- FEGLI uses simplified under-35 premium rate; actual rates vary by age band
- State tax uses simplified state standard deductions
- No local or county taxes — notably Maryland county taxes (2.25%–3.20%) and NYC income tax (3.078%–3.876%)
- States not in the V1 dataset return $0 state tax (26 states + DC covered in V1)
- Federal withholding uses annualized method, not IRS Pub 15-T exact biweekly tables (typical difference: $0–40/month)
Other Exclusions
- FERS annuity supplement — affects retirement income, not current paycheck. Note: OBBBA §90001 eliminates the FERS annuity supplement effective January 1, 2028 for individuals not yet entitled (LEOs and mandatory retirement personnel exempt).
- Federal Wage System (WG/WL/WS) pay schedules
- Senior Executive Service (SES) pay
- Foreign Service pay
Within-Year Changes
The calculator assumes a single set of inputs for the entire calendar year. In practice, many factors can change mid-year: promotions or step increases change gross pay; open season changes modify FEHB premiums; TSP contribution percentages can be adjusted at any time; and reaching the Social Security wage base mid-year causes SS tax to stop.
The calculator handles the SS wage base cap correctly (showing when year-to-date wages would reach $184,500), but does not model other mid-year transitions. For employees who expect mid-year changes, run the calculator separately for each period and prorate as needed.
Data Verification Process
All data used by the calculator went through the following verification process before publication:
- Primary source retrieval: Every rate, bracket, and salary figure was obtained directly from the authoritative source (OPM, IRS, SSA, TSP.gov, or the relevant state department of revenue).
- Cross-validation: Values were cross-checked against at least one independent secondary source (Tax Foundation, FedWeek, FedSmith, CRS reports, or r/fednews community guides).
- Calculation testing: The complete deduction chain was tested with multiple representative scenarios spanning different grades, localities, FERS tiers, TSP levels, and filing statuses. All results were verified to $0.00 precision against independent hand calculations.
- Formula-level verification: The deduction order was verified against four independent sources including the r/fednews paycheck guide, FedWeek W-2 guide, and OPM Premium Conversion FAQ.