Calculation Methodology

How every formula on SalaryCalcPro is built, which official sources feed it, and where we know the limits are.

Every calculator on SalaryCalcPro runs in your browser. Inputs you enter never leave your device. The formulas below describe exactly what happens when you press “Calculate”.

1. Salary Calculators (Gross ↔ Net)

United States (Federal)

Source: IRS Publication 15-T (Federal Income Tax Withholding Methods) for employer withholding, and the annual Rev. Proc. inflation-adjustment notice for brackets and standard deduction. FICA thresholds from the Social Security Administration.

Formula:

1. Annualize the input pay (weekly × 52, biweekly × 26, monthly × 12).
2. Subtract pre-tax deductions (traditional 401(k), HSA, Section 125 health premiums).
3. Apply federal withholding using the percentage-method tables from Publication 15-T matching filing status and W-4 step-2 checkbox state.
4. Apply Social Security (6.2% up to the annual wage base) and Medicare (1.45% on all wages; additional 0.9% above $200,000 single / $250,000 married).
5. Deannualize the result to the input pay frequency.

United States (State)

Source: Each state's department of revenue. Per-state tables (California FTB DE 44, New York NYS-50-T, etc.). For states with no income tax (AK, FL, NV, SD, TN, TX, WA, WY), state income tax returns $0.

Costa Rica

Sources: Ministerio de Hacienda (Impuesto sobre la Renta — tabla de asalariados), CCSS (cargas sociales del trabajador), MTSS (salarios mínimos, aguinaldo, liquidación).

Formula (salario neto): Salario bruto − cargas CCSS (SEM, IVM, Banco Popular, aporte obrero total 10.67%) − impuesto sobre la renta por tramos (tabla Hacienda). Aguinaldo calculado por el art. 1 de la Ley 2412 y liquidación por los artículos 28–30 del Código de Trabajo.

Mexico

Sources: SAT (tabla de ISR mensual, subsidio al empleo), IMSS (cuotas obrero-patronales), INFONAVIT.

Formula: ISR calculated using the tariff table and subsidio al empleo published by SAT for the current fiscal year. IMSS contributions apply the current Cuota Fija, excedente, prestaciones en dinero, invalidez y vida, cesantía y vejez, and guarderías percentages.

Colombia

Sources: DIAN (retención en la fuente), Ministerio del Trabajo (seguridad social, parafiscales).

Spain

Sources: AEAT (IRPF), Seguridad Social (cotizaciones).

2. Tax Estimators

Tax estimators reconstruct the year-end liability using the official brackets, standard or itemized deductions, and applicable credits for the selected jurisdiction and year. They are estimators — they do not replace filing, do not handle every credit or adjustment, and cannot account for circumstances we do not ask about.

Known simplifications, by design:

• We use the standard deduction unless the user explicitly chooses to itemize.
• We do not model the Alternative Minimum Tax (AMT) in US federal calculations unless flagged as an advanced option.
• We do not apply local / municipal taxes below the state level (e.g., NYC local income tax) unless the calculator's description explicitly says so.
• In Latin American jurisdictions, we do not model professional-fees withholdings (honorarios), only employee salary withholdings.

3. Investment & Retirement Calculators

Compound interest. Standard formula A = P × (1 + r/n)^nt with periodic contributions applied at the start or end of each compounding period as selected. No tax on growth is assumed unless the calculator explicitly models a taxable account.

Retirement projections. Deterministic single-rate projection by default. Results are scenarios, not guarantees. We do not run Monte Carlo simulation by default; when we do, we disclose the number of iterations and the assumed return distribution.

Default assumption ranges: Inflation 2–3%. Long-run equity return 6–8% nominal. Bond return 3–4% nominal. These are editable defaults, not forecasts.

4. Credit & Loan Calculators

Amortization. Standard fixed-rate amortization formula PMT = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1]. Schedules are generated month-by-month with rounding applied to the final payment. Principal, interest, and remaining balance are reported for each period.

Adjustable-rate loans. We let you model rate-adjustment scenarios but we do not predict future rates.

Credit-card payoff. Minimum-payment, fixed-payment, and avalanche / snowball strategies are supported. APR is assumed constant; promotional 0% APR periods can be entered explicitly.

5. Budget Calculators

Budget allocators apply user-entered after-tax income to common frameworks (50/30/20, zero-based). They do not prescribe a “correct” budget; they illustrate what a chosen framework would allocate.

6. Currency & Rounding

Results are displayed rounded to two decimals for currency and integer for whole-dollar / peso / colón values where appropriate. Internal calculation uses full floating-point precision; rounding is applied only at display time. When a calculation is sensitive to rounding (amortization tables), we round the final payment to reconcile the total — not intermediate payments.

7. Known Limits — Read Before Relying on a Result

• Our calculators are estimators. They do not replace filing with a tax authority or consulting a licensed professional.
• We model common scenarios, not edge cases. Non-resident taxation, dual-residency, treaty relief, equity compensation (ISO/NSO/RSU), and estate / inheritance tax are out of scope unless a calculator explicitly addresses them.
• Tax law changes frequently. A calculator may be correct today and wrong tomorrow if a government publishes mid-year changes before we update. See our update cadence for how we respond.
• For anything with material money or legal consequences, consult a licensed tax professional, accountant, or financial advisor.

8. Source Audit

We retain the official source (URL, publication ID, effective date) for every figure on the site. If you want to audit a specific number, email our contact form with the page URL and the figure in question; we will respond with the underlying source.