Methodology: Freelance Tax Estimator

1. Scope

Estimates US federal quarterly tax set-aside from freelance income, deductible expenses, and filing status. Federal-only. Not tax advice. For jurisdiction-specific planning, consult a licensed tax professional.

2. Inputs and outputs

Inputs

grossIncome number (currency/year)
deductibleExpenses number (currency/year)
filingStatus enum

single | married-joint | head-of-household.
stateTaxRate percent default: 0

Optional rough state rate.

Outputs

netSelfEmploymentIncome

gross − expenses.
selfEmploymentTax

92.35% × netSE × 15.3% (capped by SS wage base).
federalIncomeTax

Bracketed on taxable income after half-of-SE-tax and standard deduction.
quarterlySetAside

(SE tax + federal + state) / 4.

Engine source: src/lib/freelance-tax-estimator/engine.ts

3. Formula / scoring logic

net_se       = gross - expenses
se_tax       = net_se * 0.9235 * 0.153  (OASDI portion capped at SSA wage base)
federal_tax  = bracket_tax(taxable_income, filing_status)
quarterly    = (se_tax + federal_tax + state_tax) / 4

4. Assumptions

Uses 2024 US federal tax brackets and standard deduction.
Self-employment tax is computed on 92.35% of net SE income; OASDI portion caps at the SSA wage base ($168,600 in 2024).
State tax is a flat rate entered by the user — no progressive state brackets.

5. Data sources

US IRS — Self-Employment Tax (Pub. 334, Pub. 505) as of 2024
US IRS — 2024 tax brackets and standard deduction as of 2024
US SSA 2024 Social Security wage base as of 2024

6. Known limitations

Federal estimates only; state income tax uses a flat-rate approximation and misses local and franchise taxes.
Does not model QBI deduction, SEP-IRA / solo 401(k) contributions, HSA deductions, or credits.
Safe-harbour rules (110% of prior year's tax for higher earners) are not implemented — consult Pub. 505.

7. Reproducibility

Input
grossIncome = $80,000, expenses = $10,000, filingStatus = single, stateRate = 5%.

Expected output
net_se = $70,000, se_tax ≈ $9,890, fed ≈ $8,560, state ≈ $3,500, quarterly ≈ $5,490 (directional, 2024 brackets).

8. Change log

2026-04-24 methodology page first published.