How Stivenza calculates paychecks
Reviewed by Colson, Founder, ColsonSuperApps LLC · Last updated June 1, 2026.
Our approach
Every figure on Stivenza is computed in your browser from 2026 tax rules using a purpose-built, open, unit-tested tax engine. Tax tables are stored as versioned data and validated against golden reference values in continuous integration — a wrong number fails the build before it can ship. We never estimate tax figures from memory.
What we model
Federal income tax (all filing statuses), the standard deduction, Social Security (6.2% up to the annual wage base), Medicare (1.45% plus the 0.9% Additional Medicare Tax), and each state's income tax (none, flat, or progressive). Pre-tax deductions are handled by their actual treatment: 401(k) reduces income-tax wages but not FICA wages; HSA, FSA, and health premiums reduce both.
What we don't model
Local/city/county income taxes, personal-exemption credits, itemized deductions, tax credits, and deduction phase-outs. These are disclosed on each calculator. Estimates are for guidance and will differ from your actual withholding and tax return.
Sources
- IRS Rev. Proc. 2025-32 — 2026 federal rate schedules & standard deduction
- SSA — Social Security contribution & benefit base
- IRS Topic No. 751 — Social Security & Medicare withholding rates
- Tax Foundation — 2026 State Income Tax Rates and Brackets
State figures are cross-checked against each state's Department of Revenue.