What an estimate is
Payroll withholding is the tax taken from each paycheck or payslip. It is an estimate of the tax for the year, not the final bill: the final amount is settled on the annual return (US, Canada, Australia) or through the tax code and PAYE (UK). Netpaysum calculates withholding the way an employer's payroll would, from the details you enter.
Sources by country
| Country | Income tax withholding | Social contributions | Tax year shown |
|---|---|---|---|
| United States | IRS Publication 15-T (2026), percentage method tables for automated payroll systems, using the 2020-or-later Form W-4 | Social Security and Medicare: IRS Publication 15 (2026); Social Security wage base: Social Security Administration | 2026 |
| US states | Each state's revenue department withholding tables for 2026 (named on each state page) | State payroll programs where they apply (for example disability or family leave contributions), from the state agency | 2026 |
| United Kingdom | HMRC "Rates and thresholds for employers 2026 to 2027"; Scottish rates from the Scottish Government | National Insurance: HMRC, same publication | 2026/27 |
| Australia | ATO tax tables 2026–27 (Schedule 1, statement of formulas) | Medicare levy and HELP repayment thresholds: ATO | 2026-27 |
| Canada | CRA T4127 Payroll Deductions Formulas (2026 edition); Quebec: Revenu Québec TP-1015.F-V (2026) | CPP / QPP and EI: CRA, Retraite Québec | 2026 |
The tables themselves are published on Data as CSV files, each row with its source and year.
How a result is built
- Gross pay per period. Salary divided by the number of pay periods, or hours × rate, with overtime where entered.
- Pre-tax deductions. Deductions that the source rules treat as pre-tax (for example traditional 401(k) contributions, salary sacrifice, net-pay pension) are taken off before income tax.
- Income tax withholding. The official formula for the country, state or province, with the filing status, tax code or claim amounts you enter.
- Social contributions. Social Security and Medicare, National Insurance, CPP/QPP and EI, or the Medicare levy, with their caps and thresholds.
- Other deductions. Student loan or HELP repayments, post-tax deductions and local taxes where the page supports them.
- Net pay. Gross pay minus everything above, per period, per month and per year.
How we test
Every formula has test cases built from the tax authority's own worked examples and official calculators. A table does not go live until those cases match. The test cases are kept with the formulas and re-run whenever a table changes.
What is not included
- Your final tax bill: credits and deductions claimed on the annual return are outside a withholding estimate, unless a page says it includes them.
- Employer-specific items: benefits in kind, taxable fringe benefits, garnishments, union dues or insurance premiums you do not enter.
- Local taxes on pages that do not list them.
- Rounding: payroll software rounds at different steps, so a real paycheck can differ by a small amount.
When the tables change
New tables are added when the tax authority publishes them for the new year. Until then, a page says which year it uses. Each change is logged on the changelog.
Corrections
Found a difference with your payslip or with an official calculator? Send the inputs to [email protected]. Confirmed errors are fixed and logged.
Estimates only. Not tax, legal or financial advice. Your employer's payroll may differ.