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Assumptions

The v1 calculator models a deliberately narrow scenario. This page is the contract between the calculation and the reader — every assumption is here, every exclusion is here, nothing is hidden in a footer.

Modeled scenario

Every calculator on this site assumes:

  • Employment income only (not business profit, not contractor)
  • Single filer or the closest jurisdictional default
  • Full-year tax residence in the modeled jurisdiction
  • No dependents, unless the jurisdiction has a universal allowance
  • Standard or basic deductions and allowances only
  • Employee-side mandatory social contributions
  • National tax plus state or provincial tax where the platform explicitly supports it

These choices make country-to-country comparisons understandable. Additional filing statuses, dependents, self-employment, contractor structures, special expatriate regimes, and local taxes should be added only as separate, tested modules.

Excluded by design

  • Tax treaties (cross-border income)
  • Wealth, capital-gains, property, inheritance, church, municipal, special expatriate, or employer-side taxes — unless separately modeled and clearly labeled
  • Self-employment tax (Schedule SE in the US; Class 2 / Class 4 NI in the UK)
  • Pre-tax elections such as 401(k), HSA, FSA, or UK pension salary sacrifice
  • Education credits, Saver's Credit, EITC, Child Tax Credit, or other refundable credits beyond the standard deduction
  • Alternative Minimum Tax
  • Married filing jointly / dependents / head-of-household scenarios (single-filer model only)
  • Stock options, RSUs, bonuses above standard withholding rules

Jurisdiction-specific assumptions

United States (federal)

  • Standard deduction only — no itemized deductions
  • Social Security wage base ($184,500 in 2026) enforced as a cap
  • Additional Medicare Tax (0.9%) applies on wages above $200,000
  • No state income tax modeled — see separate state calculators (in research)
  • The new $6,000 senior deduction (OBBBA 2025) is not modeled in v1

United Kingdom (England / Wales / Northern Ireland)

  • Standard Personal Allowance (£12,570) — the £100,000–£125,140 taper is not enforced in v1
  • Income Tax + Class 1 employee NI for Category A (standard) only
  • Scottish Income Tax bands (six rates from 19% to 48%) are not modeled — Scottish users need a separate calculator
  • Marriage Allowance, Blind Person's Allowance, and Dividend Allowance are not modeled
  • Student-loan repayment thresholds are not modeled
  • Pension tax relief beyond the standard Personal Allowance is not modeled

Currency and rounding

  • All amounts are calculated in the jurisdiction's statutory currency (USD, GBP, …).
  • Internal calculations use 28-digit decimal precision; display values round to the minor unit (0.01) using banker's rounding.
  • Cross-country comparisons use a documented exchange-rate snapshot, not a live feed.

Status labels

Each fact record carries one of these statuses:

  • Reviewed — primary source retrieved, calculation tested against official rate tables.
  • Awaiting annual update — past data release is correct but a new tax year is in research.
  • Limited model — published with known exclusions documented (e.g., excluded deductions or credits).
  • Draft — not public; held back until evidence and review are sufficient.

Not tax advice

TaxMetria is an educational tool. It is not tax advice, not financial advice, and not a substitute for a qualified professional. If you have a tax question that matters, consult a chartered tax adviser (UK) or an enrolled agent / CPA (US).

See methodology for how the engine consumes these assumptions, and sources for the per-jurisdiction source registry.