Methodology

Tax Atlas calculates income tax by applying each country's official progressive bracket rates to a given gross income, then adding country-specific items such as tax-free allowances, credits, rebates, surcharges, payroll contributions, and levies. The calculation is deterministic: the same input always produces the same output. No AI estimation, no rounding guesses, no hidden assumptions. Bracket data comes exclusively from official government publications.

Every country page is reviewed by a credentialed tax professional in that jurisdiction before publication — a CPA, Chartered Accountant, or equivalent. The reviewer's name, credential, and review date appear at the bottom of each page. Pages in active review are labelled as preliminary estimates. These calculators are planning tools, not a substitute for professional advice.

Deterministic, client-side, pure functions

All calculations run in your browser. No income figure, no input, and no result is ever sent to any server. The engine is built from pure functions — every calculator is (input) => result with no side effects and no module-level state. The same gross income will always produce the same output, regardless of when or where you use it.

Progressive bracket math

Income is applied to each bracket in sequence. Only the portion of income within a bracket is taxed at that bracket's rate — you are never taxed on your full income at the highest rate that applies to any portion. After bracket tax is computed, country-specific items are applied in order: personal allowances or credits are deducted, then surcharges, rebates, health levies, payroll contributions, and offsets — each in the sequence mandated by that jurisdiction's tax rules.

Country-specific items

Standard bracket math handles income tax. Country-specific items are implemented as additional computation steps on top:

• UK: personal allowance taper (£100k rule), Class 1 National Insurance
• India: Section 87A rebate with marginal relief, 4% Health & Education Cess
• Canada: non-refundable Basic Personal Amount credit (federal and Ontario), Ontario surtax, CPP1/CPP2, Employment Insurance, Ontario Health Premium
• Australia: Low Income Tax Offset (LITO) against income tax only, Medicare Levy with shade-in, Medicare Levy Surcharge (MLS), HELP/HECS repayments

Marginal rate via numerical derivative

The displayed marginal rate is computed as tax(income + 1) − tax(income). This numerical derivative approach — unusual among free calculators — correctly captures every non-linearity: the UK's 60% effective rate in the £100k–£125,140 taper zone, India's Section 87A cliff and marginal relief ramp, Australia's Medicare Levy shade-in, MLS tier cliffs, and any other phase-out that would be invisible to a bracket-lookup approach.

Rounding

Internal calculations use full floating-point precision. Display values are rounded to whole currency units (nearest dollar/pound/rupee) except where a jurisdiction's official rules require otherwise. India's Health & Education Cess is applied at full precision before rounding, per Income Tax Department guidance. US bracket arithmetic follows IRS Form 1040 instructions for whole-dollar computations.

Every country calculator is reviewed by a credentialed tax professional in that jurisdiction before being marked final. The review is not a spot-check — it covers the full scope of what the calculator computes.

What reviewers check

• Bracket thresholds and marginal rates against the primary government source
• Personal allowances, standard deductions, and credit amounts
• Country-specific features: rebates, offsets, surcharges, levies, phase-outs
• Phase-out math and cliff/taper correctness at boundary incomes
• Plain-language explanations on the page for accuracy and clarity
• FAQ answers for correctness

Credential standards by country

• United States: Certified Public Accountant (CPA) or Enrolled Agent (EA)
• United Kingdom: Chartered Tax Adviser (CTA) or ICAEW Chartered Accountant
• India: Chartered Accountant (ICAI member)
• Canada: Chartered Professional Accountant (CPA Canada member)
• Australia: CA ANZ member or CPA Australia member
• New Zealand: CA ANZ (Chartered Accountants Australia and New Zealand) or Tax Agent registered with IRD
• Ireland: Chartered Tax Adviser (CTA) of the Irish Tax Institute (ITI) or Chartered Accountant Ireland (CAI) practicing personal tax
• Singapore: Accredited Tax Practitioner (ATP) or Accredited Tax Advisor (ATA) — Singapore Chartered Tax Professionals (SCTP, formerly SIATP) — or Chartered Accountant of Singapore (CA Singapore) from ISCA with personal tax practice
• South Africa: CA(SA) from the South African Institute of Chartered Accountants (SAICA), or SAIT Master Tax Practitioner (MTP-SA) / General Tax Practitioner (GTP-SA) from the South African Institute of Tax Professionals, practicing personal income tax

Current review status

Every country calculator page has been reviewed by a credentialed tax professional in that jurisdiction. Reviewer names, credentials, and review dates appear at the bottom of each country page. See the Reviewers page for the full panel.

Tax rates change every year. Tax Atlas tracks each country's budget and finance bill cycle and updates calculators on the following schedule:

Annual updates: each calculator is updated when new brackets and thresholds take effect, within 14 days of the start of the new tax year.

Mid-year updates: when a country's parliament or government passes legislation that changes rates after the tax year has begun (e.g. a supplementary budget or emergency amendment), the calculator is updated within 14 days of the change taking legal effect.

Corrections: errors reported via email ( hello@taxatlas.app) or identified through user feedback are reviewed within 3 business days and corrected if validated.

If you believe a calculation result is wrong, we want to know. Accuracy is central to this product. Every report is reviewed by someone who can evaluate it — not an automated system.

Response SLA: acknowledgment within 1 business day; fix or explanation within 5 business days if the report is validated.