Our Calculation Methodology
Transparency is central to WhatsUK. Here is exactly how we source data, build calculators, test results, and keep every tool accurate and current for the 2026/27 tax year.
Rates last verified:
Data Sources
All calculations on WhatsUK use the official rates and thresholds published by HM Revenue & Customs (HMRC) for the 2026/27 tax year (6 April 2026 to 5 April 2027). We use only primary government sources - never third-party summary sites, financial media, or community-contributed figures.
The specific HMRC publications used as the authoritative source for each calculator category are listed below, with direct links so you can verify the figures yourself.
Income Tax rates and allowances
gov.ukPersonal allowance (£12,570), basic rate (20%), higher rate (40%), additional rate (45%), personal allowance taper above £100,000.
National Insurance rates
gov.ukClass 1 employee and employer rates, primary threshold (£12,570), secondary threshold (£5,000), upper earnings limit (£50,270).
Corporation Tax rates
gov.ukSmall profits rate (19% on profits up to £50,000), main rate (25% on profits above £250,000), marginal relief fraction (3/200).
Capital Gains Tax rates
gov.ukBasic rate (18%), higher rate (24%) for all chargeable assets, including residential property and other assets.
Stamp Duty Land Tax (SDLT)
gov.ukStandard rate bands, first-time buyer relief (0% up to £300,000), 5% band to £500,000, no relief above £500,000), additional property surcharge (5%).
Student Loan repayment thresholds
gov.ukPlan 1, Plan 2, Plan 4, Plan 5, and Postgraduate Loan repayment thresholds and 9%/6% rates.
VAT rates
gov.ukStandard rate (20%), reduced rate (5%), zero rate, registration threshold (£90,000).
For the complete list of all sources used across every calculator, see our Data Sources page. For how we review and update articles, see our editorial guidelines.
How We Build Calculators
Every WhatsUK calculator starts with the official HMRC legislation. Before any code is written, James Hartley reads the relevant HMRC guidance, Finance Act provisions, and worked examples published on GOV.UK. All rates and thresholds are then stored in a single version-controlled tax config file. This means when rates change in April, every calculator on the site updates simultaneously from one edit - eliminating the risk of inconsistencies between tools.
Our build process:
- Read official HMRC guidance and Finance Act for the relevant tax year
- Record all thresholds, rates, and edge cases with source citations
- Implement calculation logic with full type safety (TypeScript)
- Write the formula explanation in plain English for the results panel
- Test against HMRC worked examples and known edge cases
- Review by a second financial professional where available
- Mark the calculator with its "last updated" date before publication
How We Calculate Each Category
Salary & PAYE
Salary calculations use the 2026/27 PAYE tax bands with personal allowance (£12,570), basic rate band (£12,571 to £50,270 at 20%), higher rate band (£50,271 to £125,140 at 40%), and additional rate (above £125,140 at 45%). The personal allowance is tapered by £1 for every £2 of income above £100,000. National Insurance is calculated using the primary threshold (£12,570) and upper earnings limit (£50,270) with 8% and 2% rates respectively. Employer NI uses the Employment Allowance of £10,500 where eligible. Scottish income tax uses six bands from £12,571 (19% starter) through to 48% top rate above £125,140 on earned and pension income.
Property (SDLT, Mortgage, Rental)
Stamp Duty Land Tax uses the current SDLT rate bands for residential property in England and Northern Ireland, including first-time buyer relief (0% up to £300,000, 5% between £300,001 and £500,000, no relief above £500,000 from April 2026) and the 5% additional property surcharge. Mortgage calculations use standard amortisation formulas. Rental yield is calculated as annual rent divided by property value.
Business & Contractor
Corporation tax uses the 2026/27 small profits rate (19% on profits up to £50,000), main rate (25% on profits above £250,000), and marginal relief for profits between £50,000 and £250,000 using the fraction 3/200. IR35 calculations use HMRC's deemed payment formula to calculate the deemed employment payment and resulting PAYE/NIC liability. CIS uses the three deduction rates: 0% (gross payment status), 20% (registered subcontractor), 30% (unregistered).
Finance & Savings
Compound interest calculations use the standard formula A = P(1 + r/n)^(nt). Student loan repayments use the income-based thresholds and rates for each plan type. Pension calculations use auto-enrolment minimum contribution rates (8% total, 5% employee, 3% employer) based on qualifying earnings. Inflation adjustments use the Bank of England's CPI inflation series from the ONS.
Accuracy and Updates
All calculator results are verified against HMRC's own online calculators and published worked examples where available. Calculators that cannot be directly verified against an HMRC example are cross-checked against the results produced by professional-grade payroll software.
Rate updates: Calculations are verified against HMRC's own online calculators where available, and rates are updated within 48 hours of official HMRC announcements each April. We maintain a checklist of every rate-dependent calculator and systematically review each one when a new tax year begins on 6 April.
Cross-verification with HMRC examples
We input the exact figures from GOV.UK worked examples and confirm our output matches to the penny.
Edge case testing
Salary at £100,000 (personal allowance taper begins), £125,140 (allowance fully withdrawn), Scottish rate boundaries, first-time buyer SDLT at £300,000 and £500,000, and corporation tax marginal relief at £50,000 and £250,000.
Automated unit tests
Core calculation functions are covered by automated test suites that run on every code change, preventing regressions when rates are updated.
Annual full review
Every April, every calculator is reviewed in full - not just updated for changed rates, but checked for legislative changes that might affect calculation logic.
Update Schedule
UK tax rates change every April following the Spring Budget. WhatsUK maintains a strict update schedule to ensure calculators are always current.
| Trigger | Action | Target |
|---|---|---|
| April Budget / new tax year | Full rate review of all calculators and blog posts | Within 48 hours |
| Autumn Statement announcement | Targeted update to affected calculators | Within 48 hours |
| Mid-year HMRC guidance update | Targeted update to affected calculators | Within 72 hours |
| User error report | Immediate investigation; correction if confirmed | Within 24 hours |
| Annual rate confirmation | Verify unchanged rates still match GOV.UK | Every 6 April |
Limitations
WhatsUK calculators are designed to provide accurate estimates for standard situations, based on the official rates and thresholds published by HMRC. However, they may not account for every individual circumstance, and results should be treated as guidance rather than definitive tax advice.
Situations that may produce different results from our calculators include: multiple sources of income, non-standard tax codes, Marriage Allowance or Blind Person's Allowance, pension lump sums, Employment and Support Allowance, High Income Child Benefit Charge, non-UK residency, offshore income, or complex trust arrangements.
We strongly recommend consulting a qualified accountant or tax adviser for any complex tax situation. For straightforward questions, HMRC's helpline (0300 200 3300) and the free GOV.UK guidance pages are also reliable resources.
Important
WhatsUK calculators provide estimates for guidance only. Your individual tax position may differ. Always consult HMRC or a qualified accountant for binding advice. Read our full disclaimer →
Questions About Our Methodology?
If you've spotted a discrepancy or want to understand how a specific calculation works, James personally responds to every report.
Contact James Hartley