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.
The 2026/27 tax rate config was last checked against GOV.UK on . That check covers the rate tables listed below, not every figure on every page.
Data Sources
Where a calculator applies a statutory rate, threshold or band, that figure comes from the body that sets it: HM Revenue & Customs for tax and National Insurance, the Scottish Government for Scottish income tax, HM Land Registry for registration fees, NHS Employers and the NHS Business Services Authority for Agenda for Change pay and pension tiers, and so on for the 2026/27 tax year (6 April 2026 to 5 April 2027). We use primary publications rather than third-party summary sites.
Not every calculator has a rate table behind it. A mortgage, loan-to-value or savings projection runs a formula on the figures you enter, and the interest or growth rate is your assumption, not a published rate. Those pages name the model instead of a source, and link to the matching section under calculation methods below. We do not attach a government source to a page that does not use one.
Each calculator names its own sources next to the result and again in a short references list at the foot of the page, with a note of exactly which table or output each source supports. A source that backs the income tax line on a salary page does not, by itself, stand behind that page's pension or student loan figures.
The HMRC publications behind the shared tax rate config are listed below, with direct links so you can check 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
A calculator starts with the published rule: James Hartley reads the relevant HMRC guidance, Finance Act provision or scheme handbook and the worked examples on GOV.UK. Rates and thresholds are then stored in a single version-controlled tax config file, so a rate change is one edit that every calculator picks up at once rather than a separate change per tool.
Our build process:
- Read the published rule for the relevant year and record the exact source URL
- Record thresholds, rates and edge cases in the shared config with that citation
- Implement the calculation as a typed, pure function separate from the interface
- Cover it with automated tests, including fixtures taken from published worked examples
- Write the plain-English explanation of the formula and its assumptions
- Bind the page to its sources, or to a calculation method on this page, in the provenance map
- Set the page's "last updated" date at publication
Steps six and seven are enforced, not aspirational: the build fails if a calculator page has neither a named source nor a valid link to a method described below.
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.
Calculation Methods
Every calculator that is not driven by a rate table links to one of the models below. Each entry states what the model does and the assumptions that change the answer. If an assumption does not match your situation, the result will not either.
Your inputs. Calculators run entirely in your browser on the figures you type. Nothing is pre-filled from your circumstances, nothing is sent to a server, and no result is stored. Where a field carries a default, that default is labelled on the page and can be overwritten.
Statutory rate tables
Rates, thresholds and bands are held in a single version-controlled config per tax year and applied as marginal slices unless the legislation is a cliff edge. The calculator applies the table; it does not decide your tax position.
Assumptions
- One tax year at a time, with no mid-year rate changes modelled
- England, Wales and Northern Ireland rates unless the page offers a nation selector
- A single, standard allowance position with no unusual tax code adjustments
Amortised loan repayment formula
Monthly payment is the standard annuity formula, P x (r(1+r)^n) / ((1+r)^n - 1), where r is the interest rate you enter divided by twelve and n is the term in months. Interest is charged on the reducing balance each month.
Assumptions
- The interest rate you enter is fixed for the whole term
- Payments are monthly, equal, and made on time
- Fees, insurance and any early repayment charge are excluded unless you enter them
- This is not a quote and no lender's actual pricing is used
Interest-only and rolled-up interest
Interest is calculated on the outstanding balance for each period. Where interest is rolled up rather than serviced, it is added to the balance and itself attracts interest in later periods, so the debt compounds.
Assumptions
- The rate you enter applies unchanged for the whole period
- Roll-up compounds monthly unless the calculator states otherwise
- Arrangement, valuation and exit fees are only included if you enter them
- No lender's actual product terms, early repayment terms or no-negative-equity guarantee are modelled
Loan-to-value ratio
Loan-to-value is the loan divided by the property value, expressed as a percentage. Band boundaries shown are the tiers lenders commonly price at, not an offer.
Assumptions
- The property value you enter is the figure a lender's valuation would agree
- No product, rate or eligibility outcome is implied by a band
Affordability multiples and ratios
Results apply an income multiple or a share-of-income ratio to the figures you enter. These are widely used rules of thumb for orientation; a lender or landlord runs its own affordability assessment.
Assumptions
- The multiple or ratio is a general convention, not any particular lender's policy
- Credit history, committed expenditure and stress-rate testing are not modelled
Compound growth projection
Balances grow by A = P(1 + r/n)^(nt), with contributions added at the start of each period where the calculator offers them. Growth rates you enter are assumptions about the future, not forecasts.
Assumptions
- The growth or interest rate you enter is constant for the whole period
- Contributions continue unchanged unless you change them
- Charges, platform fees and tax on returns are excluded unless the page states otherwise
- Figures are nominal unless the page explicitly shows a real, inflation-adjusted result
Balance run-off schedule
The balance is stepped forward month by month: interest is added at the rate you enter, your payment is subtracted, and the schedule ends when the balance reaches zero.
Assumptions
- The interest rate and payment you enter stay the same each month
- No further borrowing is added to the balance
- Promotional periods, default charges and rate changes are not modelled
Rental yield and cash flow
Gross yield is annual rent divided by property value. Net yield deducts the running costs you enter. Cash flow deducts mortgage payments as well.
Assumptions
- The rent you enter is achieved with no void periods unless you enter a void allowance
- Costs are only those you enter; nothing is assumed on your behalf
- Tax treatment depends on how you hold the property and is shown separately where relevant
Net position arithmetic
The result adds the amounts you enter and subtracts the ones you enter as outgoings. There is no external data set behind it: the output is only as good as your inputs.
Assumptions
- Every figure comes from you; nothing is estimated from national averages
- Values are point-in-time and are not tracked or stored
Unit cost and usage model
Cost is usage multiplied by the unit price you enter, plus any standing charge. Where a default is offered it is labelled and can be overwritten with your own tariff.
Assumptions
- Usage is assumed steady across the period unless you vary it
- Any default unit price is a starting point, not your tariff
Pay period conversion
Hourly, weekly, monthly and annual figures are converted using the hours and weeks you enter. Where the calculator annualises, it states the number of weeks used.
Assumptions
- Hours worked are the same in every period unless you change them
- Conversions are arithmetic and do not reproduce a payroll system's period-by-period rounding
Body mass index formula
BMI is weight in kilograms divided by height in metres squared. Category boundaries follow NHS guidance.
Assumptions
- BMI does not distinguish muscle from fat and is a screening indicator only
- It is not a diagnosis and does not replace advice from a clinician
Annual estimates and rounding
Calculators model a full tax or contract year in one step. Money is carried at full precision through the calculation and rounded once for display, to the penny for exact amounts and to the nearest pound where a figure is labelled an estimate.
Assumptions
- Pay-period rounding in real payroll can make the yearly total differ by a small amount
- Monthly figures are an annual result divided by twelve, not a payroll month
- A part-year start or leaver date is not modelled unless the page offers it
Calculator test examples
The rows below are worked estimates from WhatsUK calculator models for the 2026/27 tax year (and matching AfC pay year where stated). They are taken from automated fixtures that must pass before a figure is listed. They are not a sample of real employees, not an independently reviewed national survey, and not government-published statistics.
Model version whatsuk-calc-examples-2026-27.r1 (released 2026-09-12). Evidence is marked as source-checked rates versus model-verified fixtures. No expert reviewer is claimed for these model outputs.
NHS annualised hours and Scotland’s 36-hour week
WhatsUK estimates NHS basic hourly rates with a retained annualised convention: full-time annual pay divided by 52.2 × standard weekly hours (1,957.5 hours in England, Wales and Northern Ireland at 37.5 hours/week). That is an estimate, not exact payslip hourly rounding.
In Scotland for 2026/27 the Agenda for Change full-time week is 36 hours. Whole-time equivalent (WTE) is contracted hours ÷ 36. A 0.5-WTE salary on Band 5 entry is therefore 18 ÷ 36 × the published full-time basic (£34,544 → £17,272 in the table). Using England’s 37.5-hour week would mis-state the fraction. The full-time band salary itself does not fall when the standard week shortens. See the NHS rows below and the NHS pay calculator.
Why the Personal Allowance taper changes the income-tax result
Above £100,000 of adjusted net income, the Personal Allowance is reduced by £1 for every £2 of income. At £110,000 the allowance falls to £7,570, so more of the salary sits in the 40% band. The England £110,000 row shows income tax £33,432, NI £4,210.60 and net £72,357.40 under the stated assumptions. Do not treat that net figure as a typical outcome for every higher earner.
| Model / year / nation | Inputs | Pension / loan assumptions | Expected result | Source / mechanics |
|---|---|---|---|---|
Income tax (EW/NI bands) whatsuk-calc-examples-2026-27.r1 | Gross taxable employment income £30,000; standard Personal Allowance | No pension contribution; no student loan; no Marriage Allowance or Blind Person's Allowance | Income tax £3,486 | PA £12,570; taxable £17,430 at 20% basic rate. Rates from HMRC Income Tax rates; arithmetic verified by WhatsUK unit fixtures (F01). Evidence: model-verified fixture. Ignores Scottish bands, non-standard tax codes, and other income sources. Worked estimate only. |
Income tax + Class 1 NI (PA taper) whatsuk-calc-examples-2026-27.r1 | Gross employment income £110,000; standard tax code; Class 1 employee NI | No pension; no student loan | Income tax £33,432; NI £4,210.60; net £72,357.40 | Personal Allowance taper: PA = max(0, £12,570 − floor((ANI − £100,000) / 2)). At £110,000 ANI, PA falls to £7,570, so more income is taxed at 40%. NI uses the primary threshold and upper earnings limit. Cite this when explaining why tax rises faster than gross between £100,000 and £125,140. Evidence: model-verified fixture. Assumes ANI equals gross employment income. Does not model pension salary sacrifice, High Income Child Benefit Charge, or Scottish rates. |
Dividend tax (beneficial PA allocation) whatsuk-calc-examples-2026-27.r1 | Ordinary income £50,000; dividends £1,000 | PA allocation policy: beneficial (allocate unused PA to dividends where it reduces total tax). Dividend allowance £500. | Dividend tax ≈ £82.23; ordinary tax £7,540; PA to ordinary £12,300; PA to dividends £270; total income tax ≈ £7,622.23 | WhatsUK default for this calculator is beneficial PA allocation (not salary-first). Rates 10.75% / 35.75% / 39.35% from HMRC. Fixture F02. Evidence: model-verified fixture. Does not include corporation tax on the company side, NI on salary, or director-specific PAYE timing. Allocation policy must be stated when comparing tools. |
Dividend tax (salary-first PA allocation) whatsuk-calc-examples-2026-27.r1 | Ordinary income £50,000; dividends £1,000 | PA allocation policy: salary-first (PA used against ordinary income before dividends). Dividend allowance £500. | Dividend tax £178.75 | Same inputs as the beneficial row; salary-first leaves more dividend income taxable after the £500 allowance, so dividend tax is higher. Included to make the allocation policy explicit (F02). Evidence: model-verified fixture. Illustrates policy sensitivity only. Not advice on which allocation HMRC applies to a specific return. |
NHS take-home (AfC Band 5 entry, part-time) whatsuk-calc-examples-2026-27.r1 | Scotland Band 5 pay point 1; 18 contracted hours/week; no HCAS | Pension membership off for this gross-pay fixture; no student loan | WTE 0.5; actual gross annual £17,272 (full-time basic £34,544) | Scotland’s Agenda for Change full-time week is 36 hours in 2026/27. WTE = contracted hours ÷ 36. A 0.5-WTE salary is therefore 18 ÷ 36 × £34,544. Using England’s 37.5-hour week here would mis-state the fraction. Full-time band salary does not fall when the standard week shortens. Evidence: model-verified fixture. Gross pay only in this row (pension off). Does not generalise to every NHS worker, unsocial-hours enhancements, or overtime. Not a sample of real employees. |
NHS hourly rate (annualised convention) whatsuk-calc-examples-2026-27.r1 | England Band 5 pay point 1; 37.5 hours/week (full-time); no HCAS | Pension off for this rate fixture; no student loan | Gross annual £32,073; annualised hourly ≈ £16.3847 (same hourly for 22.5h part-time on this band/point) | Retained annualised convention: hourly = full-time annual ÷ (52.2 × standard weekly hours). England 2026/27 uses 52.2 × 37.5 = 1,957.5 hours. This is an estimate, not exact NHS payslip hourly rounding. Evidence: model-verified fixture. Annualised hours are a modelling convention. Payslips may differ. Unsocial-hours and overtime enhancements are not included. |
Council tax (Band D, single adult discount) whatsuk-calc-examples-2026-27.r1 | England Band D; Band D charge override £2,392; single adult discount (25%) | N/A | After discount £1,794; monthly (12) £149.50; monthly (10) £179.40 | Single-person discount is 25% of the bill for that band. Override used so the fixture is independent of a specific local authority’s published Band D. Fixture F06. Evidence: model-verified fixture. Local Band D amounts vary by billing authority. Student exemption and disregarded-adult 50% reduction are separate rules and must not be conflated. |
Accuracy and Updates
Accuracy work on this site is code-based. Calculation functions are covered by automated tests that run on every change, and a number of those tests use fixtures taken from published worked examples. Where a rule has no published worked example, the test asserts the arithmetic against the rule as written; that is a weaker check and we do not describe it as verification. Public rows that have passed those tests are listed in Calculator test examples.
What we do not claim. There is no daily rate check, no automated monitoring of GOV.UK, and no standing panel of reviewers. A named person's review is recorded per source in our provenance data and shown on the page only where it has actually happened. If a page shows no review line, no review has been recorded for it.
Fixtures from published examples
Where HMRC or a scheme administrator publishes a worked example, its figures are encoded as a test fixture and the calculator must reproduce them.
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.
Build-time provenance checks
The build iterates every calculator route and fails if a page has no named source and no valid calculation-method link, or if it references a method section that does not exist.
How Updates and Corrections Work
WhatsUK is run by one person, so the update process is deliberately small enough to be real. Rate changes are applied when the change is announced and confirmed on the publisher's own page. Corrections are applied when someone reports a problem or a test catches one. Each is described below with the trigger that actually starts the work.
| Trigger | What happens |
|---|---|
| A Budget or new tax year changes a rate we hold | The shared tax config is updated from the publisher's page, the effective period is changed with it, and the affected tests are re-run |
| A reader reports a wrong figure | We check the number against the source named on that page, correct it if the report is right, and add an entry to the updates log |
| An automated test or build check fails | The calculation or the content is fixed before the change can be published |
| A source has no evidence recorded yet | The value is left in place, no verification is claimed for it, and the gap is tracked privately until evidence is added |
Rounding. Money is carried at full precision through a calculation and rounded once for display. Exact statutory amounts are shown to the penny. Anything labelled an annual estimate is rounded to the nearest pound, and monthly figures are an annual result divided by twelve rather than a payroll month, so a real payslip can differ by a small amount over a year.
Three different dates. The period a figure applies to, the date a source page was last checked, and the date the page was last edited are separate and shown separately. A page edit does not move a source check date, and a rebuild with no content change does not move any of them.
Reporting a correction. Every calculator carries a "Report a correction" link to our contact page, which reaches James directly. Tell us the page, the figures you entered and the result you expected. Confirmed numeric corrections are added to the updates log with the date they were applied.
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