Methodology

How the figures are sourced, how the calculators compute, what is tested and what is not covered.

Where every figure comes from

Every rate, threshold, allowance and date lives in one rules file per tax year. Each value carries the URL of the primary source it was read from (gov.uk, HMRC, the Scottish and Welsh governments or the Student Loans Company), the date it was checked, its status, and the path of an archived copy of the source page taken that day. The build fails if any value lacks a source, a date or an archive, and warns when a value was checked more than 180 days ago.

No figure is typed into a page. The calculators, the rates pages and the worked examples all read from the same file, so they cannot disagree. If a source cannot be found the field is left empty, marked as not set, and the tool says so rather than guessing. The rules files are published as open data: 2025/26, 2026/27, 2027/28.

What the status words mean

In force: The rule applies now and is on the gov.uk page cited.
Legislated: The rule is in an Act but does not apply yet, such as a threshold fixed for future years.
Announced: The rule was announced at a fiscal event and is in a Finance Bill or awaiting one. It can still change, and a figure shown with this status is shown as announced.
Not yet set: No primary source gives the figure yet. The tool reports this rather than using an estimate.

How the take-home calculator works

The tax code sets the allowance: the number in an L, M, N or T code times ten; K codes add the number times ten to taxable pay; BR, D0 and D1 charge all pay at one rate; NT charges none. Unknown codes fall back to the standard code with a visible note.
Adjusted net income is pay less gross pension contributions. Above the taper start the allowance falls by £1 for every £2.
Taxable income is pay (after a net pay or salary sacrifice pension) less the allowance, rounded down to the pound, and taxed band by band for the nation chosen. Relief at source widens every band by the gross contribution.
National Insurance is employee Class 1, category A, on pay after salary sacrifice, using the annual thresholds.
Each selected student loan plan repays its rate on pay after salary sacrifice above its threshold, rounded down to the pound.
Take-home is pay less every deduction. The marginal rate is one minus the change in take-home over the next £1,000.

Rounding

The conventions are set out in ROUNDING.md in the repository: taxable income rounded down to the pound, tax and National Insurance to the penny, student loans rounded down to the pound, the Pension Tax Allowance maximum rounded up to the pound. A monthly payslip can differ by pence because PAYE works period by period.

The test suite

The engine runs against 37 fixtures on every build. 7 come from HMRC and gov.uk worked examples and 30 are edge cases written from gov.uk pages alone by someone who had not seen the engine: income exactly at each threshold, the taper, the point where the allowance reaches zero, the Scottish bands, each loan plan at and just above its threshold, and the K, BR, D0, D1, NT, 0T, M and N codes. The suite is public in the repository.

Every fixture and its source

HMRC explainer, Steph: income is the full new State Pension; the allowance removes all £93.28 of tax (the explainer taxes the unrounded £466.40; the engine rounds taxable income down to £466 as HMRC computations do, a difference of 8p) (/government/publications/pension-tax-allowance-explainer/pension-tax-allowance-explainer)
HMRC explainer, Anne: £12,970 of income owes £80 of tax, all removed (/government/publications/pension-tax-allowance-explainer/pension-tax-allowance-explainer)
HMRC explainer, Jonathan: £13,186.40 is in the withdrawal range, so the allowance is £44 (/government/publications/pension-tax-allowance-explainer/pension-tax-allowance-explainer)
HMRC explainer, Nadia: £13,400 owes £166 and gets no allowance because income is above £13,319 (/government/publications/pension-tax-allowance-explainer/pension-tax-allowance-explainer)
gov.uk: transferring £1,260 of allowance reduces the partner's tax by up to £252 (/marriage-allowance)
gov.uk: the giver's allowance becomes £11,310, which is tax code 1131N (/marriage-allowance)
gov.uk: the allowance is zero at £125,140 (/income-tax-rates/income-over-100000)
England: gross exactly equal to the personal allowance and the NI primary threshold gives no tax and no NI (/income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
England: one pound over the personal allowance is one pound of taxable income and one pound of NI-able earnings (/income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
England: gross 50,270 is the top of the basic rate band and the NI upper earnings limit (/income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
England: gross 50,271 puts one pound into the higher rate and one pound above the NI upper earnings limit (/income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
England: gross 100,000 exactly keeps the full personal allowance (taper starts above 100,000) (/income-tax-rates, /income-tax-rates/income-over-100000, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
England: gross 100,002 is 2 over the taper start, so the allowance drops by exactly 1 (/income-tax-rates, /income-tax-rates/income-over-100000, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
England: gross 110,000 is 10,000 over the taper start, so the allowance drops by 5,000 (/income-tax-rates, /income-tax-rates/income-over-100000, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
England: gross 125,140 exactly tapers the allowance to zero; everything is at basic or higher rate, nothing at additional rate (/income-tax-rates, /income-tax-rates/income-over-100000, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
England: gross 125,141 is the first pound at the additional rate (allowance already zero, so taxable income equals gross) (/income-tax-rates, /income-tax-rates/income-over-100000, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
England: gross 150,000, well into the additional rate with no allowance (/income-tax-rates, /income-tax-rates/income-over-100000, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Scotland: gross 15,000 ends inside the starter rate band (which runs to 16,537) (/scottish-income-tax, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Scotland: gross 30,000 spans the starter, basic and intermediate rate bands (/scottish-income-tax, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Scotland: gross 50,000 reaches the Scottish higher rate (42%) (/scottish-income-tax, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Scotland: gross 80,000 reaches the Scottish advanced rate (45%) (/scottish-income-tax, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Wales: gross 40,000 uses the same bands and rates as England and Northern Ireland (/income-tax-rates, /tax-codes/what-your-tax-code-means, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Plan 2 student loan: gross exactly at the 29,385 threshold gives no repayment (/repaying-your-student-loan/what-you-pay, /guidance/rates-and-thresholds-for-employers-2026-to-2027, /income-tax-rates)
Plan 2 student loan: 1,000 over the threshold repays 9% of 1,000 (/repaying-your-student-loan/what-you-pay, /guidance/rates-and-thresholds-for-employers-2026-to-2027, /income-tax-rates)
Plan 5 student loan: gross exactly at the 25,000 threshold gives no repayment (/repaying-your-student-loan/what-you-pay, /guidance/rates-and-thresholds-for-employers-2026-to-2027, /income-tax-rates)
Plan 5 student loan: 1,000 over the threshold repays 9% of 1,000 (/repaying-your-student-loan/what-you-pay, /guidance/rates-and-thresholds-for-employers-2026-to-2027, /income-tax-rates)
Plan 1 and Postgraduate Loan together at 35,000: the two repayments are summed (/repaying-your-student-loan/what-you-pay, /guidance/rates-and-thresholds-for-employers-2026-to-2027, /income-tax-rates)
Tax code K475 at 30,000: no allowance, 4,750 added to taxable income, tax well under the 50% cap (/tax-codes/what-your-tax-code-means, /tax-codes/k-in-your-tax-code, /income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Tax code K2000 at 10,000: the K addition would give 6,000 of tax, but the deduction is capped at 50% of gross pay (/tax-codes/k-in-your-tax-code, /income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Tax code BR at 30,000: all income taxed at the basic rate with no allowance (/tax-codes/what-your-tax-code-means, /income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Tax code D0 at 60,000: all income taxed at the higher rate with no allowance (/tax-codes/what-your-tax-code-means, /income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Tax code D1 at 150,000: all income taxed at the additional rate with no allowance (/tax-codes/what-your-tax-code-means, /income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Tax code NT at 30,000: no income tax, but NI is still due (/tax-codes/what-your-tax-code-means, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Tax code 1383M at 30,000: Marriage Allowance recipient has 12,570 + 1,260 = 13,830 allowance (/marriage-allowance, /tax-codes/what-your-tax-code-means, /income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Tax code 1131N at 30,000: Marriage Allowance giver has 12,570 - 1,260 = 11,310 allowance (/marriage-allowance, /tax-codes/what-your-tax-code-means, /income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Tax code 0T at 30,000: no allowance but normal bands apply (same result as BR here because 30,000 is inside the basic rate band) (/tax-codes/what-your-tax-code-means, /income-tax-rates, /guidance/rates-and-thresholds-for-employers-2026-to-2027)
Scotland with Plan 4 student loan at 40,000: Scottish bands plus a repayment that is rounded down to a whole pound (/scottish-income-tax, /repaying-your-student-loan/what-you-pay, /guidance/rates-and-thresholds-for-employers-2026-to-2027)

Known limits

Benefits in kind, more than one employment, and National Insurance category letters other than A are not modelled.
K codes are modelled in the basic case: the addition to taxable pay and the cap of half of pay. Codes carrying cumulative adjustments from earlier in the year are not.
Relief at source pensions are shown with the basic rate band widened by the gross contribution, which is the Self Assessment outcome; a payslip only shows it if HMRC has adjusted the tax code.
Scottish and Welsh rules are applied to income tax bands only. Interactions between the Scottish bands and the allowance taper follow the same arithmetic as the rest of the UK.
The pension annual allowance and its taper are not modelled.
Nothing that depends on live market data (mortgage or savings rates) or on a rule under legal challenge is modelled.

Review

No external review has taken place yet. When a chartered accountant or chartered tax adviser reviews a tool, this page will state who, when and the scope, and nothing beyond it. Until then every tool is checked against the fixtures above and against gov.uk's own calculators, and the corrections log records anything found wrong.

Funding

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Page reviewed 9 November 2026.