For seven of the most-used calculators on this site, HMRC publishes an official equivalent. This page tells you exactly what the official tool does, what ours does, and where we agree to the penny — and where we deliberately diverge so you understand why.
If a calculation here ever disagrees with HMRC's official tool by more than rounding, we treat that as a defect. The contact route on the About page is open to corrections, and corrections are logged in the changelog with a date and explanation.
Income Tax + NI plus personal-allowance taper, 60% trap, salary sacrifice, child benefit clawback, both student-loan plans, ISA contributions, fiscal drag
Plug a £55,000 salary into both. At 2026/27 rates the take-home after Income Tax and employee NI is about £42,457; ours matches when pension and child-benefit inputs are zero. Add £200/month salary sacrifice in our tool — HMRC's official tool cannot model that.
Test with £450,000, first-time buyer: the published rates give £7,500 (nothing on the first £300,000, then 5% on the next £150,000), and our calculator returns the same. Test with £600,000, additional dwelling: £50,000, made up of £20,000 at the standard rates plus the 5% surcharge on the whole £600,000 (£30,000). Cross-checked against the residential rates published on GOV.UK.
Capital Gains Tax (CGT)
Matches to the penny
HMRC official tool
"Tax when you sell shares" / "Tax when you sell property"
Returns headline rates and bands
The "Real Time Transaction Service" calculates and lets you pay
£20,000 gain on shares with a £45,000 salary, so £32,430 of taxable income after the Personal Allowance: after the £3,000 annual exempt amount, £17,000 is taxable. Only £5,270 of the basic-rate band is left, so £5,270 is charged at 18% and £11,730 at 24%, a total of £3,763.80, which our calculator shows as £3,764. Property scenarios add the 60-day reporting reminder, which HMRC's guidance separately spells out.
Inheritance Tax (IHT) on a typical estate
Close but simplified
HMRC official form
Form IHT400 (Inheritance Tax account), with its simple calculation or the separate IHT400 Calculation worksheet
Covers lifetime gifts, trusts, reliefs, interest and payment by instalments
Completed by executors, or their advisers, when an estate needs a full account
For a simple estate (cash, ISAs and a main home, with any unused bands transferred from a late spouse or civil partner) the calculator applies the nil-rate band, the residence nil-rate band with its taper, the transferred bands and the 40% rate. HMRC's IHT400 account also deals with lifetime gifts, reliefs, interest and payment by instalments, and its simple calculation can only be used when, among other conditions, lifetime gifts are below the nil-rate band and no tax is paid by instalments (IHTM10041). For estates that involve discretionary trusts, business property relief, or significant 7-year gifts the calculator simplifies, and the result is only an estimate — those estates usually need a STEP-qualified solicitor or accountant, not a free calculator.
Pension annual allowance (incl. taper)
Matches HMRC's formula
HMRC official guidance
"Check unused annual allowances on your pension savings"
Manual computation against worked examples in the Pensions Tax Manual
Test case using the tapered allowance formula in HMRC's Pensions Tax Manual (PTM057100): adjusted income of £290,000, with threshold income over £200,000, gives a tapered allowance of £45,000, and our calculator returns the same figure. Same for the £360,000 floor case (£10,000) and the £260,000 boundary case (full £60,000).
The SA online return is the canonical source for what you owe. Our calculators help you forecast in advance so the January bill is not a surprise — the figures should be within £50–£100 of the eventual SA computation for typical cases, but the formal return wins where they differ.
How we test
Every calculator with an HMRC official equivalent goes through a documented regression test on each tax-year change:
A standard worked example (e.g. £55,000 salary, default pension, no child benefit) is run through both tools and the results are recorded.
Three boundary cases are tested: one just below a band, one inside the band, one just above.
One known-edge case for each calculator (e.g. for SDLT: first-time buyer at £299,999 vs £300,000 vs £300,001).
Any divergence beyond £1 of rounding is investigated and either fixed or noted on the calculator page.
Results of each tax-year regression are summarised in the changelog. The methodology is also explained on the Open methodology page, including how to read the calculator JavaScript yourself in browser DevTools.
Last updated: September 2026. Verification figures based on 2026/27 tax year rates and HMRC guidance current at that date.
Pages are checked against the primary sources listed on each page and reviewed by the Editor; corrections are logged in the changelog. Pages are drafted with the help of AI tools (how AI is used). No affiliate revenue. No sponsored content. No paid placements.