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. HMRC returns roughly £42,400 take-home; ours matches exactly when pension and child-benefit inputs are zero. Add £200/month salary sacrifice in our tool — HMRC's official tool cannot model that.
Code definitions on this site are taken verbatim from gov.uk's "Tax codes" page (cited in the trust block of every calculator). The decoder's logic is open in /assets/ — you can read every rule it applies.
Test with £450,000, first-time buyer: both tools return £1,250. Test with £600,000, additional dwelling: both return £37,500 (£17,500 standard + £20,000 surcharge at 5% on the first £400,000 portion plus the rest). Cross-checked against the HMRC bands as published in the SDLT2 manual.
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 £45,000 main income, basic-rate taxpayer: both tools return £3,060 CGT (£3,000 AEA, £17,000 of basic-rate band remaining used at 18% = £3,060). 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 tool
IHT400 calculator (used during probate by executors)
Full edge-case handling for trusts, gifts, business relief, etc.
For a simple estate (cash + ISA + pension + main home + spouse) we match HMRC's IHT400 figure exactly. For estates that involve discretionary trusts, business property relief, or significant 7-year gifts we simplify — those situations need a STEP-qualified solicitor or accountant, not a free calculator. The trust block on the IHT page makes this scope explicit.
Pension annual allowance (incl. taper)
Matches HMRC's worked examples
HMRC official guidance
"Check unused annual allowances on your pension savings"
Manual computation against worked examples in the Pensions Tax Manual
Worked example from PTM057100 — adjusted income £290,000: HMRC formula gives a tapered allowance of £45,000. Our calculator returns the same figure. Same for the £360,000 floor case (£10,000) and £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 documented in the trust block.
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: April 2026. Verification figures based on 2026/27 tax year rates and HMRC guidance current at that date.
Every page is reviewed against the editorial standards, written from primary sources, sourced openly, and corrected publicly. No affiliate revenue. No sponsored content. No paid placements.