IR35 Back Tax / HMRC Settlement Calculator
Estimate a multi-year IR35 back-tax settlement – PAYE, employer and employee NI, HMRC interest and penalty bands – if HMRC reclassifies your contract.
HMRC's IR35/off-payroll settlement exposure = retrospective PAYE + employer/employee NI for each affected tax year, plus late-payment interest (currently 7.75%/year = BoE base rate 3.75% + 4pp margin, effective from 6 Apr 2025; tracks BoE base rate changes — see sourceUrl), plus behaviour-based penalties: 0% for reasonable care, up to 30% of tax due for careless error, up to 70% for deliberate non-compliance, up to 100% for deliberate-and-concealed. There is no fixed £ penalty (unlike this schema's optional fixedPenalty field, deliberately left unset here). Standard discovery-assessment look-back is 4 years, extended to 6 years for careless behaviour (used here as the representative figure widely cited for IR35 cases) and up to 20 years for deliberate non-compliance. A 2024 offset mechanism lets the fee-payer's PAYE liability be reduced by tax the contractor's PSC already paid on the same income, avoiding double taxation.
What an IR35 reclassification could actually cost you in back tax
If HMRC decides a contract was inside IR35 all along, the financial consequences aren't limited to a warning for next time — they're retrospective. HMRC can pursue the PAYE income tax and both employer's and employee's National Insurance that should have been deducted, going back as far as the standard four-year discovery window, extended to six years where the behaviour is judged careless, and up to twenty years where it's found deliberate. On top of the tax itself, HMRC charges late-payment interest — currently 7.75% a year, being the Bank of England base rate of 3.75% plus a 4-percentage-point margin, effective from 6 April 2025 — and behaviour-based penalties ranging from 0% for reasonable care up to 30% for careless error, 70% for deliberate non-compliance, and 100% where it's deliberate and concealed.
This is one of the only calculators anywhere that turns those rules into an actual multi-year figure rather than a single rough estimate: most of what exists online for 'IR35 back tax' is either law-firm commentary explaining the rules in the abstract, or an insurer's lead-generation tool with no visible methodology. Here you choose a look-back period, see the PAYE and NI recalculated year by year, interest added on top, and a penalty band applied — plus an allowance for the 2024 offset mechanism, which lets the fee-payer's PAYE bill be reduced by tax your company already paid on the same income, so you're not modelling double taxation that no longer applies.
What goes into the settlement estimate
- Choose a look-back period: the standard 4-year discovery assessment, 6 years (the figure most commonly cited for IR35 cases involving careless behaviour), or up to 20 years for deliberate non-compliance
- Enter the fee income affected in each year, and the calculator applies PAYE income tax and Class 1 employer and employee National Insurance as if the payments had always been treated as employment income
- HMRC late-payment interest is added at 7.75% a year (Bank of England base rate of 3.75% plus a 4-percentage-point margin, effective from 6 April 2025) for the period the tax has been outstanding
- A penalty band is applied on top: 0% for reasonable care, up to 30% for a careless error, up to 70% for deliberate non-compliance, or up to 100% where it's deliberate and concealed
- An offset can be applied for tax and National Insurance your company has already paid on the same income, reflecting the 2024 mechanism that prevents the fee-payer being taxed twice on it
