IR35 calculator: how the numbers really work
An IR35 calculator exists to answer one question: how much of a contract fee survives once the off-payroll working rules treat it as employment income. The mechanics behind that answer are fixed in HMRC guidance, which sets out a four-step deemed direct payment calculation before any Income Tax or National Insurance is applied [1]. The stakes are real: the reform of these rules generated an estimated £4.2 billion in additional tax between October 2019 and March 2023 [8].
This article explains what an IR35 calculator computes, the inputs it needs, the HMRC formula it applies, the 2026-27 rates that feed it, and where a calculator stops being useful. It is written for contractors comparing engagements, for engagers estimating cost, and for developers building the calculation into their own product.
A calculator is an estimate. The figure that reaches HMRC comes from a payroll system running the same arithmetic on real pay data, reported through Real Time Information. Understanding the formula is what lets a reader trust, or challenge, any number a calculator produces.
Key takeaways
- An IR35 calculator works out the deemed direct payment first, then applies PAYE and National Insurance to it [1].
- The deemed direct payment starts from the fee net of VAT, less direct materials and less expenses that would have been allowable in employment [2].
- A deemed employee's tax code is often BR, because the worker keeps their Personal Allowance against their own company [1].
- Employer National Insurance is paid on top of the fee at 15% and cannot be deducted from the worker [1].
- A calculator estimates cost, it does not decide status. Status comes from a proper determination, supported by HMRC's CEST tool [7].
- Employment Allowance cannot be used against payments to deemed employees [1].
What an IR35 calculator actually works out
Most calculators marketed for contractors do two things: they estimate the deemed direct payment, and they apply current tax and National Insurance to show an inside-IR35 net figure next to an outside-IR35 one. The second half is a comparison; the first half is where the statutory rules live.
The deemed direct payment
The deemed direct payment is the amount the deemed employer must treat as earnings for tax purposes [1]. It is not the same as the invoice value. HMRC defines it as the fee, net of VAT, reduced by the direct cost of materials and by expenses that would have been deductible if the worker were employed [2]. If the result is nil or negative, there is no deemed direct payment at all [1].
What the calculator cannot decide
A calculator cannot tell a contractor whether a contract is inside or outside the rules. That is a status question, determined by the client for medium and large engagers, or by the worker's own intermediary for a small client [6]. HMRC's free CEST service is the tool built for that decision, and HMRC stands by its result where the information entered is accurate [7]. A calculator answers "how much", never "which side".
The inputs an IR35 calculator needs
The quality of any estimate depends on the inputs. A calculator that omits VAT, materials or allowable expenses will overstate the deemed payment and understate take-home. The core inputs are set by the HMRC formula [3].
| Input | Why it matters | Source in the formula |
|---|---|---|
| Contract fee including VAT | Starting point, before VAT is stripped out | Step one [[2]](https://www.gov.uk/hmrc-internal-manuals/employment-status-manual/esm10029) |
| VAT charged | Removed in full before anything else | Step one [[2]](https://www.gov.uk/hmrc-internal-manuals/employment-status-manual/esm10029) |
| Direct cost of materials | Deducted at cost to the intermediary | Step two [[2]](https://www.gov.uk/hmrc-internal-manuals/employment-status-manual/esm10029) |
| Employment-allowable expenses | Only expenses deductible in employment count | Step three [[2]](https://www.gov.uk/hmrc-internal-manuals/employment-status-manual/esm10029) |
| Tax code | Usually BR for a deemed employee | Applied after the deemed payment [[1]](https://www.gov.uk/guidance/fee-payer-responsibilities-under-the-off-payroll-working-rules) |
A common input error is treating commuting to the engagement's workplace as an allowable expense. It is not, because a deemed employee's workplace counts as a permanent one, so those costs are excluded from the calculation [2].
How the deemed direct payment is calculated
HMRC publishes a worked example that any calculator should reproduce exactly. A contractor supplies services through a personal service company and invoices a medium-sized client £7,200 a month, which includes £1,200 of VAT [2]. The worker incurs £350 of expenses a month, of which £100 is non-allowable commuting, leaving £250 deductible, and provides £750 of materials [2]. The four steps run as follows.
| Step | Description | Amount |
|---|---|---|
| One | Fee net of VAT | £6,000 |
| Two | Deduct direct cost of materials | (£750) |
| Three | Deduct allowable expenses | (£250) |
| Four | Deemed direct payment | £5,000 |
The £5,000 is then chargeable to PAYE and National Insurance on a monthly basis, in the same way as pay for a direct employee, and reported through the normal Real Time Information process [2]. The technical basis for each step sits in HMRC's Employment Status Manual [9].
The 2026-27 rates an inside-IR35 calculation applies
Once the deemed payment is fixed, the calculator applies the tax and National Insurance rates for the tax year. These are the figures for the 2026-27 tax year.
Income Tax bands
A deemed employee usually carries tax code BR, because the worker's Personal Allowance is already used against the salary they draw from their own company [1]. A BR code applies the basic rate to every pound of the deemed payment, with no allowance and no higher-rate band. Where a different code applies, the standard bands are used [4].
| Band | Taxable income above the Personal Allowance | Rate |
|---|---|---|
| Basic rate | Up to £50,270 | 20% |
| Higher rate | £50,270 to £125,140 | 40% |
| Additional rate | Above £125,140 | 45% |
The Personal Allowance is £12,570 for the 2026-27 tax year, and the standard code is 1257L [4].
National Insurance
National Insurance is charged per pay period rather than cumulatively, so the deemed employment applies its own thresholds regardless of the worker's other income [5]. The deemed employer deducts employee National Insurance and pays employer National Insurance on top of the fee [1].
| Contribution | Band | Rate |
|---|---|---|
| Employee | Between the Primary Threshold and Upper Earnings Limit | 8% |
| Employee | Above the Upper Earnings Limit | 2% |
| Employer | Above the Secondary Threshold | 15% |
The employer rate rose to 15% on 6 April 2026, a change explained in the guide to employer National Insurance [5]. Because employer National Insurance sits on top of the fee and cannot be recovered from the worker, it is the deemed employer's cost, and any calculator built for engagers has to show it separately [1].
Inside versus outside IR35: why take-home differs
Applying the 2026-27 rates to the £5,000 deemed payment from the HMRC example produces the illustrative monthly breakdown below, assuming a BR tax code. The figures use the rates above and are for illustration, not a substitute for a payroll run.
| Line | Amount |
|---|---|
| Deemed direct payment | £5,000.00 |
| Income Tax at 20% (BR) | £1,000.00 |
| Employee National Insurance | £267.50 |
| Net reaching the worker's company | £3,732.50 |
| Employer National Insurance (paid on top by the deemed employer) | £687.45 |
Outside IR35, the same £6,000 net fee would enter the worker's company as income, and the contractor would draw a mix of salary and dividends, typically leaving more in hand than the inside-IR35 route. That gap is the entire reason an IR35 calculator exists. The deemed employer also loses reliefs it might otherwise use: Employment Allowance cannot be applied to deemed employees at all [1]. Accountants comparing scenarios across several contractor clients tend to model this inside a multi-client payroll dashboard rather than a standalone calculator, so the estimate and the eventual payroll run share one set of figures.
From calculator to payroll: running the deduction for real
A calculator ends at an estimate. The statutory obligation begins when the deemed employer reports the pay and deductions to HMRC through a Full Payment Submission, using the off-payroll worker indicator so the engagement is flagged correctly [1]. The worker does not need to be added to an existing payroll, but if the payments are not reported under the current PAYE scheme, a new one must be opened [1].
This is where a calculator and a payroll engine part company. Payroll that holds the HMRC Recognised badge applies the same four-step deemed payment logic, the correct tax code, the current thresholds and the off-payroll indicator automatically, then files the submission. Software platforms that need this inside their own product can call an HMRC-recognised payroll API and read the endpoint behaviour in the published API documentation, so the deemed payment is calculated and reported in production rather than estimated in a spreadsheet. Smaller engagers running the deduction in-house can do the same through HMRC-recognised payroll software billed per payslip, and review the model on the per-payslip pricing page.
Conclusion
An IR35 calculator is a useful estimate wrapped around a precise formula. The precision comes from HMRC's four-step deemed direct payment calculation and the fixed tax and National Insurance rates for the tax year; the estimate comes from the assumptions a user feeds in about VAT, materials, expenses and tax code. A calculator that respects the formula gives a defensible number. One that skips a step does not.
The more important shift for anyone engaging contractors is that the calculation eventually has to be run for real, reported through Real Time Information with the right indicator, and paid on the right dates. The calculator informs the decision; the payroll engine carries the liability. Treating the two as one continuous process, from estimate to submission, is what keeps an inside-IR35 engagement compliant rather than merely modelled.
Frequently asked questions
How does an IR35 calculator work out take-home pay?
It first computes the deemed direct payment by taking the fee net of VAT and deducting direct materials and employment-allowable expenses [2]. It then applies Income Tax and employee National Insurance to that figure at the rates for the tax year, using the worker's tax code, which is often BR [1]. The net of those deductions is the amount reaching the worker's company. Employer National Insurance is shown separately because the deemed employer pays it on top of the fee [1].
Why is my inside-IR35 tax code BR?
A BR code applies the basic rate to all earnings with no Personal Allowance [4]. Deemed employees often receive it because they already have a primary employment with their own intermediary, where the Personal Allowance is used [1]. The deemed employer should give the worker a new starter checklist, which determines the code that actually applies [1].
Does an IR35 calculator tell me if I am inside or outside IR35?
No. A calculator estimates the financial outcome of being inside the rules; it does not determine status. Status is decided by the client for medium and large engagers, or by the worker's intermediary for a small client, and HMRC's CEST tool is the service built to support that decision [7]. A calculator should never be treated as a status determination.
Are materials and expenses deducted before tax under IR35?
Yes, but only within the deemed payment calculation and only if they qualify. Direct materials are deducted at cost, and expenses are deducted only where they would have been allowable had the worker been employed [2]. Commuting to the engagement's workplace does not qualify, because a deemed employee's workplace is treated as permanent [2].