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Why the calculator you choose changes what you learn

Most contractors searching for an IR35 calculator want one thing: to know how much money they will actually take home. The problem is that a single figure, or even a two-scenario comparison, does not give you the full picture.

The difference between outside and inside IR35 can look catastrophic in a simple calculator. Two things are usually missing. The first is that the gap itself has compressed sharply: at £500 a day it is now £3,438 a year, not the five figures most tools report. The second is how much of what remains a salary sacrifice pension arrangement can close, which at that rate takes a sacrifice of around £6,906 a year. Both matter when you are deciding whether to accept an inside determination, negotiate a rate uplift, or challenge the assessment.

This guide compares the main IR35 calculators available in 2026, explains what each one actually models, and highlights where the important gaps are.

HMRC CEST: not a calculator

Before comparing financial tools, one point worth clarifying: HMRC's Check Employment Status for Tax tool (CEST) is not a calculator. It is a status determination questionnaire. It tells you HMRC's view of whether your engagement falls inside or outside IR35. It produces no financial output. It does not show you what you will take home under either determination. It does not model the difference between umbrella and limited company structures. If you are looking for take-home pay figures, CEST is the wrong starting point. It answers a different question entirely.

For a detailed look at what CEST does and does not assess, see our guide to why CEST gets IR35 wrong.

ContractorCalculator.co.uk

ContractorCalculator is one of the longest-established contractor resource sites in the UK and carries a substantial set of financial tools. For IR35 specifically it offers separate inside IR35 and outside IR35 calculators, an umbrella calculator, and a pension calculator, each run independently.

What this means in practice is that you run the outside scenario, note your figure, close the tool, open the inside calculator, run that, and then compare the results yourself. There is no side-by-side output. There is no integrated pension scenario that shows how contributions reduce your inside-IR35 liability within the same view.

The tools are functional and reasonably comprehensive for what they do. The limitation is structural: they are built around separate calculators for separate questions rather than a single workflow that answers the question most contractors are actually asking, which is: "what is the real financial difference between these three outcomes for my specific day rate?"

ContractorUK calculator

ContractorUK's calculator compares outside IR35 (limited company salary and dividends) against inside IR35 (PAYE), side by side. It is updated for the 2026/27 tax year and handles the key inputs: director salary, dividends, umbrella margin.

It is a useful two-scenario tool. It does not include a salary sacrifice pension scenario, so it cannot show you what inside IR35 actually costs after pension mitigation. That omission is significant for any contractor who already contributes to a pension or is considering using one to reduce the inside-IR35 hit.

IR35 Verdict calculator

IR35 Verdict's calculator runs three scenarios in a single view:

1. 1

   Outside IR35 via limited company: models corporation tax, the salary and dividend split, dividend tax, and employer and employee National Insurance where applicable.

2. 2

   Inside IR35 via umbrella: models the umbrella margin, employer National Insurance and the apprenticeship levy deducted from the assignment rate, and PAYE income tax and employee National Insurance. This is what your payslip will look like through a compliant umbrella.

3. 3

   Inside IR35 with salary sacrifice pension: the same umbrella calculation, but with salary sacrifice pension contributions applied. This scenario shows how pension sacrifice reduces both the income tax and National Insurance liability on your deemed salary, recovering a meaningful portion of the inside-IR35 penalty.

The three results appear side by side. The default working days assumption is 220 per year, adjustable with a slider. The pension contribution amount is a separate input for scenario three.

The reason the salary sacrifice scenario matters is that most calculators either ignore it or require you to run a separate pension calculator and do the arithmetic yourself. The difference it makes is not trivial. Depending on your day rate and pension contribution level, salary sacrifice through an umbrella can recover a substantial portion of the gap between outside and inside IR35 take-home pay. For contractors facing an inside determination, that number is material to the decision about whether to accept it, negotiate, or challenge it.

No other contractor IR35 calculator currently models this scenario within a three-way side-by-side comparison.

What the salary sacrifice scenario actually shows

To make this concrete: on a day rate of £500 working 220 days per year, with a £25 per week umbrella margin and £1,850 a year of accountancy and insurance costs, the difference between outside IR35 (limited company) and inside IR35 (umbrella) is £3,438 a year after all taxes. Calculators still quoting £10,000 to £15,000 at this rate are generally applying pre-2023 corporation tax, the older dividend rates, or business expense claims most contractors cannot make.

That figure is what most two-scenario calculators show you, and it is the number that understandably alarms contractors receiving an inside determination.

What the same calculators usually do not show is what happens when you sacrifice a portion of your assignment rate into a pension. Because salary sacrifice reduces the income on which both income tax and employee National Insurance are calculated, the effective cost of pension contributions for an inside-IR35 umbrella contractor is significantly lower than the nominal contribution amount. Depending on the contribution level, this can close a meaningful portion of the outside-vs-inside gap.

This does not make an inside determination painless. The financial difference between the two statuses remains real. But it does mean the net impact is often less severe than the headline calculator figure suggests. Understanding the real gap, after pension mitigation, is important for any contractor making a decision about their engagement.

For a detailed walkthrough of how salary sacrifice works inside IR35, see our guide to salary sacrifice pension inside IR35.

Which calculator is right for your situation

If you want a quick outside-vs-inside comparison: any of the tools above will give you a ballpark. ContractorUK's side-by-side view is clean and straightforward for this purpose.

If you want to understand the full financial picture including pension mitigation: the IR35 Verdict calculator is currently the only tool that models all three scenarios in a single comparison. Use it when an inside determination is on the table and you need to know the real net impact before making a decision.

If you want to understand your IR35 status, not your take-home pay: use a status tool, not a financial calculator. The IR35 Verdict contract checker analyses your contract against six IR35 dimensions grounded in UK tribunal case law, including substitution, control, mutuality of obligation, and financial risk.

The status question and the financial question are separate

It is worth being explicit about this because the two questions often get conflated. A take-home pay calculator, including IR35 Verdict's, does not tell you whether you are inside or outside IR35. It shows you what each scenario means for your pay, on the assumption that you already know (or are trying to evaluate) your status.

The financial calculation is relatively mechanical once you know the inputs. The status question is a legal and factual assessment that depends on your contract terms and your actual working practices.

Recent case law makes the status question more nuanced than it used to be. The Supreme Court's 2024 ruling in HMRC v Professional Game Match Officials Ltd [2024] UKSC 29 confirmed that mutuality of obligation and a degree of control are both capable of being satisfied more easily than many contractors had assumed. The subsequent First-tier Tribunal decision in May 2026 (Professional Game Match Officials Ltd v HMRC [2026] UKFTT 654 (TC)) found the referees were nonetheless self-employed, but on the basis of the third Ready Mixed Concrete limb (the multifactorial in-business evaluation) rather than because the first two limbs failed. The FTT's decision carries persuasive weight but is not binding precedent, and HMRC has indicated it is considering its position.

For contractors, the practical implication of both decisions together is that status now turns primarily on the third limb: whether you genuinely operate as a business on your own account. That assessment requires looking at your contract and your working practices together. A calculator cannot do that for you.

Running the numbers

The IR35 Verdict calculator is free and requires only your day rate and working days. The pension input is optional and applies only to scenario three.

Use the IR35 Verdict calculator to see your outside IR35, umbrella, and salary sacrifice figures side by side. We also publish every rate and formula we use, so you can check the workings behind each figure.

If you want to go further and understand what your contract says about your IR35 status, the contract checker analyses your contract text against the case law framework and produces a probability-based determination rather than a binary inside or outside verdict, with a reasoned assessment across six dimensions.

Frequently asked questions

Is there a free IR35 calculator? Yes. The IR35 Verdict calculator, ContractorCalculator, and ContractorUK all offer free calculators. IR35 Verdict is free and models three scenarios including salary sacrifice pension.

Does HMRC have an IR35 take-home calculator? No. HMRC's CEST tool is a status determination questionnaire, not a financial calculator. It does not produce take-home pay figures.

Which IR35 calculator includes salary sacrifice pension? The IR35 Verdict calculator is currently the only tool that models the salary sacrifice pension scenario as a third side-by-side comparison alongside outside IR35 and standard umbrella.

How accurate are IR35 calculators? All calculators are estimates based on the inputs you provide and the tax rates they apply. For outside IR35, the actual figure depends on your accountancy structure, expenses, and dividend decisions. For inside IR35 via umbrella, the figure depends on your umbrella's margin and any additional deductions. Treat calculator outputs as informed estimates and consult a contractor accountant for your specific position.

Can a calculator tell me if I'm inside or outside IR35? No. IR35 status is a legal and factual question assessed against UK case law. A calculator shows you the financial impact of each status. It cannot determine which status applies to you. Use a status tool such as the IR35 Verdict contract checker for that assessment.