IR35 Calculator UK — Inside vs Outside

IR35, formally known as the Intermediaries Legislation, is a set of tax rules introduced by HM Revenue and Customs (HMRC) that targets individuals who supply their services to clients through an intermediary, such as a personal service company (PSC), partnership, or other entity, who would otherwise be classified as an employee if the intermediary did not exist. The legislation exists to combat disguised employment, where workers effectively perform the same role as employees but avoid paying the correct levels of Income Tax and National Insurance Contributions by operating through a company structure.

History of IR35

The IR35 legislation was first introduced in the Finance Act 2000, coming into effect on 6 April 2000. The name derives from the press release numbered IR35 that announced the measure. The rules were significantly strengthened through subsequent legislation, most notably the Income Tax (Earnings and Pensions) Act 2003 (ITEPA 2003), which consolidated and codified the intermediary legislation into Chapters 8 and 9 of Part 2 of that Act. The Finance Act 2022 further refined the framework, particularly around the Health and Social Care Levy provisions and the alignment of dividend tax rates.

Inside IR35 vs Outside IR35

When a contractor is determined to be inside IR35, the engagement is treated as one of employment for tax purposes. This means the income is subject to the same taxes as a regular employee's salary: Income Tax at basic rate (20%), higher rate (40%), and additional rate (45%), plus Employee National Insurance Contributions (8%), Employer National Insurance Contributions (13.8%), and where applicable, the Apprenticeship Levy (0.5% on pay bills exceeding £170,000). The contractor's take-home pay is therefore significantly lower compared to operating outside the legislation.

When a contractor falls outside IR35, they are genuinely self-employed and can operate through their company with a more tax-efficient structure. The typical approach involves taking a small salary up to the National Insurance Primary Threshold (£12,570 for 2024/25), which is free of both Income Tax and NI, and extracting the remaining profit as dividends. The company pays Corporation Tax on its profits at 19% for profits up to £50,000, with marginal relief applying between £50,000 and £250,000, and the full main rate of 25% on profits above £250,000. Dividends are then taxed at 8.75% (basic rate), 33.75% (higher rate), or 39.35% (additional rate), with a £500 dividend allowance. This structure typically results in a substantially higher take-home income.

The Off-Payroll Working Rules

The landscape changed dramatically with the introduction of the off-payroll working rules. Originally applied only to the public sector from April 2017, these rules were extended to the private sector from 6 April 2021. Under the off-payroll rules, the responsibility for determining IR35 status shifted from the contractor's personal service company to the fee-payer (usually the recruitment agency) or the end client, depending on the supply chain. Medium and large-sized clients must issue a Status Determination Statement (SDS) explaining their decision and the reasoning behind it.

This shift fundamentally changed the contracting landscape. Many organisations adopted risk-averse blanket determinations, placing all contractors inside IR35 regardless of the actual working relationship. While this approach reduces compliance risk for the engager, it can result in contractors bearing a significantly higher tax burden than is warranted by their genuine employment status.

How IR35 Status Is Determined

HMRC uses several key tests to determine employment status for IR35 purposes. The primary factors examined include:

• Control: Does the client control what, when, where, and how the contractor performs their work? A high degree of client control suggests employment.
• Substitution: Can the contractor send a substitute to perform the work, or must they provide their personal service? A genuine right of substitution is a strong indicator of self-employment.
• Mutuality of Obligation (MOO): Is the client obligated to offer work, and is the contractor obligated to accept it? The absence of MOO supports self-employment status.
• Financial Risk: Does the contractor bear financial risk, such as having to correct defective work at their own expense? Self-employed workers typically bear greater financial risk.
• Provision of Equipment: Does the contractor provide their own equipment, or does the client supply everything needed?
• Integration: Is the contractor integrated into the client's organisation as if they were an employee?

HMRC provides an online tool called Check Employment Status for Tax (CEST) to help determine IR35 status. However, CEST has been widely criticised for producing inaccurate results, particularly around mutuality of obligation, and many tax professionals recommend seeking specialist advice rather than relying solely on the tool.

Financial Impact

The financial difference between inside and outside IR35 can be substantial. A contractor earning £400 per day working 5 days a week for 48 weeks generates £96,000 in annual revenue. Inside IR35, after Income Tax, Employee NI, and Employer NI, the take-home might be approximately £52,000. Outside IR35, using a salary-plus-dividends strategy with Corporation Tax at the small rate, take-home could be approximately £68,000 — a difference of roughly £16,000 per year. This gap widens further at higher day rates.

Compliance and Penalties

Non-compliance with IR35 carries serious consequences. HMRC can investigate previous tax years and demand payment of deemed direct payments including Income Tax and National Insurance that should have been paid. Penalties can reach up to 100% of the tax due where HMRC determines that a contractor knew they were inside IR35 but failed to comply. Under the off-payroll rules, the fee-payer bears the liability for unpaid taxes, while the client may bear the liability if they failed to take reasonable care in making the status determination.

Recent and Future Developments

The IR35 landscape continues to evolve. The removal of the 5% expense deduction for deemed employees, the alignment of dividend tax rates, and ongoing HMRC enforcement activity all impact contractors. HMRC has significantly increased its compliance activity in recent years, with dedicated IR35 teams conducting targeted reviews of sectors with high contractor populations including IT, engineering, oil and gas, and financial services. Staying informed about legislative changes and obtaining professional advice remains essential for any contractor working through an intermediary in the UK.