Understanding UK Company Taxation: How Corporation Tax Is Calculated and Assessed

If you’ve just set up your own company in the UK-or you’re thinking about taking that step-sooner or later, you’ll encounter the topic of corporation tax. It’s one of those things all UK company owners need to get right, but the rules can feel confusing and even a little intimidating if you’re new to business ownership. Don’t worry – this guide breaks down the essentials so you can approach your company’s tax with clarity, confidence, and a plan for compliance.

Corporation tax is a fundamental part of running a company in the UK. Making sense of how it’s calculated and assessed is crucial, whether you’re a first-time founder or managing a growing venture. Understanding this system can help you avoid unexpected surprises and stay on the right side of HM Revenue & Customs (HMRC)-the government body in charge of assessing and collecting company taxes.

In this guide, we’ll cover who pays corporation tax, how your business’s accounting period impacts your tax bill, what counts as taxable income or gains, and which rates apply (and when). We’ll also share some practical tips for keeping your company legally compliant while maximising efficiency. Ready to demystify UK company taxation? Let’s dive in.

Who Pays Corporation Tax in the UK?

Corporation tax is a tax on a company’s profits-so it applies to incorporated entities like limited companies, not to individuals running a business as a sole trader or to members of a traditional partnership. If you’ve registered your business as a company with Companies House, then it’s the company itself (as a separate legal entity) that’s responsible for reporting and paying corporation tax, not you as an individual.

• Private companies limited by shares or guarantee (Ltd).
• Public limited companies (PLCs).
• Some clubs, societies, and other associations with corporate status.

It’s important to stress this distinction: corporation tax doesn’t apply to individuals, even if you pay yourself a salary or dividends from the company. Each type of business structure attracts different tax and legal obligations. If you’re unsure which structure is right for you, check out our guide on business structures or chat to a legal expert for tailored advice.

What Is an Accounting Period and Why Does It Matter?

One thing that often trips up new business owners is the concept of the “accounting period”. Unlike the UK tax year for individuals (which runs 6 April to 5 April), companies choose their own accounting period. This is usually a 12-month stretch that suits how you run your books-often aligned with your company’s financial year, but not required to match the government’s financial year (which goes from 1 April to 31 March).

• The accounting period is the timeframe over which your company’s corporation tax liability is measured.
• It starts when your company begins trading or when your previous accounting period ends.
• Usually, your first accounting period will end on your company’s “accounting reference date”-set when you register with Companies House.

Getting your accounting period right is vital, especially since tax rates can change between government financial years. If your accounting period straddles a point where the corporation tax rate changes, you’ll need to apportion your profits accordingly-more on that shortly.

How Is the Taxable Income of a Corporation Calculated?

Your corporation tax bill is based on your company’s “taxable profits.” But what actually counts as taxable profit? Broadly speaking, it’s all the money your business made during the accounting period, minus allowable expenses (the costs of running your company). However, HMRC breaks this down into three key buckets:

• Trading Profits:
  This is the income your company generates from normal business activities-like selling products or providing services-after deducting ordinary business expenses (such as staff salaries, rent, office supplies, and advertising).
• Investment Income:
  Profits from things like investments, dividends received from other companies, or interest.
• Chargeable Gains:
  If your company sells a business asset (for example, property, machinery, or shares) for more than it was worth when you bought it, the profit (known as a “chargeable gain”) is included as taxable income.

You’ll need to carefully track and document all sources of company income, as well as all deductible costs and any chargeable gains. Accurate reporting will not only make tax time easier, but is also legally required-mistakes can lead to penalties or an HMRC investigation.

If you’re keen to understand which expenses are allowable (and which aren’t), this is often a good area for a chat with your accountant or a legal advisor with experience in company tax compliance.

What About Loss Relief?

Not every company makes a profit every year-and that’s something HMRC recognises, too. If your company makes a loss, you may be able to set off that loss against profits from other income or carry it forward to offset future profits. This is known as “loss relief.”

• Trading Losses: Losses from your core business operations can usually be offset against other profits in the same accounting period, carried back to the previous period to reclaim tax paid earlier, or carried forward to reduce future corporation tax bills.
• Capital Losses: Losses made when selling company assets can typically only be offset against capital gains (not trading profit).

Making full use of loss relief is one of the ways you can legally optimise your tax situation. It’s worth getting tailored advice to ensure you’re claiming everything you’re entitled to-a professional can help you work through what’s possible.

What Is the Current Corporation Tax Rate in the UK?

The big question: what percentage of profit will your company actually have to pay?

As of 1 April 2023, the government introduced a main rate of 25% for company profits above £250,000. For companies with profits under £50,000, a “small profits rate” of 19% applies. If your profits fall between £50,000 and £250,000, you’ll pay “marginal relief,” which means your rate is calculated on a sliding scale between 19% and 25%.

• Small Profits Rate (under £50,000 profit): 19%
• Main Rate (over £250,000 profit): 25%
• Marginal Relief (between £50,000–£250,000): See HMRC’s Marginal Relief guidance.

But don’t forget: If your accounting period falls across more than one tax year (for example, if it starts before and ends after a government rate change in April), you’ll need to “split” your profits for tax purposes, applying the relevant rate to each. This calculation can get fiddly, so it’s wise to plan for this with your accountant.

Rates and bands can change each tax year, so always check the latest information from HMRC or speak to a tax professional. For most small companies starting out, the small profits rate is likely to be most relevant-at least until your profits exceed £50,000.

How Is Corporation Tax Assessed and Paid in Practice?

Your company is responsible for working out its own corporation tax liability each year, telling HMRC how much is owed, and paying it by the deadline. This process is called “self-assessment.”

1. Prepare your annual accounts: Your company will need to prepare a set of accounts for each accounting period, showing all income, expenses, and profits. This forms the basis of your tax return.
2. File a Company Tax Return: You'll submit a Company Tax Return (CT600) online to HMRC-usually within 12 months after the end of your accounting period. This details your taxable income, allowable deductions, and the calculation of your corporation tax bill.
3. Pay Corporation Tax: Payment is due 9 months and one day after the end of your accounting period (not the same as your filing deadline!). Even if your tax calculation is late, the payment must be on time.

HMRC may query your return, and penalties can apply for late filing or underpayment, so accuracy is key. If you need further guidance, our guide to filing accounts with Companies House can help.

Why Does the Accounting Period and Tax Year Alignment Matter?

Let’s look at a common scenario: What happens if your accounting period doesn’t match the government’s financial year, and the corporation tax rate changes mid-way through your period? You’ll need to split your profits for that accounting period in proportion to the days that fall before and after the rate change, and pay tax at the relevant rate on each “slice” of profit.

Here's how it works:

• Calculate total profits for your accounting period.
• Apportion those profits by the number of days under the old and new rates.
• Apply each rate to the appropriate share of profits.
• Add both tax figures for the total due.

While this might sound complicated, your accountant or a tax professional (and most modern accounting software) can handle this for you. Just know that your accounting period can impact your company’s tax liability, so plan accordingly (especially if you anticipate a governmental rate change).

What About Other Taxes and Considerations?

Corporation tax isn’t the only tax a UK company may face. You’ll need to think about:

• VAT (Value Added Tax): If your annual turnover exceeds the VAT threshold, you’ll need to register and charge VAT on applicable sales. Read our guide to VAT in the UK for more on this.
• Payroll Taxes: If you have employees, you’ll need to operate PAYE (Pay As You Earn), deducting income tax and National Insurance from salaries and paying employer’s contributions.
• Dividends and Personal Tax: When taking money out of the company as dividends, the recipients will pay personal tax on those dividends-the company itself only reports and pays corporation tax on its own profits.

Each of these areas comes with its own set of rules and reporting requirements, so it’s vital to set up systems for compliance from the start. For additional tips on laying the right legal and tax groundwork for your company, have a look at our business startup checklist.

How Can You Stay Legally Compliant and Efficient?

Keeping up with all the rules and deadlines as a UK company can quickly feel overwhelming. Here are a few steps you can take to stay on track:

• Keep accurate records:
  Store receipts, invoices, bank statements, and details of any asset sales securely and in an organised system-these will back up your tax filings in case of an HMRC query.
• Plan for tax payments:
  Set aside money throughout the year for your corporation tax bill, rather than scrambling at the last minute. Consider using a separate bank account for this purpose.
• Work with professionals:
  A good accountant or legal advisor can help you make the most of allowable tax reliefs, keep you compliant, and save time-allowing you to focus on running your business.
• Understand the importance of key legal documents:
  Having well-drafted documents (like a shareholders agreement or company constitution) can support smooth company operations and future fundraising or investment. Learn more in our guide on ongoing compliance and reporting.

Getting the legal basics in place can save you headaches down the line. For more on company setup and compliance, visit our article on what are the legal requirements for starting a business.

Key Takeaways

• Corporation tax is a tax on company profits-paid by the company itself, not its individual owners or shareholders.
• Your company defines its own accounting period (normally 12 months), which forms the basis for tax calculation and assessment.
• Taxable profits include trading income, chargeable gains, and investment income, less allowable business expenses and any applicable loss relief.
• The current main corporation tax rate is 25%, but small profits (under £50,000) are taxed at 19%, with marginal relief for profits between £50,000 and £250,000.
• If your accounting period covers a change in tax rates, profits must be apportioned to apply the correct rates to each period segment.
• Pay close attention to records, deadlines, and filing obligations to avoid penalties and keep your company legally compliant.
• Working with professional advisors can help you stay compliant and improve your financial efficiency-from day one.

Need Help With UK Company Taxation?

Understanding corporation tax is a key part of running a successful UK company, but you don’t have to figure it all out alone. If you need advice about setting your accounting period, allowable expenses, or preparing for growth, our friendly legal team is here to help.

Reach us at team@sprintlaw.co.uk or call 08081347754 for a free, no-obligations chat about company taxation or any other legal questions for your business.