A lot of limited company directors get their first corporation tax bill and feel completely lost. The numbers make sense, but the rules behind them do not. Here is something worth knowing: corporation tax is the tax UK limited companies pay on their annual profits.
HMRC charges it on trading income, investment returns, and gains from selling assets. It is separate from income tax, and companies get no personal allowance against it. Every penny of profit counts. The rates, deadlines, reliefs, and penalties all follow a set of rules that many business owners only discover when something goes wrong. So, what do you actually need to know before that bill lands?
What Is Corporation Tax?
Corporation tax is charged on the taxable profits a UK company makes during its accounting period. This comprises the money earned by selling goods or services, income earned by investments, and profits earned by selling business assets.
The tax is imposed only on profit, not on total income. When a firm earns £100,000 and incurs £60,000 in real allowable business expenses, it is only the amount of £40,000 that is taxed.
That one fact alone changes how many businesses think about spending and planning. Company formation done correctly from the start puts a business in the best position to manage this from day one.
Who Has to Pay It?
All UK limited companies must register with HMRC for corporation tax within three months of starting to trade. The company director is legally responsible for making sure the company tax return gets filed, and the tax gets paid on time.
It does not matter if an accountant does the work. The director still carries the responsibility. Limited companies, partnerships, and startups all have different needs when it comes to tax, but the obligation to register and pay is the same.
UK Corporation Tax Rates and Thresholds for 2025/26
How much a company pays depends on how much profit it makes. There are three bands, and knowing which one applies to your business shapes every tax decision you make.
Each band below applies to a different level of profit, and the jump between them is not always as sharp as people expect.
Small Profits Rate
Companies with taxable profits of £50,000 or below pay the small profits rate of 19%. This is the lowest band and applies to many smaller UK businesses.
Marginal Relief Band
When profits fall between £50,001 and £250,000, marginal relief kicks in. The effective UK corporation tax rate rises gradually between 19% and 25%. It does not jump all at once. This protects growing businesses from a sudden tax spike as they scale up.
Main Rate
Profits above £250,000 are taxed at the full corporation tax main rate of 25%. This replaced the flat 19% rate in April 2023 and currently applies to larger UK companies.
How Is Taxable Profit Calculated?
This is where many business owners get confused. The figure on your profit and loss account is not always the same as your taxable profit.
The formula is simple:
Total business income minus allowable business expenses = taxable profit
Allowable business expenses are costs that are wholly and exclusively for business use. Staff wages, office rent, software, and work travel all count. Client entertaining and personal costs do not. The tricky part is knowing the difference. Clean, accurate bookkeeping throughout the year makes this calculation far easier and far less risky at return time.
Tax Reliefs That Can Lower Your Bill
HMRC offers several legal ways to reduce your corporation tax liability. These are not tricks. These are created within the system to motivate businesses to invest and expand.
Three of the most popular reliefs are listed below, and each of them functions differently based on how your business spends and functions.
Capital Allowances and the Annual Investment Allowance
When a company buys equipment, machinery, or vehicles, it can deduct the cost from taxable profits through capital allowances. Annual Investment Allowance (AIA) allows most businesses to fully and immediately deduct up to 1 million pounds in a year, in the year of purchase.
R&D Tax Relief
Companies that develop new products, systems, or processes may qualify for Research and Development tax relief. This cuts the corporation tax bill and can even result in a cash repayment from HMRC. Many businesses qualify without realising it.
Loss Relief
If the company made a loss in a past accounting period, that loss can be carried forward. It then offsets taxable profits in future years, which directly reduces what gets paid to HMRC.
Filing Your Corporation Tax Return (CT600)
Every UK company files a CT600 form with HMRC each year. This is the official company tax return, and it goes in alongside the annual accounts. The two key dates every director must know are:
Payment deadline: 9 months and 1 day after the accounting period ends.
Filing deadline: 12 months after the accounting period ends.
Note that these are two different deadlines. Missing the payment date costs money in interest even if the return itself goes in on time. Support with accounts and CT600 filing keeps both on track. A free tax calculator is also available to get an early estimate of what is owed.
Penalties for Late Filing or Payment
HMRC does not give much grace when deadlines are missed. The fines start small but build up fast.
One day late brings a £100 fine. Being over three months late adds another £100. Past six months, HMRC adds 10% of the unpaid corporation tax on top. Past twelve months, another 10% is added again. Interest runs on top of all of this for every day the tax remains unpaid.
Errors on the return carry their own penalty scale, too. Accidental mistakes that are disclosed carry a 0% to 30% fine. Deliberate ones can reach 100%. Consistent bookkeeping and timely CT600 filing remove almost all of this risk before it starts.
Stay on Top of Your Corporation Tax Year-Round
Corporation tax does not have to be confusing or stressful. The key is having the right support in place well before deadlines arrive. Whether you need help with VAT returns, payroll, self-assessment, CIS returns, or CGT returns, having all your corporate tax obligations managed in one place means nothing gets missed and your business stays fully compliant all year long.