If you run a UK limited company, you've probably heard the term "CT600" thrown around. Maybe your accountant mentioned it, maybe HMRC sent you a letter, or maybe you just stumbled across it while trying to figure out your tax obligations.
Either way, here's everything you need to know — in plain English.
What is a CT600?
A CT600 is the form your company uses to tell HMRC how much money it made, how much Corporation Tax it owes, and any reliefs or deductions it's claiming.
Think of it like a self-assessment tax return, but for your company rather than for you personally.
Every UK limited company has to file one if HMRC has issued a notice to deliver a return — even if the company made no profit that year. Dormant companies are a special case: once you have notified HMRC that your company is dormant, no CT600 is required unless HMRC sends you a notice specifically asking for one.
Who needs to file a CT600?
If you have a limited company registered with Companies House, you need to file a CT600 with HMRC. This includes:
- Active trading companies — even if you made a loss
- Loss-making companies — zero profit still means a return is due
- Companies with no Corporation Tax to pay — you still need to file the return
Dormant companies are treated differently. Once you notify HMRC that your company is dormant, no CT600 is required unless HMRC issues a notice to deliver one (form CT603). If HMRC does send that notice, you must file — but absent a notice, a properly notified dormant company has no CT600 filing obligation. We have a separate guide for dormant companies.
What information goes into a CT600?
The CT600 covers your company's financial year (called an "accounting period" by HMRC). Here's what you'll typically need:
Basic company details
- Your company name and registration number
- Your Unique Taxpayer Reference (UTR) — the 10-digit number HMRC gave you
- The start and end dates of your accounting period
Income and expenses
- Turnover — the total money your company brought in (sales, fees, etc.)
- Costs and expenses — what it cost to run the business (rent, supplies, salaries, software subscriptions, etc.)
- Profit or loss — the difference between income and expenses
Tax calculation
- Taxable profit — your profit after any allowable deductions
- Corporation Tax rate — currently 19% for small profits (under £50,000) and up to 25% for profits over £250,000, with marginal relief in between
- Tax due — the amount your company needs to pay
Allowances and reliefs
- Capital allowances — tax relief on things like equipment, computers, or vehicles
- R&D tax credits — if your company does qualifying research and development
- Losses — if your company made a loss, you can carry it forward or back
What about company accounts?
Along with your CT600, you also need to submit your company accounts in a special digital format called iXBRL (inline eXtensible Business Reporting Language).
If your company qualifies as a micro-entity — most small one-person companies do — then your accounts are simpler. A micro-entity is a company that meets at least two of these three conditions:
- Turnover of £1,000,000 or less
- Balance sheet total of £500,000 or less
- 10 employees or fewer
(These thresholds were increased in April 2025. If your accounting period began before 6 April 2025, the previous limits of £632,000 turnover and £316,000 balance sheet apply.)
If that sounds like your company, you're in luck. Micro-entity accounts are the simplest type, with fewer disclosure requirements.
When do you need to file?
You need to file your CT600 within 12 months of the end of your accounting period. So if your company's financial year runs from 1 April 2025 to 31 March 2026, your CT600 is due by 31 March 2027.
But watch out — you need to pay your Corporation Tax much earlier. The payment deadline is 9 months and 1 day after the end of your accounting period. Using the same example, that's 1 January 2027.
Miss either deadline and HMRC will charge you. For returns due from 1 April 2026, late filing penalties are £200 initially, plus another £200 if still outstanding after 3 months. On top of that, tax-geared penalties apply — 10% of any unpaid tax at 6 months late, and a further 10% at 12 months. Interest also runs on any tax paid late.
How do you file a CT600?
There are a few ways:
- HMRC's own online service — this closed in March 2026 and is no longer available
- Commercial software — this is what most companies use, either through an accountant or directly
- An accountant — they'll use software on your behalf, but you're paying for their time on top of the software cost
The key thing to know is that CT600s must be filed electronically. You can't post a paper form to HMRC.
Common mistakes to avoid
Here are the mistakes we see most often:
- Wrong accounting period dates — make sure they match what HMRC has on file for your company
- Forgetting to include bank interest — even £3.42 of interest from your business bank account counts as income
- Missing the payment deadline — remember, payment is due before the filing deadline
- Not notifying HMRC of dormancy — if your company becomes dormant, tell HMRC. Once notified, you won't need to file a CT600 unless HMRC sends you a specific notice. Failing to notify means HMRC may still expect a return
- Mixing up personal and company money — director's loan accounts need to be reported correctly
Can you file your own CT600?
Absolutely. Thousands of company directors file their own CT600 each year. If your company is relatively straightforward — say, a freelancer or contractor operating through a limited company — the numbers aren't complicated.
What makes it tricky isn't the maths. It's the software and the format. HMRC requires everything in specific digital formats, and most commercial software is designed for accountants, not regular people.
That's exactly why we built Taxpipe. It walks you through the CT600 step by step, in plain English, and handles all the technical formatting behind the scenes. You answer simple questions about your company, and it generates your CT600 and micro-entity accounts, ready to submit to HMRC.
Filing your CT600 doesn't have to be stressful. With the right tool and a bit of understanding, it's something most directors can handle themselves in under an hour.
File your CT600 with Taxpipe
Now you know what a CT600 is — filing one is easier than you think. Taxpipe's guided wizard walks you through each section in plain English. £59 one-time, no accountant needed.
Property company? Read our guide to CT600 for property companies →
Related articles
- How to File a CT600 Online — Step by Step
- Your First Company Tax Return
- Filing a CT600 for a Dormant Company
- 10 Common CT600 Mistakes
- How to Register for Corporation Tax
- Corporation Tax Losses: Carry Forward and Carry Back
- Do I Need to File a CT600?