Corporation Tax (CT600)
Corporation Tax is the tax UK limited companies pay on their profits. The CT600 is the annual return you file with HMRC to report your taxable profits and calculate your tax bill.
Filing Deadline
12 months after year-end
Tax Rate 2024/25
19% - 25%
Payment Deadline
9 months + 1 day
Corporation Tax Rates 2024/25
Since April 2023, Corporation Tax has a tiered structure based on your company's profits. Small companies pay less than larger ones.
£0 - £50,000
Small profits rate
£50,000 - £250,000
Marginal relief
£250,000+
Main rate
What is Marginal Relief?
If your profits are between £50,000 and £250,000, you get "marginal relief" — a gradual increase from 19% to 25% rather than a sudden jump. AccountsOS calculates this automatically.
Important Deadlines
Filing Deadline
12 months
after your accounting period ends. You must file your CT600 return with HMRC.
Payment Deadline
9 months + 1 day
after your accounting period ends. You must pay any Corporation Tax owed.
Example: If your accounting year ends on 31 March 2025:
• Payment due: 1 January 2026 (9 months + 1 day)
• CT600 filing due: 31 March 2026 (12 months)
Reducing Your Tax Bill
Corporation Tax is calculated on your taxable profits — not your total revenue. You can deduct allowable business expenses to reduce your tax bill.
Late Filing Penalties
HMRC charges penalties for late CT600 returns. These are in addition to interest on any unpaid tax.
Interest: HMRC also charges interest on late payments at the Bank of England base rate + 2.5%. This adds up quickly on large tax bills.
Calculate Your Corporation Tax
Use our free calculator to estimate your Corporation Tax liability. Enter your profit and see your tax bill instantly.
- Automatic marginal relief calculation
- Shows small profits vs main rate
- Includes payment deadline
- Compare with previous years
Example Calculation
Effective rate: 19.8%
How AccountsOS Helps
We track your Corporation Tax liability all year, not just at year-end.
Real-Time Tax Estimate
See your estimated CT liability update as you record transactions. No year-end surprises.
Deadline Reminders
Automatic reminders for both filing and payment deadlines based on your accounting period.
Expense Tracking
AI categorizes your expenses correctly to maximize legitimate deductions.
CT600 Preparation
We prepare your figures for CT600 filing. Just download and submit to HMRC.
Frequently Asked Questions
When do I need to register for Corporation Tax?
Within 3 months of starting to trade. If you've just incorporated, HMRC will contact you — but you should register proactively at gov.uk.
Can I change my accounting year-end?
Yes, you can shorten or extend your accounting period by filing with Companies House. This affects your CT deadlines, so plan carefully.
What if my company makes a loss?
You don't pay CT on losses. You can carry the loss forward to offset against future profits, or in some cases, carry back to previous years.
Do I still need to file if my company is dormant?
Dormant companies may not need to file CT600 returns if they've told HMRC they're dormant. But you still need to file annual accounts with Companies House.
Related Resources
Track Your Corporation Tax Year-Round
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