Corporation Tax & CT600
Understand the company tax computation, CT600 structure, supplementary pages and how figures flow from accounts into the return.
Corporation Tax & CT600 explained
Corporation Tax is charged on a company’s taxable total profits, not simply the accounting profit shown in its statutory accounts. A Company Tax Return brings together the tax computation, CT600 and, where relevant, supplementary pages.
This learning path covers UK rules and ReturnDeck’s UK form set. Check the effective period and official source before using a rule for a live filing.
Learn it in stages
Beginner · From accounts to taxable profit
Start with the company accounts, identify trading and other income, then distinguish accounting expenses from deductions allowed for Corporation Tax. Depreciation is normally added back and qualifying capital expenditure is dealt with through capital allowances. Learn the difference between accounting profit, adjusted trading profit, chargeable gains and taxable total profits.
Intermediate · Rates, periods and reliefs
Understand the accounting period, associated-company effects on profit limits, the small-profits and main-rate framework, marginal relief, losses, capital allowances and other reliefs. Keep the calculation period-specific: the return and computation must use the rules applying to that accounting period.
Advanced · CT600 structure and supplements
Follow the computation into the CT600: company information, turnover and profit figures, tax calculation, reliefs, repayments and declarations. Then determine whether supplementary pages are required for items such as loans to participators, group relief, R&D or other specialist claims.
Filing · Return versus payment deadline
A Company Tax Return is normally due 12 months after the end of the accounting period. Corporation Tax is usually payable earlier — normally 9 months and 1 day after the end of the accounting period for companies outside the quarterly-instalment regime. Treat filing and payment as separate obligations.
Example: accounts to CT600
A company has £180,000 accounting profit before tax. Its accounts include £12,000 depreciation and £4,000 client entertaining, and it has £20,000 qualifying plant expenditure. The learning task is to add back non-deductible/accounting-only items, calculate the appropriate capital allowance, derive taxable profits, then carry the resulting figures into the relevant CT600 boxes. ReturnDeck Practice Mode shows the source documents and expected field flow rather than asking you to memorise a single tax figure.
Apply this in Practice Mode →Common mistakes to avoid
- Using accounting profit as taxable profit without adjustments
- Forgetting that payment and filing deadlines are different
- Claiming an expense because it appears in the accounts without checking tax deductibility
- Ignoring associated companies when considering Corporation Tax limits
- Completing CT600 boxes without reconciling them to the computation and accounts
Records and preparation checklist
Before calculating or filing, assemble the evidence that supports the position. This makes the return easier to complete and easier to explain later.
- Final statutory accounts and trial balance
- Detailed profit-and-loss and balance-sheet schedules
- Fixed-asset additions/disposals and capital-allowance pool
- Expense analysis for disallowables and private/non-business items
- Losses, reliefs, tax credits and prior-period information
- CT600 plus any supplementary pages that actually apply
Key terms
The profits on which Corporation Tax is calculated after the relevant tax adjustments and reliefs.
An accounting expense that is not deductible, or not fully deductible, for Corporation Tax.
A tax deduction for qualifying capital expenditure, used instead of accounting depreciation.
The period for which Corporation Tax is calculated; it cannot normally exceed 12 months.
Follow the lesson into the form
ReturnDeck’s 1,139 mapped UK fields connect form codes to plain-English meaning, applicability, source figures, calculation notes, examples, common mistakes, interactions and HMRC sources.
Calculate and check
Use the related ReturnDeck calculator or checker to turn the lesson into a practical result where the verified rules support deterministic calculation.
Open related tool →Practice what you learned
Move from learning into a fictional case file, source documents, return completion, validation and a worked answer. Practice Mode never submits anything to HMRC.
Continue to practice →Official HMRC / GOV.UK sources
These are the primary official sources for this learning path. ReturnDeck summarises them for learning; the official material remains the authority.
Source set reviewed for ReturnDeck’s 2026 UK learning layer. Tax rules change; verify the effective date for a live decision.