CT600 Box by Box — Which Boxes Do I Actually Need to Fill In?

The CT600 Looks Intimidating — It Doesn't Have to Be

The CT600 form has over 200 numbered boxes. Open the HMRC guidance and it's 100+ pages. But here's the secret: most companies only fill in about 25-35 boxes. The rest are for specialist situations (oil companies, insurance companies, ring-fenced profits, etc.) that don't apply to the vast majority of UK limited companies.

This guide tells you exactly which boxes matter for the three most common company types.

Micro-Entity Contractor (Single Director, Under £632k Turnover)

This is the most common type filing through Taxpipe. A single director-shareholder, typically contracting or freelancing, with turnover under £632,000.

Company Information (Boxes 1-50)

Box 4 is a code for your company type — not an accounting standard. For a standard UK limited company, enter 0. The full list: 0 = normal company, 1 = unit trust/OEIC, 2 = close investment-holding company, 3 = company in liquidation (2nd+ year), 4 = qualifying asset holding company, 5 = insurance, 6 = members' club or voluntary association, 7 = property management company, 8 = charity, 9/10 = REIT, 11 = non-resident. If none of those apply, enter 0.

Turnover (Boxes 145-155)

How to calculate Box 155: Start with your accounting profit, add back any expenses HMRC doesn't allow (depreciation, client entertaining, personal costs), then deduct capital allowances. The result is your taxable trading profit. For a straightforward service company with no fixed assets, this is close to turnover minus allowable business expenses.

Tax Calculation (Boxes 235-440)

Box 235 is a calculated subtotal — the sum of net trading profits plus other income (interest, property, gains) minus any brought-forward non-trading deficits. Boxes 295 to 315 step through total deductions to arrive at the profits actually chargeable to Corporation Tax. Box 326 records the total number of associated companies for the entire accounting period; this affects the thresholds for the small profits rate and marginal relief.

Final Tax Figures (Boxes 510-528)

Box 515 is Income Tax already withheld from income your company received — for example, patent royalties paid by individuals or certain interest payments. For most small trading companies it is zero. Box 525 is your self-assessed Corporation Tax payable: box 510 minus box 515.

That's it. About 20 boxes for a straightforward micro-entity. Everything else is zero or doesn't apply.

Company with Marginal Relief (Profits £50k-£250k)

If your profits are between £50,000 and £250,000, you get marginal relief. The boxes are mostly the same as above, with these additions (example: £100,000 profit, no associated companies):

The marginal relief calculation (Box 435) uses the statutory formula from CTA 2010 s.18B:

3/200 × (Upper limit − Augmented profits) × Profits / Augmented profits

For most companies with no exempt distributions, augmented profits equal taxable profits, simplifying to 3/200 × (Upper limit − Profits). For £100,000 profit with no associated companies: 3/200 × (£250,000 − £100,000) = £2,250, giving £22,750 net.

Your software calculates this automatically. See our marginal relief guide for worked examples.

Dormant Company (Nil Return)

The simplest CT600 — almost everything is zero:

About 10 boxes total. Taxpipe's dormant shortcut auto-zeros everything.

Boxes You Can Safely Ignore

Unless your accountant tells you otherwise, these box ranges usually don't apply to small companies:

• Boxes 95-144: Supplementary pages (CT600A through CT600N) — for close company loans, controlled foreign companies, tonnage tax, etc.
• Boxes 172-210: Specialist income types (non-trading loan deficits carried forward, non-UK dividends, income subject to Income Tax deduction at source, intangible asset gains, tonnage tax profits)
• Boxes 480-509: S455 tax on loans to participators, CFC tax, bank levy, bank surcharge, RPDT, energy levies, ring fence supplementary charges
• Boxes 530-615: R&D credits, creative industry relief, life assurance
• Boxes 790-965: Losses, supplementary page details, excess management expenses, repayment instructions

Note: Box 190 (UK property rental income) and boxes 210-220 (chargeable gains) may be relevant if your company receives rental income or disposes of capital assets during the period.

Capital Allowances Boxes

If you bought equipment, these boxes come into play:

For most small companies, only Box 690 (AIA) matters — it gives you 100% deduction on equipment purchases up to £1,000,000.

Split Period Boxes

If your accounting period spans two financial years (e.g., calendar year ending 31 December), you need both FY columns:

Box 326 (associated companies for the whole period) is used when the period falls entirely within a single financial year. When profits span two financial years, boxes 327 and 328 hold the associated company count for each year separately.

The profit is apportioned by days in each financial year.

Let Software Handle It

You don't actually need to memorise any of this. Modern CT600 software — including Taxpipe — maps your plain-English inputs (turnover, expenses, equipment purchases) to the correct boxes automatically.

The value of this guide is knowing what's happening behind the scenes, so you can verify the output makes sense.

Calculate your corporation tax now →