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 | What It Is | Example |
|---|---|---|
| 1 | Company name | SMITH CONSULTING LTD |
| 2 | Company registration number | 12345678 |
| 3 | Tax reference (UTR) | 1234567890 |
| 4 | Company type | 6 (micro-entity) |
| 30 | Period start date | 01/04/2024 |
| 35 | Period end date | 31/03/2025 |
Turnover (Boxes 145-155)
| Box | What It Is | Example |
|---|---|---|
| 145 | Total turnover | £80,000 |
| 155 | Trading profits | £45,000 |
| 160 | Trading losses brought forward | £0 |
| 165 | Net trading profits | £45,000 |
How to calculate Box 155: Turnover minus all allowable business expenses (director salary, office costs, travel, equipment, professional fees, etc.)
Tax Calculation (Boxes 235-440)
| Box | What It Is | Example |
|---|---|---|
| 235 | Charges/expenses against profits | £0 |
| 295 | Profits before qualifying donations | £45,000 |
| 300 | Qualifying donations | £0 |
| 305 | Profits before group relief | £45,000 |
| 315 | Total profits chargeable | £45,000 |
| 326 | Associated companies (FY1) | 0 |
| 330 | Financial year 1 | 2024 |
| 335 | FY1 profit | £45,000 |
| 340 | FY1 tax rate | 19% |
| 345 | FY1 tax | £8,550 |
| 430 | Corporation tax | £8,550 |
| 440 | Tax chargeable | £8,550 |
Tax Payable (Boxes 510-515)
| Box | What It Is | Example |
|---|---|---|
| 510 | Tax chargeable | £8,550 |
| 515 | Tax payable | £8,550 |
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:
| Box | What It Is | Example |
|---|---|---|
| 326 | Associated companies (FY1) | 0 |
| 430 | Corporation tax (at 25%) | £25,000 |
| 435 | Marginal relief | £1,950 |
| 440 | Tax after marginal relief | £23,050 |
The marginal relief calculation (Box 435) uses the formula:
(Upper limit - Profits) × Profits/Augmented Profits × 3/200
Your software calculates this automatically. See our marginal relief guide for worked examples.
Dormant Company (Nil Return)
The simplest CT600 — almost everything is zero:
| Box | What It Is | Value |
|---|---|---|
| 1-4 | Company details | As normal |
| 30, 35 | Period dates | As normal |
| 145 | Turnover | 0 |
| 155 | Trading profits | 0 |
| 315 | Total profits | 0 |
| 430 | Corporation tax | 0 |
| 515 | Tax payable | 0 |
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: Non-trading income types (loan relationships, intangible assets, tonnage tax)
- Boxes 480-527: CFC tax, bank levy, bank surcharge, supplementary charges
- Boxes 530-615: R&D credits, creative industry relief, life assurance
- Boxes 790-965: Supplementary page details (losses from overseas trade, excess management expenses, etc.)
Capital Allowances Boxes
If you bought equipment, these boxes come into play:
| Box | What It Is | When to Use |
|---|---|---|
| 690 | Annual Investment Allowance (AIA) | Bought equipment up to £1m |
| 695-700 | Special rate pool | Long-life assets, integral features |
| 710 | Main pool allowances | 18% writing down allowance |
| 725 | Zero-emission car allowances | Electric company cars |
| 730 | Structures & buildings | Commercial property |
For most small companies, only Box 690 (AIA) matters — it gives you 100% deduction on equipment purchases.
Split Period Boxes
If your accounting period spans two financial years (e.g., calendar year ending 31 December), you need both FY columns:
| Boxes | FY1 | FY2 |
|---|---|---|
| 330-345 | First financial year | |
| 350-365 | Second financial year | |
| 326, 328 | Associated companies FY1 | Associated companies FY2 |
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.