CT600 Box 155: Trading Profits Explained
Box 155 on the CT600 is where you enter your company's net trading profit (or loss). It's arguably the most important single number on the entire return — it drives your corporation tax calculation.
Getting it right means paying the correct amount of tax. Getting it wrong means penalties, interest, or overpaying.
What Box 155 Represents
Box 155 = Adjusted trading profit for the accounting period.
This is NOT simply your accounting profit from your profit and loss account. It's your accounting profit with tax adjustments applied.
The Formula
Accounting profit (from P&L)
+ Disallowable expenses (added back)
- Non-taxable income (deducted)
- Capital allowances (deducted)
= Box 155: Adjusted trading profit
Step-by-Step Calculation
1. Start with Your Accounting Profit
Take the net profit figure from your profit and loss account. This is revenue minus expenses as shown in your accounts.
2. Add Back Disallowable Expenses
Certain expenses that reduce your accounting profit are not allowed for tax purposes:
| Expense | Tax Treatment |
|---|---|
| Entertaining clients/customers | Add back — never deductible |
| Depreciation | Add back — capital allowances claimed instead |
| Fines and penalties | Add back — not deductible |
| Political donations | Add back |
| Non-business proportion of mixed expenses | Add back the personal portion |
| Provisions (unless specific) | Add back general provisions |
| Capital expenditure in P&L | Add back — claim via capital allowances |
3. Deduct Non-Taxable Income
Some income in your accounts isn't trading income:
- Bank interest — reported separately in box 160 (non-trading loan relationships)
- Rental income — reported separately if from investment property
- Gains on asset disposals — reported as chargeable gains, not trading profit
- Government grants — may be spread over multiple periods
4. Deduct Capital Allowances
Instead of depreciation (which you added back), claim capital allowances:
| Allowance | Rate | Limit |
|---|---|---|
| Annual Investment Allowance (AIA) | 100% | £1,000,000 |
| Full Expensing (plant & machinery) | 100% | Unlimited (main rate items) |
| Writing Down Allowance (main pool) | 18% | No limit |
| Writing Down Allowance (special rate) | 6% | No limit |
| Structures & Buildings Allowance | 3% | No limit |
5. The Result is Box 155
Your adjusted trading profit goes in box 155. If it's a loss, enter it with a minus sign (or in box 275 for trading losses).
Worked Example
XYZ Ltd — Year ended 31 March 2025
| Item | Amount |
|---|---|
| Revenue | £200,000 |
| Expenses (per accounts) | £150,000 |
| Accounting profit | £50,000 |
Tax adjustments:
| Adjustment | Amount |
|---|---|
| Add back: depreciation | +£5,000 |
| Add back: client entertaining | +£2,000 |
| Add back: parking fine | +£100 |
| Deduct: bank interest (→ box 160) | -£500 |
| Deduct: capital allowances (AIA on equipment) | -£8,000 |
| Box 155: Adjusted trading profit | £48,600 |
Common Box 155 Mistakes
1. Including Bank Interest
Bank interest is NOT trading income. It goes in box 160 (income from non-trading loan relationships), not box 155.
2. Forgetting to Add Back Depreciation
Depreciation is an accounting concept, not a tax deduction. Always add it back, then claim capital allowances separately.
3. Double-Counting Capital Allowances
If you've already deducted equipment costs as expenses in your P&L AND claimed capital allowances, you've deducted them twice. Capital allowances replace the accounting depreciation — they don't add to it.
4. Including Non-Trading Income
Rental income, investment gains, and other non-trading income have their own boxes. Box 155 is for trading activities only.
5. Negative Numbers
If your adjusted trading profit is negative (a loss), you should enter zero in box 155 and report the loss in the losses section (boxes 275-310). However, some filing software handles this automatically.
Related CT600 Boxes
| Box | Description |
|---|---|
| 145 | Turnover (total revenue) |
| 155 | Trading profits (what we're discussing) |
| 160 | Trading losses brought forward |
| 165 | Net trading profits |
| 170 | Bank and other interest income |
| 190 | Income from property |
| 235 | Profits before deductions |
How Taxpipe Helps
Taxpipe's guided wizard walks you through each adjustment:
- Enter your accounting profit
- Answer questions about disallowable expenses
- Enter your asset purchases for capital allowances
- The software calculates box 155 automatically
No accounting knowledge required — the wizard explains each step in plain English.
File your CT600 → — £59, with guided box-by-box support.