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CT600 rounding: should you round company figures up or down?

Last updated: 8 September 2026

Short version. Keep pounds and pence in your underlying records. When a field asks for a whole-pound figure, follow its entry instructions. Nearest-pound rounding means below 50p rounds down and 50p or above rounds up for positive amounts. This is different from rounding the Corporation Tax bill, which can include pence. A whole-pound field alone does not tell you which rounding direction to use.

1. Does HMRC require CT600 figures in whole pounds?

Some CT600 fields use whole pounds. Others allow pounds and pence. Turnover in box 145 uses a whole-pound format, while tax amounts can include two decimal places. The HMRC CT600 technical specifications contain the exact definitions. In the CT600 V3 (2026) Artefacts V1.994 download, open CT-2014-v1-994.xsd and look for Turnover, its Total element and the IRnonNegativeWholeUnitsMonetaryStructure definition. Turnover's restriction is at line 431. Following its base types leads to the .00 pattern at line 3930. These line numbers refer to V1.994.

That defines what the return can contain. It does not say that turnover must always be rounded down, always up or to the nearest pound. We do not present this schema as an HMRC instruction to use a 50p threshold.

A software field labelled Other charges may be an accounts input rather than a separate CT600 box. Do not assume every accounts line has an identically named box on the tax return.

2. How should I round figures for my CT600 filing?

Follow the instructions for the particular field and filing software. Where you are asked for amounts rounded to the nearest pound, add up the transactions belonging to each line first, then round that total. For positive figures, the convention works like this:

Nearest-pound examples for a category total
Total including penceWhole-pound entry
£1.49£1
£1.50£2
£1.99£2
£12,345.49£12,345
£12,345.50£12,346

For SimpleCompanyTax's whole-pound financial input fields, enter category totals using this nearest-pound convention. The fields do not accept pence. We calculate from the numbers you enter, so we cannot include pence left in your spreadsheet. This is our input convention, not a claim that HMRC has published a universal nearest-pound rule for CT600 inputs.

3. Should I round individual transactions or category totals?

Two sales in the same turnover line: £1.49 + £1.49 = £2.98. Rounded to the nearest pound, turnover is £3. Rounding each sale to £1 before adding them would give £2 and lose information unnecessarily.

Turnover and other income in separate lines: suppose each is £1.49 and there are no costs. The underlying accounting profit is £2.98. If each separate input is rounded to the nearest pound, the software receives £1 and £1, which add to £2. Rounding the original combined £2.98 instead would produce £3. These are different rounding stages, not different answers to £1.49 + £1.49.

This example explains a reconciliation difference. It is not a rule that taxable profit must be £2. Keep the original figures and reconcile the accounts, computation and return. Taxable profit also depends on any tax adjustments. See accounting profit versus taxable profit.

4. Does FRS 105 say to round up or down at 50p?

No. These paragraphs in the Financial Reporting Council's FRS 105 address accounts presentation and accounting policies, not a specific CT600 rounding algorithm:

These links open the September 2024 edition. They do not prescribe a 50p threshold or establish an ACCA-approved or Chartered Accountant-approved CT600 rounding rule. Calling nearest-pound presentation a convention is more precise than claiming it is required by these paragraphs.

5. Is Corporation Tax rounded to the nearest pound or penny?

The tax amount can include pence. For a simple illustration, if taxable profit is £10,001 and the applicable rate is 19%, with no further adjustments or reliefs, the tax is £1,900.19, not £1,900.

SimpleCompanyTax uses nearest-penny rounding for calculated tax and marginal-relief amounts, producing two decimal places. It does not always round tax up or always down to a whole pound. Whole-pound return fields are formatted separately using nearest-pound rounding. Not every intermediate value is rounded in the same way.

Rounding is not confined to the final bill. For example, where a tax calculation spans financial years, individual tax components can be rounded before they are added together. A spreadsheet that retains every fraction until its final cell can therefore differ. Compare the component amounts in the computation, not just the final total.

6. Why is my CT600 tax calculation different from Excel?

Formatting an Excel cell to show no decimal places does not remove its pence. A cell can display £1 while calculations still use £1.49. To model a nearest-pound entry for a positive category total in cell A1, use =ROUND(A1,0). Make sure A1 is the total for that line, not one transaction.

When comparing a spreadsheet with filing software, check:

  • The actual values supplied to both calculations, including hidden pence.
  • The same accounting period and any financial-year split.
  • The same tax adjustments, rates and reliefs.
  • Whether each tax component or only the final total is rounded to pence.

A difference below £1 can come from rounding, but its size alone does not prove the calculation is correct. If it remains unexplained, contact support before filing and provide the relevant figures and workings.

7. Why do some guides say to round income down and expenses up?

Different returns and concessions have different instructions. HMRC BIM31047, under SA returns describes rounding income and gains down and tax credits and deductions up. Do not treat that Self Assessment instruction as a universal CT600 input rule.

HMRC CTM93220, Computations: in round thousands discusses a conditional concession for computations in £1,000 units. It expressly says not to round on the CT600 itself or in preceding arithmetic. Read that statement in its round-thousands context. It does not instruct micro-entities to round turnover to the nearest pound.

Neither a software field rejecting decimals nor a reference to standard accounting guidance is, by itself, evidence of an HMRC-approved rounding direction. Check the specific source and the field it addresses.

8. What should I check before filing my CT600?

  • Keep the original records with pence and total each income or expense category before rounding.
  • Use the field's required precision and apply the stated convention consistently.
  • Reconcile differences between the original records and the figures used in the filing.
  • Review the accounts, tax computation and CT600 together before approving them.

This is general guidance, not advice on your company's specific tax position. Ask an accountant or tax adviser if you cannot reconcile the figures or are unsure which treatment applies.

Payments already made have their own reconciliation. See CT600 box 595 explained and partial-payment examples with pence.