Back to Basics: Corporation Tax Computation

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By Duncan McKellar, Tax Tutor

23 January 2017

In this guide we aim to provide you with a checklist of the steps to allow you to confidently calculate a corporation tax liability from the start point of a set of accounting results.

Calculating the corporation tax (CT) liability of a company involves a number of steps, starting with calculating the tax-adjusted trading profit or loss of the business and progressing through to applying the appropriate tax rate to Taxable Total Profits (TTP).

1. Adjust the accounting profit

Adjust the accounting profit or loss to arrive at a tax-adjusted trading result for the period. This adjustment will usually include a calculation of capital allowances as a trade deduction.

2. Check Chargeable Accounting Period

Consider the dates of, and length of, the Chargeable Accounting Period (CAP) of the business and whether this spans more than one financial year or not. A CAP falling entirely in FY15 and FY16 will result in a tax charge of 20% x TTP as there is only a single rate of CT from 1 April 2015.

3. Apply relevant tax rate

Apply the relevant tax rate (20% for FY15 / FY16)  to TTP.

Following these steps should assist you in calculating corporation tax liabilities and you will find that not every computation involves every step. However, you should get used to working through each of the steps, in turn to avoid missing anything crucial.

Once you have completed the corporation tax liability it is good practice to follow up with filing and payments dates. The CT600 will generally be due 12 months after the end of the CAP.

The payment date will depend on whether the company is ‘large’ or not. Large companies pay their CT liability in quarterly instalments, while other companies will pay 9 months and 1 day after the CAP.

Calculating tax

4. Dividend income

Identify any dividend income that must be included in the calculation of Augmented Profits (AP), being TTP plus dividends received. Dividends from non-group companies will generally be included in this figure.

5. Compare AP

Compare the AP figure to the adjusted corporation tax limits of £1,500,000, above which a company will have to pay tax in instalments. This figure is adjusted for short CAPs and is divided by the total number of '51% group companies'.

Note on quarterly instalments:

Corporation tax quarterly payments are introduced at TC Principles of Tax and remain examinable at TPS Taxation. A taxpayer company will not only want their adviser to calculate the tax but to be clear about when this is paid.

For large companies this is quarterly. Here's a question we’ve received on this:

  • “I can't seem to get the right dates when working out quarterly payments of corporation tax for large companies, particularly when it's not a 12-month period. Can you help?”

Fiona Winter, one of our tax tutors explains:

The approach is the same whether you are dealing with a 12-month period or a short CAP (remember that a company cannot have a CAP greater than 12 months). Always fix the first and last payment dates in stone - these can’t be moved:

  • First payment = 6 months and 14 days (we will call this 6 ½ months) after the start of the CAP.
  • Final payment = 3 ½ months after the end of the CAP.

Now go back to the first payment and add three months - this will be your second payment date as long as it is not later than the final payment.

Move three months on from the second payment to identify the date of the third payment - again as long as it is not later than the final payment.

You will never have more than four instalment payments but you may have fewer than four.

The amount due on each date is 3/n of the estimated CT liability, or the balance of tax if lower. "n" refers to the number of months in the CAP.  So for a 12 month period each payment would be 3/12 (or 1/4) of the liability. For an 8-month period they would be 3/8, 3/8 and finally the balance.


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