When you sell or otherwise dispose of a capital item for more than you paid for it, making a 'capital gain', that gain may be taxable.
If your company or organisation is liable to Corporation Tax, you don't pay Capital Gains Tax separately on any gains. Instead, you pay Corporation Tax on your 'chargeable gains'. In your Company Tax Return you calculate your net chargeable gains and add that to your net income.
Including chargeable gains on your Company Tax Return
You enter your gross chargeable gains, if any, in Box 16.
Then enter your capital losses for your current accounting period and any brought forward from previous accounting periods in Box 17. You can only enter a figure here that's less than or equal to your gains in Box 16.
Deduct Box 17 from Box 16.
If the result is positive or zero, put that figure in Box 18. This is your 'net chargeable gain' figure. You can't have a negative figure in Box 18.
If you have made capital losses for this accounting period, you can't deduct those losses from your profits in the same accounting period. You leave Boxes 16 and 17 blank. Instead, you put the amount of capital losses in Box 131.
You can then 'carry forward' those capital losses to offset against any capital gains you make in a future Corporation Tax accounting period(s). You would include those losses in Box 17 of a future Company Tax Return. You can carry forward capital losses indefinitely.