Hi,
You should keep a record of your transactions, so that you know which result in a gain and those that resulted in a loss in each tax year. Capital gains tax is payable where the disposal value, minus allowable costs and losses, still result in a gain. Where the gain exceeds the annual exempt allowance, then you will have tax to pay.
There is a calculator below to help with this, as well as a link to the realtime capital gains service (RTTCGT), where you report and pay your Capital Gains Tax.
Tax when you sell shares
Please note that you need to have declared your gains using the RTTCGT service no later than 31 December, after the tax year that the gain arises, has ended. If you miss this deadline date, you are required to declare the gain in a Self Assessment Tax Return. Interest arising from savings in an ISA are not taxable, while they remain in the ISA.
Thank you.