How do I calculate the CGT on shares Im now selling

I have been buying shares in an investment trust ( outside of an ISA) most months over the last 10+ years ( so 120 +purchases) When I want to sell some, how do I calculate the CGT? Do I need to find the paper work from when I brought the shares and calculate the difference in the shares price? When I sell, are the "the oldest", or the newest shares sold?

Hi cp3,
If the share are of the same company and same kind, you can pool them together and use an average value for the acquisition cost.  This will be deducted from the disposal value, along with broker fees etc, to work out your gain/loss.  In this way it does not matter whether the share disposed of is older or newer than any other disposed at the same time.  This is known as a section 104 holding.  
Please have a look at the guidance at helpsheet HS284 for more information. Shares and Capital Gains Tax (Self Assessment helpsheet HS284).
Thank you.

Thank you for your reply. Looking at example 3, it looks like I need to go through every purchase of shares in a company to determine the cost of the pool? ... then it gets easier because of the pooling. I can only find the contract notes from the last 7 years. If I am unable to find the contract notes, what should I do.

Hi cp3,
You would need to contact the company direct od the London Stock Exchange.
Thank you. 

How does it work with sale of non UK shares when an average value is used for the acquisition cost of shares acquired over different dates, as far as the exchange rate for conversion back to sterling is concerned? What date of acquisition should be assumed when there is a partial disposal over the total holding? Please clarify this point as the question seems to have been asked by a few users already but no clear answer has been provided. Example: I own 1,000 shares in A Inc (US company) with 600 shares acquired in 2015, 300 shares acquired in 2017 and the remaining 100 shares acquired over a number of different dates between 2015 and 2024 due to dividend reinvestment. I sold 700 shares in 2023-2024. If I were to use the s104 pooling method to calculate the average cost base for my disposal, what date of acquisition should I assume for the purpose of converting the cost base from USD back to GBP?

Hi Pomhk,
Under the terms of Self Assessment, we do not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate for each acquisition and disposal. For your convenience, there are exchange rates at
Exchange rates from HMRC in CSV and XML format
and for older rates at
Foreign exchange rates and spot rates: 1 January 1989 to 31 March 2009.
You are free to use any of the supplied rates or one of your own choosing.  
You can use the date of the earliest share acquisition and use an exchange rate of your choosing.
Thank you.