How to work out how much tax and national insurance to pay on a certain amount?

I have some shares available to sell... I know how much the shares are worth but they are subject to tax and national insurance. How do I work out how much my shares are worth after deducting the tax and national insurance.
(i.e. what are the percentages of tax and national insurance)

ANSWERS FROM UK RESIDENTS ONLY PLEASE!

Answer: 
Sale of shares is not subject to income tax or national insurance. You will be liable for capital gains tax on the profit you make (the selling price less the buying price less any expenses such as commission paid on both transactions). You can make up to £10,100 of gains in a year tax free, any gain after that is taxed at 18% if you are a basic rate income tax payer or 28% if you are a higher rate payer. I will hazard a guess that if you don't know which tax is due on the profits you probably do not have that many shares so your profits will be under the £10,100 limit and all your profit will be tax free.

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