I want to give rs.1lacs (100,000 rupees) to my sister's son who resides in the UK as my sister died recently and he has no father. My sister's son is an adult and is earning. So is this gift taxable to me or him? If so how do I calculate it?
The UK doesn't have a gift tax. In limited circumstances if the giver is also in the UK and dies within 7 years, then some inheritance tax might be payable, but if you're in India that won't apply.
India also appears not to have any gift tax if the giver is an uncle of the recipient, so no tax will be payable by either party here.
There's also no tax deduction for gifts in either the UK or India, so if this is out of your income you'll probably already have paid tax on the money in some form.