Just like employed person pays PAYE and NI, every self-employed person must complete a self assessment tax return showing how much tax and Class 4 National Insurance contributions a person must pay. It is paid on the basis of business profits or revenue earned by an individual.
In order to calculate the profit, self employed person must first keep a record of his income and expenditure. It is sometimes difficult for non finance person to work out the actual profit earned during a tax year.