how is class 4 national insurance calculated
You pay a flat rate of Class 2 National Insurance contributions regardless of how profitable your business is (though low-earning businesses can be exempted altogether). You also pay Class 4 National Insurance contributions calculated as a percentage of your annual profits above a basic threshold.National Insurance contributions are calculated and collected as part of your self-assessment tax return.ou pay a flat rate of Class 2 National Insurance contributions regardless of how profitable your business is (though low-earning businesses can be exempted altogether). You also pay Class 4 National Insurance contributions calculated as a percentage of your annual profits above a basic threshold.