Class 4 NIC and Class 1 NIC interaction

Hi, If you pay a maximum amount of annual NIC by way of class 1 contributions, I understand that you do not need to pay the full amount of class 4 NIC on self-employed profits. The Class 1 NIC liability is calculated first, then you can pay 2% class 4 NIC on all profits above the level of £12,570 for the current Tax year. Can you please explain how this works and how I can calculate this for my Self-Assessment tax return? thanks. Paolo.

Hi,
If you are employed, but also do self-employed work, your employer will deduct your Class 1 National Insurance from your wages.  
You may also have to pay Class 4 National Insurance for your self-employed work.  
How much you pay depends on your combined wages and your self-employed work.  
HMRC will let you know how much National Insurance is due after you’ve filed your Self Assessment tax return.
Thank you.