National Insurance Help

Hi, I just wanted to confirm how national insurance works. Is it based on my monthly income or my overall annual income? I worked 3 different jobs (2 months 2 months 1 months) and got varying amounts of NI taken from them but earnt under 13k thanks!

Hi
National Insurance is charged individually on each job if you have multiple employments during a tax year.
There are weekly, monthly and annually thresholds and if you hit the Primary Threshold during the week, month or year you are paid, you will be charged National Insurance.
National Insurance will be charged how ever often you are paid and receive a payslip from your employer.
For example one month your gross pay could be well above the Primary Threshold in which case you will be charged national insurance for that month even though your Annual Gross earnings are below the Annual Primary threshold.  
Rates and allowances: National Insurance contributions 
There is also a National Insurance calculator you can use to check to see if your employer has took off the correct amount of National Insurance during the Week, Month or Your Annual Pay.
How much are you paid? - PAYE Tax Calculator 
Thank you.