Thanks for your reply.
Its 2011 - i'm really confused, why is the taxcalc website saying zero?
Because I presume you've put in an annual figure. PRSI for employments is calculated weekly, on a standalone basis, it's not cumulative.
Consider 2 people:
Person 1:
Week 1 - 51: €351.99 p.w. - Class A0 = Pays no PRSI.
Week 52: €353 - Class AX - Pays 4% on €226 = €9.04
Total earnings €18,304.49 - PRSI paid €9.04.
Person 2:
Week 1 - 51: €352.50 p.w. - Class AX - Pays PRSI on €225.50 = €9.02 p.w. = €460.02
Week 52: €200 - Class A0 - pays no PRSI.
Total earnings €18,177.50 - PRSI paid €460.02.
So in that example both people have worked 52 weeks, and earned almost exactly the same amount gross over the course of the year, but the one who actually earned less will have paid about €9 a week more in PRSI.