Manual steps and calculations for working our your net take home pay per fortnight

Hi
I trying to work out what my net fortnightly take home salary should be and how the tax/Prsi/usc calcs are worked out

Im using some of the online salary calculators e.g. http://download.pwc.com/ie/2018-income-tax-calculator/index.htm

The figures they are generating for net fortnightly take home salary are less that what my payslip is saying

I am not on emergency tax

The figures on my payslip and on the online calculators match for Taxable income but the amounts for income tax etc. differ between payslip and online calculator - the online calculator is showing more tax deducted than payslip is and hence online calculator net take home salary is less than what payslip is showing

Does anyone know the manual steps/calculations/formulas to figure out your net fortnightly take home pay or is there another post on here that shows this

Thanks

Income tax is based on your gross pay taxed at 20% or 40% less your tax credits.

So if youre single then it’s the first €34,800 at 20% with the balance at 40% less the Personal Tax Cr of €1,650 and PAYE Tax Cr of €1,650.

PRSI is 4% of your salary.

USC is .5% on 12012
Then 2% on 7,360
And 4.75% in the balance

Put up your gross pay,credits etc and there are people who will happily take you through the calculations.