Use calculator:
sedlak.pl/Kalkulator_wynagrodzen.html
Just type there gross salary and in next view you will see your NET monthly salary in: Umowa o pracę > Koszty pracownika > Netto (column).
Examples:
1680zł gross = 1237zł net
4000zł gross = 2854zł net
8000zł gross = 5640zł net
15000zł gross = 10518zł net
So as you see net salary is between 74% of gross down to 70% gross in case of high earnings. (That's not so bad, because in Germany it's around 60%)
The difference is:
Health insurance
Sick insurance (worker get payed when sick)
Disability insurance
Retirement Insurance
Advance payment of income tax
Foreigner must pay that money, cannot get them back when leaving Poland. Without permanent permit to stay foreigner could take advantage of health and accident insurance, but contributions for retirement found will be wasted except if foreigner decides to retire in European Union.
In some cases foreigner has to pay extra tax at home. That depends on agreements between countries. You're lucky because there is favorable agreement between India and Poland
przepisy.gofin.pl/przepisy,4,15,121,99,,,umowa-miedzy-rzadem-polskiej.html
It says in article 16th that earning of Indian worker in Poland can be taxed in Poland only:
1. Z zastrzeżeniem postanowień artykułów 17, 18, 19, 20, 21 i 22 pensje, płace i podobne wynagrodzenia, które osoba mająca miejsce zamieszkania w Umawiającym się Państwie osiąga z pracy najemnej, mogą podlegać opodatkowaniu tylko w tym Państwie, chyba że praca wykonywana jest w drugim Umawiającym się Państwie. Jeżeli praca jest tam wykonywana, to osiągane za nią wynagrodzenie może być opodatkowane w tym drugim Państwie.
will i have to pay ZUS over this 18% or is that something that my employer will pay and will not be deducted from my salary.
Your employer will be contributing to your insurances too. But that's his extra cost. It's around 20% of gross salary in Poland. It means, that when you get employed for average salary 4000zł gross, it will cost your boss 4824zł. Employer would get only 2853zł of that. That means that total taxation of work in Poland is around 41%. It's possible to lower amount of taxes payed, by working as a contractor:
pl.wikipedia.org/wiki/Umowa_%C5%9Bmieciowa
But that's usually not available for foreigners, because employment status is usually needed in order to get visa.