What is the ruling on one who withholds Zakat and is negligent in paying it due to his ignorance of how to calculate it? And how is Zakat calculated?

Not knowing how to calculate zakat is not an excuse for not paying it. It is obligatory for anyone who does not know to ask. The amount of zakat varies depending on the asset being zakated. Zakat on gold, silver, paper currencies, and trade goods is one-quarter of one-tenth, meaning the wealth is divided by forty, and one part out of these forty is given.