Is Zakat on money calculated upon reaching the nisab or upon the passing of a hawl (full lunar year)? And upon what amount is it calculated if the amount differs between reaching the nisab and the passing of the hawl?

For Zakat (definition) to be obligatory on money, two conditions must be met: reaching the nisab (definition) (minimum threshold) and the passing of a hawl (one lunar year) on it. The nisab is equivalent to 85 grams of gold or 595 grams of silver. The amount obligatory to be given out is a quarter of a tenth (2.5%). If the wealth increases during the hawl, then if the increase resulted from the original capital, it is subject to Zakat along with the principal. If it did not result from the original capital, then a separate hawl is calculated for it, or it can be given as Zakat along with the principal by way of anticipation. If the increase is gradual (like saving a salary), you have the option of either giving Zakat on the entire amount at the end of the hawl for the first sum, or making a separate hawl for each addition.