Q. What is the amount of Nisab? How is the Nisab amount calculated? Is it a fixed dollar amount?
A. The amount of Nisab is calculated by you finding out if you have the value of the following: if the silver you have is 52 1/2 Tolas (634 grams), or as in the case of gold, 7 1/2 Tolas (87.48 grams), then you are eligible to pay Zakat. Thus, the Nisab for cash would be calculated in accordance to whether that cash amount is above the value of 634grams of silver or 87.48grams of gold. (If you have gold and silver which is less than the mentioned weight, however, when they are combined, and its dollar value equals the amount of Nisab, then too you are obligated to pay Zakat.)
To calculate, go here. Kindly place ounces in ounces box, grams in the grams box, silver in the silver box, etc.
Allah certainly knows best.