Nisab
In one sentenceThe minimum amount of wealth on which Zakat is due, equal to the value of 85 grams of 24-karat gold or 595 grams of silver.
Simply put
The nisab value in SAR changes with the gold price, so you calculate it on the day the hawl completes: 85 × the price of one gram of 24-karat gold on that day.
If your zakatable wealth is below this value, no Zakat is due; if it reaches or exceeds it, you pay 2.5% of the whole amount, not only of the excess.
Example
An individual whose hawl completed today multiplied 85 by today's published price of one gram of 24-karat gold to find the nisab, then compared his cash and savings against it.
Don't confuse it with
HawlThe nisab is a condition on the amount of wealth; the hawl is a condition on how long it is held.