Why the nisab changes every day
The nisab was set as 20 gold dinars or 200 silver dirhams. Those weights do not change, but the price of gold and silver does, so the nisab in pounds, dollars or lira moves with the metal markets. It is worth checking on your zakat date rather than reusing last year's figure.
Gold or silver
The silver nisab is far lower than the gold nisab, because silver is much cheaper. Zaytuna College treats both as valid and calls silver the safer approach, since it does not leave out anyone who may owe zakat. Under the gold standard, many more people fall below the threshold.
Why sites show different figures
Bodies convert the old weights differently: the National Zakat Foundation uses 87.48 g of gold and 612.36 g of silver, Zaytuna College about 85 g and 595 g, and Diyanet 80.18 g and 561.2 g as cited from its handbook. Prices differ too. The National Zakat Foundation, for example, adjusts the market price towards a real-world selling price. This page uses the unadjusted market price and shows the time it was taken.
Using the nisab
Add up your zakatable wealth, take off the debts you are allowed to deduct, and compare the result with the nisab on your zakat date. If it equals or exceeds the nisab, and you have held it for a lunar year, zakat is 2.5% of it.