I started paying zakat last year, 2012. In it I paid $2,500 zakat on a $10,000 saving. This year, 2013, my savings have increased to $12,000.
Should I now pay zakat on $12,000 or only the net increase in savings i.e. $2,000?
Zakat is due on the entirety of your savings, so long as they have maintained at least the nisab for an entire year. So in your case — and this is according to the Hanafi madh'hab — you would owe zakat for the whole $12,000 (which at 2.5% works out to $300).