How do Capital Gains Taxes work?

-Suppose if I held a stock throughout one year and it went up 30%. If I still haven't sold the stock after one year, would I still have to pay 15% in taxes on my capital gains?Normally no. There is no capital gains tax until either (a) you sell, or (b) you constructively accomplish the same thing through futures contracts or other means.
You don't pay any tax on change of value until you sell it. It shows on schedule D on your tax return for the year you sell it.
You go from your buy cost to your sell cost. Ups and downs in the meantime don't matter.



GC
only when you sell it