tax Free long term capital gain

My husband and I will have a total of about 55k in income from, social security, unemployment, interest, dividends, and wages this year. We have some stock we'd like to sell this year that we've had for many years. When I put that amount of profit (about 1,000.)into the turbotax calculator, it changed the amount of my Federal Tax liability, but I thought LT gains were tax free based on our income?