taxable income = income + short term gain + long term gain.

Capital gains tax rate	Single (taxable income)	Married filing jointly (taxable income)
0%	Up to $44,625	Up to $89,250
15%	$44,626 to $492,300	$89,251 to $553,850
20%	Over $492,300	Over $553,850

 

Let's say your salary is 200K, you sold a house with LT capital gain of 400K, your total taxlable income is 700K, the long term capital gain of 400K would be taxed as 20%. 

If your income is 0, your LT gain is 400K, it will be taxed as 15%.