Hi and welcome to Just Answer!
Gains and losses from the sale or exchange of US real property interests (whether or not they are capital assets) are taxed as if you are engaged in a trade or business in the United States. You must treat the gain or loss as effectively connected with that trade or business. Thus - the gain is taxed the same way as for US persons - depending on your total income from US sources, filing status, deductions and how long you held the property.If you held the property more than a year - the long term capital gain is taxed not more than 15%.
So its 15% if held more than a year?
and I heard there is a state capital gains tax also on top of the 15% right?
That is correct for federal taxes. Hawaii has additional state taxes - that are between 1.4% and 11% depending on total income.
how do I find the exact figure for both federal and state?
To determine the exact amount of tax liability - you need to prepare tax returns - that is the purposes of preparing tax returns - to calculate taxes.
I also know there is a 10% witholding on sale of property, how long will they release it after the sale