Depth for pot leeks

Usually it's between 6"-8" so you get the white shank that is more tender to use. It depends on the amount of top soil you have, plus some varieties of leek have a longer white shank such as Tornado which I now grow because I don't have a great depth of top soil to plant them in, and found with other varieties that, no matter how deeply I planted them, I couldn't achieve the white stem my OH wants for cooking.