Any ideas on the recommended depth of sand for a turnout pen?

[alisonpook]

I sometimes use a sand pen for restricted turnout for my Cushings horse but it now needs topping up with sand and I can't remember the ideal depth. I need this to work out the tons needed. Does anyone have any ideas. I was thinking around 4" to 6" compacted depth. Would this be about right?

 

[mygeorge]

an ordinary sand riding surface has 6" of top so that sounds about right