The depth of sand in a riding arena is dependent on the sand quality and the riding discipline. Typically, 3-4 inches of fine sand is used for dressage and jumping arenas with a geotextile sand additive. For plain sand arenas, 2-3 inches of fine sand is recommended. A consult with an experienced riding arena builder will help you select the correct sand and its depth.
For more information on arena footing, read Riding Arena Footing FAQ.