This depends partly on the size of the pond, but a minimum depth of 75 cm (two and a half feet) in the deepest part will give the pond everything it needs.
Most of the wildlife usually lives in the top 30cm (one foot) although in the depths of winter the deeper parts can provide an important refuge when the pond is frozen over. A deeper, cooler area of water also helps to prevent the pond becoming too warm in hot weather.
Profile is also vital. Gently sloping edges will offer good access to wildlife, and will allow birds to drink and bathe. Emergent plants such as purple loosestrife, flowering rush, and arrowhead will thrive as the profile shelves to depths of 20 to 30 cm (eight to twelve inches).