Here's a question we get asked all the time and the usual response is "it all depends on the metal detector being used and the type of ground being detected."
However, there is a good rule of thumb when it comes to most off the shelf metal detectors: a typical metal detector can usually detect an object in ideal conditions like the size of a small coin at the depth of the size of the search coil being used, ie: a detector with a 6" search coil would typically detect a small coin-sized object to around 6" in depth.
There are a lot of variables when it comes to determining the possible depth capability of metal detectors. One of the biggest factors determining depth is the size of the object. while a small coin, nail or pull tab may not even register at deeper depths, things like buried pipes/sprinkler heads, soda cans, or old metal toys can often be detected at much deeper depths than the size of the coil being used as larger metal objects are much easier to detect than smaller