It's hard to say exactly how far a dog can hear a whistle because it varies depending on a range of features, including the type of whistle, the frequency, the terrain, and how good your dog's hearing is. However, it's safe to say that most dogs can hear a whistle from at least a couple of hundred feet away, and often much farther. You can buy dog whistles that are specially shaped to encourage the sound to travel farther. It's also worth noting that sounds at lower frequencies travel farther than those at higher frequencies, so the sound from an audible dog whistle will be heard at a greater distance than the sound from a silent dog whistle.