The Rat Terrier is a small dog, available in two AKC-recognized size varieties: the standard, standing 33 to 46 cm at the shoulder and weighing 4.5 to 11 kg; and the miniature, standing 25 to 33 cm and weighing under 4.5 kg. The build is compact and athletic — similar to a small, leggy terrier — with a smooth, dense short coat, erect bat-shaped ears, and a bright, alert expression.
The coat is short and smooth, easy to care for, and comes in a wide variety of patterns and colors including pied, brindle, solid, and tricolor. White with patches is common. Natural tails of various lengths are acceptable by AKC breed standard. The overall impression is of a compact, quick, and lively dog well-adapted for farm and outdoor work.