To be legal a blade has to be less than 3 inches and cannot be fixed. Only folding knives with a blade under 3 inches are legal unless it's an essential tool for your job.
FYI: Wisconsin Criminal Code 941.23-4 only refers to gravity knives & switchblades, and carrying concealed weapons. No mention is made of blade length or fixed v. folding blades. I'm not a lawyer; so feel free to research it yourself.