The classical warrant pricing formula requires knowledge of the variance of the firm value process, and the firm value. When warrants are outstanding the firm value itself is a function of the warrant price. Firm value and the variance of the firm value are then unobservable variables. I develop an algorithm for pricing warrants using stock prices, an observable variable, and variance of stock returns. The method also enables estimation of the variance of firm value. A proof of existence of the solution is provided.