We propose a new method to derive invariants for LTI systems with uncertain inputs, i.e. systems of the form _x(t) =Ax(t)+Bu(t) with state vectorx(t)2Rnand uncertain inputu(t)2Rmbounded by u(t)2U\022Rmfor allt\0250. Our approach is based on the real canonical form and the resulting invariants are conjunctions of bounds on linear and quadratic forms in the state variablesx(t).