A conjugate gradient solver apparatus is provided for generating data defining a solution vector x for a linear system represented by Ax=b where A is a predetermined matrix and b is a predetermined vector. The apparatus includes solver circuitry and a precision controller. The solver circuitry processes input data, defining said matrix A and vector b, in accordance with an iterative conjugate gradient method to generate said data defining the solution vector x. The solver circuitry is adapted to process data items, corresponding to vectors used in said conjugate gradient method, having a variable fixed-point data format. The precision controller determines the fixed-point data formats of respective said data items adaptively during progress of the conjugate gradient method in the solver circuitry.