Superconducting quantum interference devices (SQUIDs) are highly sensitive magnetometers and they have found application in ultra-low field MRI. However, they require cryogenics and their noise performance is hindered by external noise sources and the strong fields employed in pre-polarised MRI experiments. Air-core magnetometers provide an attractive alternative, as they are highly sensitive, robust and relatively cheap to manufacture. Our goal is to provide of a method to optimise the sensitivity of these devices. In this work we propose an accurate numerical model and the use of a genetic algorithm to consider previously unexplored coil configurations.