Regulators, litigators and cyber security standards require that cyber security controls should be ‘reasonable’. But rarely do these authorities define what the word means. Lawyers and regulators have long stated that reasonableness is a balance between protecting others from harm and using controls that are no more burdensome than the risks they reduce. They have illustrated this concept with a calculation that is remarkably similar to risk calculations used in cyber security risk management. This paper explores an accidental collaboration between the cyber security community, judges and regulators to define reasonableness, and demonstrates to readers how they can use risk analysis to defend their security programmes as reasonable.