Abstract: A calculator and method for simply and quickly selecting appropriate spot type fire detectors and determining their preferred locations in a chosen space division in accordance with standard spacing ratings. The slide rule-like calculator includes a body member, a slide member slidably positioned within said body member, and in one embodiment a cursor slidably encompassing both the body member and slide member. The body member comprises at least one logarithmic scale representing one dimension of the space division and the slide member comprises at least one logarithmic scale representing the second dimension of the space division. Additionally, at least one fire detector scale, established with reference to standard space detector ratings, is included either on said body or slide member and an index or zero line is included on the other member.