There are many computer programs that perform lighting calculations. They vary enormously in speed, sophistication and cost. Occasionally, surveys are published which compare programs in terms of their features. While these can be of direct benefit to purchasers, they tend to go out of date quickly, as new programs and new versions of old programs are released. This guide, therefore, does not focus on individual programs but on the issues which are common to all programs, and in particular the algorithms that underly them. Guidance is given on the features that are common to each type of program and on some of the extra features that may be useful to designers. This guide helps lighting designers to select appropriate interior lighting calculation software. 8 pages.