Scientists, engineers and product developers face daily phone calls and hallway visits from peers who want quick answers to complex questions: "If I improve X, how will it affect Y and Z?" We tend to want to answer these questions with a wall full of equations, even when questioners just want a rough estimate to gauge whether the change would cause more problems than it would fix. The "rules of thumb" in this magazine feature provide the framework for this kind of "guesstimate," with shortcuts to relatively accurate answers. They are not designed to replace the complex equations that you'll need to produce commercial or laboratory instruments, systems and components, but to provide a starting point for determining whether a project is even feasible.