An Engineering Rule of Thumb is an outright statement regarding suitable sizes or performance of equipment that obviates all need for extended calculations. Because any brief statements are subject to varying degrees of qualification, they are most safely applied by engineers who are substantially familiar with the topics. Nevertheless, such rules should be of value for approximate design and cost estimation, and should provide even the inexperienced engineer with perspective and a foundation whereby the reasonableness of detailed and computer-aided results can be appraised quickly, particularly on short notice such as in conference.
In other words, an Engineering Rule of Thumb is an educated guess.
Attached is a nice compilation of Engineering Rules of Thumb with a ChE slant, but can benefit other Engineering disclipines too:
heuristics in CHE.pdf (255.8 KB)