Just about every profession has rules of thumb that allow you to estimate quickly something that needs a lot of research or a large calculation. Even with tools like calculators, applications and excel, rules of thumb allow you to tell if you’ve made an entry error. They also help you remember certain things that might be important, like “volts hurt, amps kill”.
A gentleman called Tom Parker wrote a couple of books collecting together rules of thumb. These books are now out of print and he has put together a wbesite with the ambisious goal of collecting together every rule of thumb in the world. You can browse the existing ones in the collection, contribute new ones, and vote on whether newly submitted rules qualify for inclusion. This is a site it is easy to lose yourself in for a long period of time.
Found via Cool Tools