A working library of the short, memorable rules people use to make decisions without doing the maths: how long money takes to double, how to season a sauce, how many copies a backup needs, how far to lean a ladder. Search it, filter it, take the rule of the day — and read why each one works, because a rule you cannot explain is a rule you will misapply.
A rule of thumb is a heuristic: a shortcut that is usually right, cheap to apply and good enough to act on. It is not a law. The rule of 72 is a few months out at very high rates; the two-pizza team is wrong for a team maintaining a nuclear control system; "measure twice, cut once" does not save you if you measured the wrong thing twice.
Three ways they go wrong. They go stale: most of these were written for a world of 400 kB photos and 4% savings rates. They hide the hard part: "eat the frog" is easy to say and hard to do, and no rule fixes that. And they get applied outside their range: a rule tuned for one job quietly becomes wrong when the stakes, the scale or the timescale changes.
Nothing here is financial, medical, legal or engineering advice — it is a memory aid for everyday judgement. Where a number really matters, do the calculation properly, or ask someone who does it for a living. The list runs entirely in your browser.