Quote from a reader: “I've learned that if you ask a physicist a hypothetical question, they will most likely try to give you a real answer. As a result, never ask a physicist a hypothetical unless you're ready for a real explanation and potentially hand-drawn diagrams on napkins. I love you guys.”
A back of the envelope calculation is a rough estimate performed on a random scrap of paper (like the back of an envelope). They are synonymous with physicists. http://en.wikipedia.org/wiki/Back-of-the-envelope_calculation
Let's go through an example of one of these calculations:
Estimate the number of pizzas consumed by all the students at the Northwestern University during one quarter. (Adapted from University of Maryland)
Estimate the number of pizzas consumed by all the students at the Northwestern University during one quarter. (Adapted from University of Maryland)