A back-of-the-envelope calculation is a rough calculation, typically jotted down on any available scrap of paper such as the actual back of an envelope. It is more than a guess but less than an accurate calculation or mathematical proof. The defining characteristic of back-of-the-envelope calculations is the use of simplified assumptions. A similar phrase is "back of a napkin", which is also used in the business world to describe sketching out a quick, rough idea of a business or product. In British English, a similar idiom is "back of a fag packet".

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In English law, estovers is wood that a tenant is allowed to take, for life or a period of years, from the land he holds for the repair of his house, the implements of husbandry, hedges and fences, and for firewood.