"Napkin math" is the act of performing estimates (usually of a financial nature) to roughly determine the feasibility or validity of an outcome or fact. It is usually performed in an informal setting.

Napkin is an allusion to the disposable nature of the information.
"I've done napkin math Dave, even if you get a $10k/year raise you still can't afford that mortgage".
by silentguardian September 13, 2013
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