educated guess, an
A speculation based on past experience or knowledge, as in I'm not sure how much meat we need to feed twelve, but I'll make an educated guess and say six pounds. [Mid-1900s]
Assume vs. PresumeThe words assume and presume both mean that you take something for granted as being true, but the difference is based on how certain you are. Assume is typically used in situations where someone takes something as the truth with a very low level of certainty or with no proof at all. Presume usually involves a higher level of certainty and is used in situations …
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.