Why is a rough estimate sometimes referred to as a "ballpark figure"? What is the background behind this?

Answer 1

The term ball park figure has its roots in the field of astronautics in the late 1950's. When splashing down back on earth, a spacecraft was said to be in the ballpark if it landed within a designated area. This was taken from baseball, of course, where balls which were hit within the ballpark were still playable, versus those hit out of the ballpark, which were usually home runs. Therefore, a ball landing inside the park was considered a good thing for the team in the field. Ballpark first appears in print in this astronautics context in 1960, when the San Francisco Examiner noted that "[t]he Discoverer XIV capsule...came down 200 miles from the center of its predicted impact area, but still within the designated ‘ballpark’ area."

The term spread from the astronautics industry and came to be applied to anything which was "close" to correct. The San Francisco Examiner used the full phrase in the ballpark in 1968 with that meaning. Interestingly, the term ballpark figure appears in print in 1967 in the Wall Street Journal! A ballpark figure is, of course, one which is within range of the true figure.

Answer 2

The first ballpark, Union Grounds (Brooklyn, N.Y.), opened on May 15, 1862. It started when a hit was called out of bounds or in the "ball park."
The Random House dictionary citation files show the term first started out as 'in the ballpark' (1962),as when talking about figures, estimates, etc., with 'I hope that's in the ballpark.' ♥

Answer 3

A “ballpark figure” is an acceptable, roughly accurate approximation. This idiom alludes to a baseball field. “In the ballpark” is considered a reasonable range. A “figure” is another way to say “number”.

Answer 4

Well, there you are. Two different versions of the origin of the saying. So we're still none the wiser. Interesting though.