When the 12-inch long-playing album was first introduced in the summer of 1948, it was a scientific breakthrough. But why did anyone choose 12 inches when the format of choice at the time was the 78, which was ten inches across? A couple of reasons.
First, 12 inches is a foot, a nice familiar unit of measurement. Second, the goal set for engineers was to get at least 22 minutes per side. Third, 22 minutes was determined to be a nice length for classical recordings. Anything written that needed to be recorded usually didn’t require any more than 22 minutes. And fourth, technology at the time allowed for grooves to be cut at anywhere from 224 to 300 per inch.
If you work out the math, a 12-inch record with that many grooves per inch spinning at 33 1/3 RPM works out to 22 minutes.
Part one of the story about the first LP can be found here.