Bar stools come in many styles, including different heights. The so-called 'standard' height is the 30" bar stool. Why is that? There are actually several reasons why 30 inches has been deemed the proper seat height standard for bar stools. If you think of the typical adult human's body dimensions and then consider the height of most kitchen counters and restaurant bar tables, it begins to make a lot of sense.
Unless you're really short or really tall, having a 30"-high seat when sitting on a stool is just about the most comfortable position. Most bar stools have a foot rail about halfway between the seat and the floor. This is to provide us with a comfortable place to rest our feet and take pressure off the backs of our thighs. It's purely a matter of being able to sit for extended periods in the stool without leg and lower back pain. It works well for adults who fall within the average range of leg length.
Another reason why a 30" bar stool is considered standard is to match the manufacturing standards for home kitchen counters. Those typically run 40-44 inches, so a 30" seat is just right for eating, sipping wine, smoking, or just resting your elbows as you chat.
If you use the kind of counters that separate the kitchen from the living room or breakfast nook, they often have overhangs to extend the surface area. This is another reason why 30" stools work so well. Most adults have about 8-12 inches of thigh on both legs that must fit between the stool seat and the bottom of the overhang of the counter. A 30" stool is just right for a snug fit that doesn't pinch.
In restaurants, many bars have a few tables for patrons who wish to drink alcohol and enjoy a meal. These tabletops sit higher off the ground than regular restaurant tables, so they require bar stools or tall chairs. The tables are, just like kitchen counters, generally about 40-44 inches high, so 30" bar stools are, again, a good fit.