According to mlb.com, the strike zone is that area over home plate the upper limit of which is a horizontal line at the midpoint between the top of the shoulders and the top of the uniform pants, and the lower level is a line at the bottom of the knees. The Strike Zone shall be determined from the batter's stance as the batter is prepared to swing at a pitched ball.

Strike zones vary with different umpires and it also depends on how well the catchers can frame the pitch. the strike zone is usually from the bottom of your shoulders to waist or the letters on your uniform to waist

basically armpits to knees is a general rule for the strike zone, so it varies on the batter.

The strike zone is very simple. It is where the home plate umpire decides it is. As a little leaguer you may have been told that it was from the armpits to the knees, but as you develop as a player you realize that one of the most important things you can do as a pitcher or a batter is to pinpoint the strikezone of an umpire.

from the knees to the shoulders and above the plate

There is the "book" strike zone and then there is the "called" strike zone. Or you could call it the "expected" zone. This is a long answer, but here is my perspective (as an umpire) on the strike zone. Calling balls and strikes is more of an art than a science and many factors go into deciding whether a pitch is a ball or a strike. Most players and coaches realize the zone will vary from one umpire to another but want each umpire to have a "consistent" strike zone. Because we are all human and all different, we see things differently. Even slight differences will alter the zone from one umpire to another. Consistent pitching and consistent catching helps the umpire have a consistent strike zone. If the pitcher is all over the place or if the catcher can't catch a pitch, it usually results in the umpire having an inconsistent strike zone. Umpires should not have "their own strike zone." We strive for consistency but we are not perfect. Even pro umpires "miss" pitches. The key to calling pitches is working the slot properly, proper head height (the umpire's chin should be no lower than the top of the catcher's helmet, remaining motionless once you "lock in" and following the pitch with your eyes all the way from the pitcher's hand into the catcher's mitt (while not moving your head). Once the pitch is caught, you consider everything you saw and make the decision whether the pitch was a ball or strike. If the umpire is calling the pitch at the same time the ball hits the catcher's mitt, then he is calling the pitch too quickly and making up his mind before the ball crosses the plate. Calling pitches too quickly results in an inconsistent strike zone and missed pitches. Timing is everything. Tim McClelland is one of the best ball and strike umpires in the Majors, he is also the slowest to make the call - that is what makes him so consistent. For younger players, the "called" zone is usually more generous. Most umpires will call a higher pitch (armpits or shoulder) a strike. Most will also give a little more off the corners. A pitch below the knees should never be called a strike at any levle because it is not a "hit-able" pitch and causes players to develop the bad habit of swinging at low pitches. As you progress, the top of the zone moves lower with the mid-point between the shoulders and the waist being the upper limit. If you umpire high school, Legion, college, or higher, calling a pitch more than 2 or 3 inches above the belt will get you nasty looks. Is that lower than the "book" strike zone? Yes, but it is the zone players and coaches at that level expect. The bottom of the knee (and no lower) is generally the accepted lower limit of the zone. A number of factors actually go into whether a pitch is called a strike or a ball. Those include the catcher jerking or moving his mitt - that is not "framing" the pitch. If the catcher pulls the pitch back over the plate, he must have thought it was off the plate so why should I call it a strike? If he catches a low pitch with his mitt turned over (back handed) and pointing down or touching the dirt, the pitch was probably too low. If he catches a pitch below his own knees, it was probably below the batter's knees when it crossed the plate. If he catches a high pitch above his head, the pitch was probably too high. If he moves the elbow on his mitt arm across the middle of his chest to catch a pitch on his throwing arm side, it is probably off the plate. If he moves his elbow away from his body when catching a pitch on his mitt side, the pitch was probably off the plate. At the varsity high school level and above, if a pitch is not caught by the catcher, it probably won't be called a strike. Coaches and players expect called strikes to be caught.

from about your knees, to about your elbows. it also depends on the umpire.

From your knees to your elbows.