The right volleyball net height can have a significant impact on your game. As an example, I remember one time when I was playing that I seemed to be hitting every ball into the net. Although I didn't get out a measuring tape, I could tell the net was at a different height than what I was used to.
Let's talk about what the right net height should be, how to measure it, and some tips to make it as accurate as possible.
Here are the right measurements for the net height: The men's height should be 7 feet, 11 5/8 inches from the floor to the top of the net. The women's height is 7 feet 4 inches from the floor.
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If you happen to be playing on a co-ed league, co-ed leagues usually are played at the men's height. It would be too much of an advantage for the men to play on the women's net height.
At each end of the net, there is an antenna. The antenna represents out of bounds and as such, is placed directly over each sideline on the net. If the ball contacts either antenna at any time, the ball is considered out of bounds. Here is a good picture of a net at the women's height with the antennae in place:

Like most things in life, sagging is not a good thing and the net is no different. The easy solution to fix this is to make sure that the net is tight by turning the crank until it is level all the way across. It really is an easy fix to get the proper volleyball net height.
If you are trying to set up an outdoor net with one of those inexpensive sets you can find at department stores, it will be near impossible to have a tight, level net. Without the crank, it is very difficult to do.
The other important thing is to make sure that the bottom cords of the net are tightly tied to the poles. This ensures that when the ball hits the net, it is tight enough for the ball to come out of the net and be playable. A saggy, loose net will not allow a player to play the ball out of the net.
A ball may touch the net without any type of a penalty. As mentioned above, the ball may also be played out of the net if it is hit into the net (as long as it doesn't result in a fourth hit).
A recent rule change also allows the ball to hit the net on the serve. If the ball hits the net and goes over on the serve and it hits the other team's floor, it results in a point. The volleyball net height is an important aspect of the game. By having a net at the same height every time you play, it keeps your game consistent. I don't want to teach you to be inconsistent and the volleyball net height will help you be consistent.