This article is from the Ballet and Modern Dance FAQ, by Tom Parsons twp@panix.com with numerous contributions by others.
For a studio, the short answer is, 3 feet 6 inches (107 cm) to the
top surface of the barre, at least in the studios I've heard about or been
able to measure myself. It's a good idea to have a second barre 8 to 10
inches (20 to 25 cm) below the first for children, shorter dancers, or
those with limited extension.
For private use, the answer is less clear-cut. Waist-high is one
answer. Robert Joffrey, on the other hand, used to say that the hand on
the barre should be the same height as the other hand when the free arm is
in second position. This would place the barre even with the bottom of
your breastbone. In any case, it's best if the height is adjustable to
accommodate the dancer's height, especially if it is for a growing child
(or children).
 
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