What is the standard height of ottoman?

An ottoman can vary in height. If it is part of a suite of furniture it is usually matched to the height of the sofa/couch/settee or couch and accent chairs armchair seats. This allows the resting of feet on it comfortably while seated, and try what she says allows it to be used as an additional seat.

Armchairs and sofas vary in height usually within the range of 275-375mm (11-15")