From wikipedia
In aerodynamics, the aspect ratio of a wing is essentially the ratio of its length to its breadth (chord). A high aspect ratio indicates long, narrow wings, whereas a low aspect ratio indicates short, stubby wings.
For most wings the length of the chord is not a constant but varies along the wing, so the aspect ratio AR is defined as the square of the wingspan divided by the area S of the wing. [which is equal to the length-to-breadth ratio for a constant chord wing.
Aspect Ratio's calculations for wings have been around for a while so all Kite companies should be calculating the same way.
For Example a 6m x 3m rectangle has a aspect ratio of 2. The length is 2 times the width.
By formula = (6^2 )/ (6x3) = 36 / 18 = 2.
The denominator in kite terms is the size of the kite (square metreage of the kite, measured accurately).