There are two methods to calculate equivalent square.
One is Sterlings approximation, this is better found from 4*Area/Perimeter of the field.
The other method is Days method. This was published in the British Journal of Radiology and is the common method of interpolation between field sizes and involves lookup in a table. This is definitely preferred for more rectangular fields (ex 12x36 or the like) as Sterlings approximation gives a improper equivalent square size under those conditions.
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