Gross floor area is the total floor surface of a space, measured wall to wall. Net floor area is what's left for your guests. Subtract all space-eating elements: stage, bar, cloakroom, technical area, escape routes and aisles.
In practice, net is 60 to 75% of gross. A 600 m² gross hall effectively offers you 360 to 450 m² for your guests. At every venue, check whether the communicated size is gross or net. That difference determines whether a hall is suitable.
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