Formula is correct, and the easy way to measure (but numbers are not...) Aspect Ratio = span / average width. Area = span * average width. Meaning AR = span^2 / area. So your 12m2 kite with an AR of 5.4 has a span = sqrt(AR*area) = sqrt(5.4*12m2) = 8.0 meter. And an average width = span / AR = area...