How to calculate the pipe diameter, such as 200 cubic meters of water per hour. How to calculate the formula

How to calculate the pipe diameter, such as 200 cubic meters of water per hour. How to calculate the formula

Pipe diameter d = √ [4 flow D / (π economic velocity V)]
Among them: pipe diameter D is in meter, economic flow velocity V: 1.2m/s for pipes below DN300 and 1.5m/s for pipes above DN300; flow unit is m3 / s
Therefore, pipe diameter d = √ [4x (200 / 3600) / (π x1.2)] = 0.243 (m) = 243mm