Course it's calculated.
Are you heating, cooling or chilled, what's your system filled with, what's your output, what's your delta t,what's your flow rate, what's you pipe sizes, system layout, what's your appliance KPa, what's your valves KPa, what's your index circuit KPa???????????????
A plumber may guess but he'd be excused for not understanding heating!
Nearly as open ended question as asking a forum "how much will it cost??"