You can generally calculate it using this formula: Calculate Temperature Differential (?T°F) ?T°F = Incoming Water Temperature (°F) - Required Chill Water Temperature. Calculate BTU/hr. BTU/hr. = Gallons per hr x 8.33 x ?T°F. Calculate tons of cooling capacity. Tons = BTU/hr. ÷ 12,000. Oversize the chiller by 20%.