BTU/hr. = 40gpm x 60 x 8.33 x 12°F = 239,904 BTU/hr. Ton Capacity = 239,904 BTU/hr. ÷ 12,000 = 19.992 Tons. Oversize the chiller = 19.992 x 1.2 = 23.9904. A 23.9904 or 25-Ton chiller is required. There also industry specific rules of thumb for chiller sizing depending on