Chiller Tonnage Calculator
Size a chiller by cooling tons from water flow (GPM) and temperature difference, with BTU/hr and kW conversions.
Quick Answer
To size a chiller, use Tons = (GPM × ΔT) ÷ 24, where GPM is the chilled-water flow and ΔT is the temperature drop in °F. 50 GPM at a 10°F ΔT ≈ 20.8 tons. One ton of cooling equals 12,000 BTU/hr or 3.517 kW.
Chiller Tonnage Calculator Guide
The chiller tonnage calculator works out the cooling capacity a chiller needs, in tons of refrigeration, from two inputs: the chilled-water flow rate in gallons per minute (GPM) and the temperature difference (ΔT) between the return and supply water. It also converts that result into BTU/hr and kilowatts so you can compare it against manufacturer ratings.
In cooling, a “ton” has nothing to do with weight — it is a unit of heat-removal rate. One ton of refrigeration equals 12,000 BTU/hr, or about 3.517 kW, and originally described the cooling produced by melting one ton of ice over 24 hours. (This is a different “ton” from the weight tons used for gravel and stone elsewhere on this site.)
How to Size a Chiller
The cooling load carried by water is Flow (GPM) × 500 × ΔT (°F) in BTU/hr, and since one ton is 12,000 BTU/hr, that reduces to a simple shortcut: Tons = (GPM × ΔT) ÷ 24. A design ΔT of 10°F is common for comfort cooling, while process cooling often runs tighter or wider depending on the equipment.
Chiller tonnage conversions (1 ton = 12,000 BTU/hr = 3.517 kW):
| Tons | BTU/hr | kW | Flow at 10°F ΔT |
|---|---|---|---|
| 1 | 12,000 | 3.5 | 2.4 GPM |
| 5 | 60,000 | 17.6 | 12 GPM |
| 10 | 120,000 | 35.2 | 24 GPM |
| 20 | 240,000 | 70.3 | 48 GPM |
| 50 | 600,000 | 175.8 | 120 GPM |
| 100 | 1,200,000 | 351.7 | 240 GPM |
What Affects Chiller Sizing
The GPM-and-ΔT method gives the load the water is actually carrying, but the chiller you select should include headroom. Glycol antifreeze mixes carry less heat than plain water, heat-exchanger fouling reduces capacity over time, and peak loads or high ambient temperatures push demand up. Engineers typically add a safety factor of 10–20% on top of the calculated tonnage, and confirm flow, ΔT, and fluid type against the manufacturer’s performance curves before finalizing a unit.
Frequently Asked Questions
What is a ton in a chiller?
A ton of refrigeration is a rate of cooling equal to 12,000 BTU per hour, or about 3.517 kW. It is named for the cooling once produced by melting a ton of ice over 24 hours and is unrelated to weight.
How do you calculate chiller tonnage?
Use Tons = (GPM × ΔT) ÷ 24, where GPM is the chilled-water flow rate and ΔT is the temperature difference in °F between return and supply water. For 50 GPM at a 10°F ΔT, that is (50 × 10) ÷ 24 ≈ 20.8 tons.
How many BTU is a ton of cooling?
One ton of cooling equals 12,000 BTU per hour. So a 5-ton chiller provides 60,000 BTU/hr and a 20-ton chiller provides 240,000 BTU/hr of cooling capacity.
How much water flow does a chiller need per ton?
At the common 10°F design ΔT, a chiller needs about 2.4 GPM of chilled water per ton (24 ÷ ΔT). A wider ΔT needs less flow per ton; a narrower ΔT needs more.
Should I add a safety factor to the calculated tonnage?
Yes. The formula gives the water-side load, but real installations add roughly 10–20% for glycol, fouling, peak demand, and ambient conditions. Always verify the final selection against the manufacturer’s performance data.