Chiller Tonnage Calculator
Measure the chilled-water flow and the difference between return and supply temperatures, and you know how many tons the chiller is delivering.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Cooling capacity
- 100tons
- Heat removed
- 1,200,000BTU/h
240 gpm at 10 °F ΔT = 100 tons.
- Chilled-water flow
- 240gpm
- Flow per ton
- 2.4gpm/ton
- Thermal capacity
- 351.7kW
- Water ΔT
- 10°F
Show the working
- ΔT = return − supply54 − 44 = 10 °F
- Q = 500 × GPM × ΔT500 × 240 × 10 = 1,200,000 BTU/h
- Tons = Q ÷ 12,000 (for water: GPM × ΔT ÷ 24)1,200,000 ÷ 12,000 = 100 tons
Where GPM × ΔT ÷ 24 comes from
Water weighs about 8.33 lb per gallon and needs 1 BTU per pound per °F, so one gpm carrying a 1 °F temperature change moves 8.33 × 60 ≈ 500 BTU/h. One ton of refrigeration is 12,000 BTU/h, and 12,000 ÷ 500 = 24.
So tons = GPM × ΔT ÷ 24: 240 gpm cooled from 54 °F to 44 °F is 100 tons, or 2.4 gpm per ton at a 10 °F difference.
Measurement matters
Errors carry straight through. At a 10 °F temperature difference, a 0.5 °F error in either thermometer is a 5 % error in tonnage. Use matched, calibrated sensors in wells close to the chiller, and a flow reading you trust.
For glycol, enter the product of the fluid’s specific gravity and specific heat at the operating temperature from the supplier’s data.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Calculate
- Tons from flow and ΔT
- Chilled-water flow
- 240 gpm
- Return (entering chiller) water temperature
- 54 °F
- Supply (leaving chiller) water temperature
- 44 °F
- Fluid
- Water (factor 500)
Result
- Cooling capacity
- 100 tons
- Chilled-water flow
- 240 gpm
- Heat removed
- 1,200,000 BTU/h
- Flow per ton
- 2.4 gpm/ton
- Thermal capacity
- 351.7 kW
- Water ΔT
- 10 °F
240 gpm at 10 °F ΔT = 100 tons.
Assumptions
- Water: 500 = 8.33 lb/gal × 60 min/h × 1 BTU/(lb·°F); 1 ton = 12,000 BTU/h.
Limitations
- Flow and temperature measurement errors carry straight through: a 0.5 °F error at a 10 °F ΔT is 5 % of the tonnage.
- Glycol needs the supplier’s properties.
Frequently asked questions
How many gpm per ton for chilled water?
24 ÷ ΔT: 2.4 gpm per ton at a 10 °F difference, 2 gpm per ton at 12 °F.
Is this the same as the GPM to BTU calculator?
It uses the same equation, laid out for chillers in tons with supply and return temperatures.
Why is my measured tonnage lower than the nameplate?
Chiller capacity depends on the water and condenser temperatures and the load. Compare with the manufacturer’s performance data at the same conditions.
Sources
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
- Water Density — U.S. Geological Survey Water Science School. Water’s density is roughly 1 g/mL (about 62.4 lb/ft³) and decreases as it warms; the page tabulates density by temperature.
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