Superheat and subcooling calculator
Superheat is the suction line temperature minus the saturation temperature at the suction pressure. Subcooling is the saturation temperature at the liquid pressure minus the liquid line temperature. Choose the refrigerant, enter the gauge pressure and the line temperature, and the calculator reads the saturation temperature from the pressure-temperature chart.
- Saturation temperature, suction39.8 °F
- Superheat10.2 °F
- Saturation temperature, liquid104.6 °F
- Subcooling9.6 °F
Superheat: 50.0 °F on the suction line minus 39.8 °F saturation at 118.0 psig = 10.2 °F.
Subcooling: 104.6 °F saturation at 340.0 psig minus 95.0 °F on the liquid line = 9.6 °F.
The target is not one fixed number. It comes from the charging table or the rating plate of the unit.
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What are superheat and subcooling?
Superheat is how many degrees the refrigerant vapor in the suction line is above its saturation temperature. Subcooling is how many degrees the liquid in the liquid line is below its saturation temperature. The saturation temperature is the temperature at which the refrigerant boils or condenses at the pressure you measured, and it is read from a pressure-temperature chart.
Superheat = suction line temperature − saturation temperature at suction pressure
Subcooling = saturation temperature at liquid line pressure − liquid line temperature
Manufacturers’ service manuals and the ENERGY STAR installation checklist use the same two subtractions.
Which reading checks the refrigerant charge?
It depends on the metering device at the indoor coil. Trane’s product data puts it in one line: verify the charge by subcooling on a system with a thermostatic or electronic expansion valve (TXV or EEV), and by superheat on a system with a fixed orifice, also called a piston.
| Metering device | Charge is checked by | Measured on |
|---|---|---|
| Fixed orifice (piston) | Superheat | The suction line: the larger, insulated line |
| TXV or EEV | Subcooling | The liquid line: the smaller line |
What should the reading be?
There is no single correct number. The target comes from the manufacturer of the unit.
- Subcooling: Carrier prints the desired subcooling on the rating plate of the unit. Goodman and Daikin give it in the specification sheet for the model. One Daikin manual for R-32 units asks for 8 °F, give or take 1 °F. One Carrier table for an R-454B unit asks for 17 to 18 °F.
- Superheat: on a fixed-orifice system the target moves with the weather. The manufacturer’s charging table gives it from the outdoor temperature and the wet-bulb temperature of the air entering the indoor coil.
ENERGY STAR’s Version 3 inspection checklist for certified homes accepts a measured superheat within 5 °F of the manufacturer’s target, and a measured subcooling within 3 °F. Its 2011 guide to quality installation notes that the test typically needs an outdoor temperature of at least 55 °F for superheat and 60 °F for subcooling.
Why do some refrigerants have two pressures at one temperature?
A refrigerant that is a blend of several fluids boils over a small range of temperature. Its chart prints two pressures at each temperature: the liquid, or bubble, pressure and the vapor, or dew, pressure. Arkema, which makes Forane refrigerants, states the rule: use the vapor pressure for superheat and the liquid pressure for subcooling. The calculator does this for you.
R-454B and R-407C are blends with a clear gap between the two pressures. R-410A is also a blend, but its two pressures are never more than 1.1 psi apart on the chart. R-22, R-32 and R-134a are single fluids with one pressure at each temperature.
Pressure-temperature chart
Saturation pressure in psig at sea level, for the four refrigerants found in home air conditioners and heat pumps. The calculator holds the same chart in 5 °F steps, together with R-407C and R-134a.
| Temperature (°F) | R-410A | R-454B liquid | R-454B vapor | R-32 | R-22 |
|---|---|---|---|---|---|
| 0 | 48.4 | 45.3 | 42.7 | 49.3 | 24.0 |
| 10 | 62.4 | 58.5 | 55.4 | 63.5 | 32.8 |
| 20 | 78.7 | 73.9 | 70.3 | 80.0 | 43.1 |
| 30 | 97.4 | 91.7 | 87.4 | 99.1 | 55.0 |
| 40 | 118.8 | 112.0 | 107.0 | 121.0 | 68.6 |
| 50 | 143.2 | 135.0 | 129.3 | 145.9 | 84.1 |
| 60 | 170.7 | 161.1 | 154.6 | 174.1 | 101.6 |
| 70 | 201.8 | 190.5 | 183.1 | 205.8 | 121.4 |
| 80 | 236.5 | 223.4 | 215.2 | 241.5 | 143.6 |
| 90 | 275.4 | 260.1 | 251.0 | 281.3 | 168.4 |
| 100 | 318.6 | 300.8 | 290.9 | 325.7 | 195.9 |
| 110 | 366.4 | 345.9 | 335.2 | 374.9 | 226.4 |
| 120 | 419.4 | 395.7 | 384.3 | 429.3 | 260.0 |
| 130 | 477.9 | 450.5 | 438.7 | 489.5 | 296.9 |
| 140 | 542.5 | 510.7 | 498.8 | 555.8 | 337.4 |
| 150 | 613.9 | 576.8 | 565.4 | 628.8 | 381.7 |
Values are from Arkema’s Forane pressure-temperature charts, which Arkema generated with the NIST REFPROP database. The R-410A column is the liquid pressure. Chemours’ own tables for R-454B and R-22 agree with these to within 0.2 psi.
Who may connect gauges to an air conditioner?
In the United States, a certified technician. The Environmental Protection Agency counts attaching and detaching gauges to measure the pressure in an appliance as technician work under Section 608 of the Clean Air Act, and that work needs certification. Only certified technicians can buy refrigerant for home air conditioning, and releasing refrigerant on purpose is prohibited.
If you are not certified, use this calculator to check the numbers on a service report, or to follow what your technician is measuring.
Limits of this calculator
- It gives the measured superheat and subcooling. It does not know the target for your unit.
- A pressure-temperature chart is printed for sea level. A gauge at 5,000 feet reads about 2.5 psi higher than the chart expects, which the elevation field corrects.
- Between two rows of the chart the calculator interpolates along the shape of a vapor-pressure curve. The result is within 0.1 °F of the printed chart.
- Charts from different publishers differ slightly. One equipment maker’s chart prints R-454B up to 4.6 psi higher at 150 °F than Arkema and Chemours do, which is less than 1 °F of saturation temperature.
- The readings mean something only when the system has run long enough to settle, under the conditions the manufacturer’s charging procedure sets.
Sources
- Arkema, Forane Refrigerants Pressure Temperature Chart, PPC/SDC 05-2025: https://forane.arkema.com/files/live/sites/shared_arkema/files/downloads/fluorochemicals/Forane%20Ref%20PT%20Regular%20Chart%20EN
- Arkema, Forane 410A Pressure Temperature Chart: https://forane.arkema.com/files/live/sites/shared_arkema/files/downloads/fluorochemicals/forane-410a-pressure-temperature-chart.pdf
- Chemours, Opteon XL41 (R-454B) thermodynamic properties, used to cross-check the R-454B chart: https://www.opteon.com/en/-/media/files/opteon/opteon-xl41-thermo-properties-eng-031522.pdf
- Daikin Comfort Technologies, A1 and A2L pressure-temperature chart PM-A2LPTC, the equipment maker’s chart compared above: https://apps.goodmanmfg.com/brochures/files/666324eb72fb9PM-A2LPTC-USA_04-24.pdf
- Chemours, Freon 22 thermodynamic properties, used to cross-check the R-22 chart: https://www.freon.com/en/-/media/files/freon/freon-22-thermodynamic-properties-eng.pdf
- Arkema, tech tip on liquid and vapor pressure columns: https://forane.arkema.com/en/forane-refrigerants/forane-tech-center/tech-tips/liquid-or-vapor-pressure
- Daikin Comfort Technologies, service manual RS6200301 for R-32 units: superheat and subcooling formulas, method by metering device, subcooling target: https://daikincomfort.com/docs/default-source/amst/sm-rs6200301.pdf
- Trane, split system heat pump product data 22-1765-13J-EN: charge verified by subcooling (TXV or EEV) or superheat (fixed orifice): https://www.trane.com/content/dam/Trane/Commercial/global/products-systems/equipment/unitary/split-systems/split-systems-1-1-2-to-5-tons/22-1765-13J-EN.pdf
- Carrier, service manual SM10-R454B-03: desired subcooling on the rating plate, superheat charging table, required subcooling table: https://www.shareddocs.com/hvac/docs/1009/Public/0C/SM10-R454B-03.pdf
- Goodman and Daikin, service manual RS6200007 for R-410A units: design subcooling from the specification sheet: https://partnerlinkmarketing.goodmanmfg.com/Lit/RS6200007r25.pdf
- ENERGY STAR certified homes inspection checklists: superheat within 5 °F and subcooling within 3 °F of the manufacturer’s target: https://www.energystar.gov/sites/default/files/asset/document/Inspection_Checklists.pdf
- ENERGY STAR HVAC quality installation sponsor guide: method by metering device and minimum outdoor temperatures: https://energystar.gov/ia/home_improvement/downloads/ESQI_sponsor_guide.pdf
- US Environmental Protection Agency, Section 608 technician certification requirements: https://www.epa.gov/section608/section-608-technician-certification-requirements
- US Environmental Protection Agency, refrigerant sales restriction: https://www.epa.gov/section608/refrigerant-sales-restriction
- NOAA, NASA and US Air Force, US Standard Atmosphere 1976, for air pressure at elevation: https://ntrs.nasa.gov/citations/19770009539
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The refrigerant in a unit follows from its age: find it with the AC unit age lookup. Read the size of the unit from its model number with the model number decoder. For the airflow across the indoor coil, use the CFM calculator.
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Calculator last reviewed on October 5, 2026. Method and sources.