Superheat and Subcooling Calculator
Turn your gauge and thermometer readings into superheat or subcooling, see the evaporator or condensing temperature, and check the result against the manufacturer’s target.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
Within target
- Superheat
- 10.2°F
- Evaporator (dew-point) temperature
- 39.8°F
10.2 °F superheat (within target); evaporator at 39.8 °F.
- Difference from target
- 0.16°F
Show the working
- Absolute pressure = gauge + atmospheric118 + 14.696 = 132.7 psia
- Dew-point (saturated vapor) temperature at that pressure39.84 °F
- Superheat = suction-line temperature − saturation temperature50 − 39.84 = 10.16 °F
- Compared with the target10.2 vs 10 ± 5 °F → within target
Superheat
Superheat is how far the suction vapor has warmed above the temperature at which it boiled. Read the suction pressure and convert it to the saturation (dew-point) temperature; then measure the suction-line temperature near the same port. Superheat = suction-line temperature − saturation temperature.
Systems with a fixed metering device (a piston or capillary tube) are usually checked by superheat, against a target that depends on the indoor wet-bulb and outdoor dry-bulb temperatures. The manufacturer’s charging chart gives that target.
Subcooling
Subcooling is how far the liquid has cooled below the temperature at which it condensed. Read the liquid-line pressure, convert it to the saturation (bubble-point) temperature, and measure the liquid-line temperature. Subcooling = saturation temperature − liquid-line temperature.
Systems with a thermostatic or electronic expansion valve (TXV or EEV) are usually checked by subcooling, against a target printed on the outdoor unit’s nameplate or in its installation instructions.
Taking good readings
Let the system run for at least 15 minutes, or as long as the manufacturer specifies, with clean filters and the correct indoor airflow. Measure the line temperature with a clamp probe on clean, bare copper next to the service port, insulated from the surrounding air. Small reading errors matter: near 40 °F, R-410A’s saturation pressure changes by about 2.3 psi per °F, so a 2 psi gauge error shifts the result by almost a degree.
What an off-target result means
A result outside the target is a symptom, not a diagnosis. High superheat can come from low charge, a restriction or low heat load; low superheat from overcharge or low indoor airflow. Airflow, metering devices and measurement errors all have to be ruled out before adding or removing refrigerant. Follow the manufacturer’s procedure. Only EPA Section 608 certified technicians may handle refrigerant in the U.S.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Refrigerant
- R-410A
- Calculate
- Superheat (suction side)
- Suction pressure (gauge)
- 118 psi
- Suction-line temperature
- 50 °F
- Target superheat
- 10 °F
- Allowed difference from target (±)
- 5 °F
- Outdoor air temperature at the condenser
- 85 °F
- Site elevation
- 0 ft
Result
- Superheat
- 10.2 °F
- Evaporator (dew-point) temperature
- 39.8 °F
- Difference from target
- 0.16 °F
10.2 °F superheat (within target); evaporator at 39.8 °F.
Assumptions
- Superheat uses the dew-point (saturated vapor) temperature at the suction pressure; subcooling uses the bubble-point (saturated liquid) temperature at the liquid pressure.
- Saturation temperatures come from the same validated tables as the PT calculator, corrected to absolute pressure for the site elevation.
Limitations
- The target and the charging procedure come from the equipment manufacturer. A result outside the target can come from airflow, metering-device, measurement or charge problems — it does not by itself say how much refrigerant to add or remove.
- Only certified technicians may handle refrigerant (U.S. EPA Section 608). Wear eye and hand protection when connecting gauges.
Frequently asked questions
What is a normal superheat?
There is no single normal value. For a fixed metering device it depends on the indoor and outdoor conditions; for a TXV system it is controlled by the valve. Use the manufacturer’s chart or target.
What is a normal subcooling?
It varies by model. The correct target is the one on the outdoor unit’s nameplate or in its installation instructions, and it may depend on the line-set length.
Which saturation temperature do I use for R-410A?
The dew point for superheat (suction side) and the bubble point for subcooling (liquid side). For R-410A they differ by only about 0.2 °F; for blends with more glide the difference matters more.
Why does it warn below 32 °F?
When the refrigerant in the indoor coil boils below 32 °F, water condensing on the coil can freeze. That often points to low airflow, a dirty filter or low charge, and the coil can ice over.
Sources
- CoolProp — open-source thermophysical property library (version 8.0.0) — CoolProp developers (MIT licence). Equations of state for pure fluids and mixtures, incompressible fluids and brines, and IAPWS-IF97 water/steam; free for commercial use under its licence. Used here to generate the refrigerant saturation tables.
- Refrigerant PT charts and how they were validated — FreeHVACCalculators.com. Each published refrigerant’s generated table agrees with at least two independent published tables (worst differences and tolerances listed); R-454B is withheld because it does not.
- Refrigerant Charge Verification, form CEC-CF2R-MCH-25-H (2025 Single-Family Standards, RA3.2) — California Energy Commission. Superheat method (fixed metering devices) passes when measured superheat is within ±5 °F of target; subcooling method (variable metering devices) passes within ±3 °F of the manufacturer’s target; both require outdoor air of at least 55 °F and no more than 115 °F; otherwise superheat must be 4–25 °F when no manufacturer specification is known.
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