Refrigerants · Calculator
Superheat and Subcooling Calculator: Formulas, Targets and Readings
Superheat is the suction line temperature minus the evaporator saturation temperature. Subcooling is the condensing saturation temperature minus the liquid line temperature. TXV systems are usually charged to about 8 to 12 °F of subcooling, while fixed orifice systems are charged to a target superheat taken from the manufacturer's chart.
Superheat and subcooling calculator
The two formulas
Superheat = Suction line temperature − Evaporator saturation temperature
Subcooling = Condensing saturation temperature − Liquid line temperature
Both saturation temperatures come from converting gauge pressures with a refrigerant PT chart. The line temperatures come from a clamp or pipe thermometer strapped to clean, insulated copper.
Superheat is how far the vapor leaving the evaporator has warmed above its boiling point. A little superheat proves all the liquid has boiled off before reaching the compressor. Subcooling is how far the liquid leaving the condenser has cooled below its condensing point. A little subcooling proves a solid column of liquid is feeding the metering device.
Worked example
An R-410A system with a TXV is running on a warm afternoon. The technician records:
- Suction pressure 118 psig, which the PT chart converts to 40 °F
- Suction line temperature 52 °F
- Liquid line pressure 341 psig, which converts to 105 °F
- Liquid line temperature 95 °F
- Nameplate target subcooling 10 °F
Superheat = 52 − 40 = 12 °F. Subcooling = 105 − 95 = 10 °F, exactly on target. These are the calculator's default inputs, and it returns the same result: subcooling within 3 °F of target and superheat in the normal range.
Which number to charge by
| Metering device | Charge by | Typical target |
|---|---|---|
| TXV (thermostatic expansion valve) | Subcooling | Nameplate value, commonly 8 to 12 °F |
| EEV (electronic expansion valve) | Subcooling or manufacturer's procedure | Per installation manual; many inverter units use a dedicated charge mode |
| Fixed orifice or piston | Superheat | From the target superheat chart using indoor wet bulb and outdoor dry bulb, often about 5 to 20 °F |
A TXV adjusts its opening to hold evaporator superheat roughly steady, usually somewhere around 8 to 12 °F, so superheat on a TXV system mainly tells you whether the valve is working. A fixed orifice cannot adjust, so its superheat changes directly with charge, load and airflow. That is why the method has to match the device. The background on each method is in how AC refrigerant charge is verified.
Reading the results together
| Superheat | Subcooling | Most likely cause |
|---|---|---|
| High | Low | Undercharge (leak) |
| Low | High | Overcharge |
| High | High | Restriction such as a blocked filter drier or a stuck-closed TXV |
| Low | Low or normal | Low indoor airflow, dirty evaporator or oversized metering device |
Always confirm airflow before adjusting charge. A clogged filter, closed returns or a slow blower lower suction pressure and superheat and can look like a charge fault. Our filter guide and AC not blowing cold checklist cover the airflow checks.
Getting accurate readings
- Run the system at least 10 to 15 minutes with doors closed and the indoor fan on high cooling speed.
- Measure line temperatures on bare copper, cleaned and insulated over the clamp so outdoor air does not affect it.
- Take suction readings near the outdoor unit's service valve, not at the indoor coil.
- For blends with temperature glide such as R-454B, use the dew-point value for superheat and the bubble-point value for subcooling.
- Avoid charging in very mild weather (outdoor air below roughly 60 to 65 °F) unless the manufacturer gives a low-ambient procedure; weigh in instead.
Attaching gauges and adjusting refrigerant charge requires EPA Section 608 certification. This calculator is for technicians, students and homeowners checking a service report, not a guide to adding refrigerant yourself.
Why superheat protects the compressor
Compressors pump vapor, not liquid. If superheat falls toward zero, droplets of liquid can reach the compressor, wash oil off bearings and, at worst, break valves. Too much superheat is a problem too: the returning vapor cools the compressor motor, so a starved evaporator runs the compressor hot and shortens its life. Keeping superheat in the expected band is one of the cheapest ways to avoid a compressor replacement.
Common reading mistakes
- Wrong refrigerant column: R-410A and R-32 are close but not identical, and R-22 is very different. Check the nameplate.
- Probe on painted or dirty pipe: paint and oxidation insulate the sensor and read several degrees off.
- Reading too soon: pressures and temperatures drift for the first 10 minutes or more of a cycle.
- Low-loss fittings and hose losses: long hoses and partly open valves can drop pressure between the port and the gauge, especially on the suction side.
- Ignoring glide: on R-454B and other zeotropic blends, using a single mid-point saturation value can shift superheat or subcooling by a degree or more.
- Charging a dirty system: a dirty condenser raises head pressure and changes subcooling; clean first, then measure.
If readings do not make sense, recheck airflow and the measurements before concluding anything about charge.
Frequently asked questions
What is a good superheat for an AC?
On a TXV system, evaporator superheat is usually about 8 to 12 °F. On a fixed orifice system the right value depends on indoor wet bulb and outdoor temperature and comes from the manufacturer's chart, often between 5 and 20 °F.
What does zero subcooling mean?
The liquid line contains a mix of liquid and vapor, which typically means a significant undercharge. The metering device then feeds bubbles and capacity drops sharply.
Can superheat be negative?
Not in a real measurement. A negative result means a reading error, such as using the wrong refrigerant column, a gauge not zeroed or a probe on the wrong line.
Last reviewed 2026-10-28