Superheat & Subcooling Calculator
Enter your gauge pressure and line temperature. The tool reads the built-in R-410A / R-22 PT chart, computes superheat and subcooling, and compares them against normal charging targets.
Fill one side or both. Take readings with the system running steady for 10–15 minutes, temps from an insulated clamp probe on the line.
What superheat and subcooling mean
Superheat is how many degrees the suction line vapor is above its boiling (saturation) temperature at the measured pressure. Subcooling is how many degrees the liquid line is below its condensing temperature. Convert gauge pressure to saturation temperature on the PT chart, then: superheat = suction line temp − suction saturation temp, and subcooling = liquid saturation temp − liquid line temp.
Together, the two numbers describe what the refrigerant is doing at both ends of the system. Superheat protects the compressor by proving no liquid is returning to it. Subcooling proves a full column of liquid is feeding the metering device. Charging by these numbers — instead of “beer-can cold” guesswork — is the difference between a system that runs at rated capacity and one that quietly eats itself.
The formulas
Superheat (°F) = Suction line temperature − Saturation temperature at suction pressure
Subcooling (°F) = Saturation temperature at liquid pressure − Liquid line temperature
Saturation temperatures come from the refrigerant’s pressure–temperature chart. This calculator carries the R-410A and R-22 tables internally (both printed below), and interpolates between chart rows for pressures that fall in between.
A real R-410A reading, step by step
- Gauges on a running R-410A system show
118 psigsuction. The PT chart says 118 psig saturates at about40 °F. - The clamp probe on the suction line reads
50 °F. Superheat = 50 − 40 = 10 °F. - Liquid side shows
382 psig, which saturates near110 °F. The liquid line probe reads100 °F. Subcooling = 110 − 100 = 10 °F. - 10 °F superheat with 10 °F subcooling on a TXV system sits inside normal targets — this charge is healthy.
Normal superheat and subcooling ranges
| Metering device | Superheat target | Subcooling target |
|---|---|---|
| TXV / EEV | 8–14 °F at the condenser | 8–12 °F (check nameplate) |
| Fixed orifice / piston | Varies with conditions — use a target-superheat chart | 5–15 °F typical, informational |
Manufacturer data always wins. Many condensers print the required subcooling on the nameplate — charge to that number, not a generic range.
Quick diagnosis patterns
| Reading | Commonly points to |
|---|---|
| High SH + low SC | Undercharge, or a restriction starving the evaporator |
| Low SH + high SC | Overcharge, or a TXV overfeeding |
| High SH + high SC | Liquid-line restriction — plugged filter-drier or underfeeding TXV |
| Low SH + low SC | Low evaporator airflow or a compressor not pumping |
Before touching the charge, verify airflow: filter, blower wheel, evaporator coil, duct static. Half of “charge problems” in the field are airflow problems wearing a disguise.
Built-in PT charts
These are the same tables the calculator reads, printed for field reference. Values are saturation pressures in psig at sea level, interpolated from standard PT charts and rounded to 0.1 — always verify critical charging decisions against the equipment manufacturer’s chart.