Enter your gauge pressures and line temperatures to get superheat, subcooling and the saturation temperatures for the refrigerant in your system. It also gives the target superheat for fixed-orifice systems, compares subcooling to the nameplate target for TXV systems and shows a full pressure–temperature chart.
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Low side (suction / vapor line)
Clamp at the large line near the outdoor service valve
High side (liquid line)
Clamp at the small line near the liquid service valve
Charging target
Printed on the outdoor unit; often 8–12 °F (4–7 °C)
Return air at the indoor coil
Air entering the condenser
Enter your values to see the result.
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How to use the Superheat and Subcooling Calculator
Choose the refrigerant and the metering device (TXV/EEV or fixed orifice).
Enter the suction pressure and the suction line temperature measured near the outdoor unit.
Enter the liquid line pressure and liquid line temperature.
Add the target: nameplate subcooling for a TXV, or indoor wet bulb and outdoor dry bulb for a piston system.
Press Calculate to read superheat, subcooling, the target comparison and the PT chart.
Superheat and subcooling formulas
Superheat = suction line temperature − saturation temperature at the suction pressure (dew point for blends). It shows how much the vapor has warmed after all the liquid boiled off in the evaporator.
Subcooling = saturation temperature at the liquid line pressure (bubble point for blends) − liquid line temperature. It shows how much the liquid has cooled below condensing temperature, and how much liquid is stacked in the condenser.
Measure on a system that has run for at least 10–15 minutes with clean filters and coils. Strap the temperature clamp to bare, clean pipe and insulate it from ambient air.
Reading the results
Superheat
Subcooling
Common cause
High
Low
Low refrigerant charge (possible leak)
Low
High
Overcharge
High
High
Restriction in the liquid line, filter drier or metering device
Low
Low
Low indoor airflow, or a TXV that is overfeeding / weak compressor
Normal
Normal
Charge is probably correct; check airflow and temperature split
These patterns are starting points, not a full diagnosis. Airflow problems (dirty filter, wet coil, slow blower) are the most common reason readings look wrong, so check airflow before adding or removing refrigerant.
Saturation points at a glance
Refrigerant
40 °F evaporator
100 °F condensing
Glide
R-410A
≈ 118 psig
≈ 317 psig
under 0.5 °F
R-32
≈ 122 psig
≈ 329 psig
none
R-454B
≈ 108 psig (dew)
≈ 307 psig (bubble)
about 3 °F
R-22
≈ 68 psig
≈ 196 psig
none
R-134a
≈ 35 psig
≈ 124 psig
none
R-404A
≈ 87 psig (dew)
≈ 243 psig (bubble)
about 1 °F
R-407C
≈ 63 psig (dew)
≈ 225 psig (bubble)
about 9–11 °F
Frequently asked questions
How do you calculate superheat?
Superheat = suction line temperature − saturated suction temperature. Read the suction pressure, look up the saturation (boiling) temperature for that pressure on the refrigerant's PT chart, and subtract it from the temperature measured on the suction line. Example: R-410A at 118 psig saturates at about 40 °F; a 50 °F suction line means 10 °F of superheat.
How do you calculate subcooling?
Subcooling = saturated condensing temperature − liquid line temperature. Convert the liquid line pressure to its saturation temperature, then subtract the measured liquid line temperature. R-410A at 365 psig condenses at about 110 °F, so a 100 °F liquid line has about 10 °F of subcooling.
Should I charge by superheat or subcooling?
Systems with a thermostatic or electronic expansion valve (TXV/EEV) are charged by subcooling, because the valve holds superheat roughly constant. Systems with a fixed orifice or piston are charged by superheat, using a target that depends on indoor wet bulb and outdoor temperature. Always follow the manufacturer's charging chart when one is available.
What is the target superheat formula?
A widely used approximation for fixed-orifice systems is target superheat = (3 × indoor wet bulb − 80 − outdoor dry bulb) ÷ 2, in °F. With 63 °F wet bulb and 85 °F outdoors the target is about 12 °F. If the result is below 5 °F, conditions are outside the chart and you should not charge by superheat.
What are normal superheat and subcooling values?
For most residential TXV systems, subcooling is typically 8–12 °F (check the nameplate) and superheat 5–15 °F. Fixed-orifice superheat targets range from about 5 °F on hot, dry days to 30 °F or more in mild, humid conditions.
Why use dew point and bubble point for R-407C and R-454B?
Blends such as R-407C and R-454B boil over a range of temperatures (temperature glide). Superheat is measured from the dew point, where the last liquid evaporates, and subcooling from the bubble point, where the first vapor would form. Using the wrong one can misjudge the charge by several degrees on high-glide blends.
How accurate are the pressure–temperature values?
Saturation data are computed with the Peng–Robinson equation of state and agree with published PT charts to within about 1 °F (0.5 °C) for most of the working range. They are meant for diagnosis and learning; confirm against the manufacturer's chart, and remember that refrigerant handling in the US requires EPA Section 608 certification.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.