Superheat and Subcooling Calculator
This superheat and subcooling calculator finds the system superheat and subcooling from gauge saturation temperatures and line temperatures, then compares them with your targets. Add the indoor wet bulb and outdoor dry bulb and it works out the target superheat for a fixed orifice system, so you can check the refrigerant charge on an air conditioner or heat pump in Fahrenheit or Celsius.
Results
The Superheat and Subcooling Calculator is part of our Cooling & Heating collection. Browse more cooling & heating to size, convert, and check your work alongside this tool.
What the superheat and subcooling calculator shows
Superheat is how far the suction gas has warmed above its boiling point; subcooling is how far the liquid has cooled below its condensing point. Both tell you whether the evaporator and condenser hold the right amount of refrigerant. The superheat and subcooling calculator subtracts the saturation temperatures from the measured line temperatures, then compares the results with the target superheat for a piston system and the target subcooling for a TXV system.
Superheat formula and subcooling formula
Both values are simple differences:
The target superheat formula applies to fixed orifice (piston) metering devices. TXV and EEV systems hold superheat on their own, so you charge them by subcooling.
Superheat and subcooling calculator example
A piston system shows 45°F suction saturation and a 57°F suction line, 115°F liquid saturation and a 104°F liquid line, with 63°F indoor wet bulb and 85°F outdoors:
Superheat matches the target and subcooling sits 1°F above a 10°F target, so the charge is right.
How to use the superheat and subcooling calculator
Run the system for at least 15 minutes. Read the suction and liquid saturation temperatures from your gauge set on the scale for the refrigerant in the system, and clamp a thermometer on the suction line near the outdoor service valve and on the liquid line. Enter all four temperatures. Add the indoor return wet bulb and the outdoor dry bulb for a target superheat, and the target subcooling from the unit data plate.
Reading superheat and subcooling
On a piston system, superheat above target usually means low charge: check the filter and airflow first, then add refrigerant in small amounts. Superheat below target points to overcharge or low indoor airflow. On a TXV system, low subcooling suggests undercharge and high subcooling suggests overcharge or a restriction. Use the two readings together: low superheat with high subcooling is a classic overcharge, high superheat with low subcooling a classic undercharge.
When to use a superheat calculator
- Checking the refrigerant charge during a service call
- Commissioning a new air conditioner or heat pump
- Diagnosing low cooling or a frozen evaporator coil
- Confirming a TXV system is charged by subcooling
- Training apprentices on how to calculate superheat
Common superheat calculator mistakes
- Reading the wrong refrigerant scale. Use the saturation scale for the refrigerant in the system.
- Measuring too soon. Wait 15 minutes after start and after each adjustment.
- Using dry bulb as wet bulb. The target formula needs indoor wet bulb.
- Charging a TXV by superheat. Use subcooling for TXV and EEV systems.
Tips for superheat and subcooling checks
- Check airflow and filters before you touch the charge.
- Add or recover refrigerant in small steps and recheck.
- Record readings with the outdoor temperature for the next visit.
Target superheat and why it changes
A fixed orifice passes a set amount of refrigerant for a given pressure difference. When indoor air is warm and humid, the evaporator boils more refrigerant and the gas leaves with little superheat to spare, so the target rises. When the outdoor air is hot, head pressure climbs, the piston feeds more refrigerant, and the target falls. The formula captures both effects in one line. When the result drops below about 5°F, the system runs too close to flooding the compressor, and most charging charts say not to adjust charge until conditions change. Indoor wet bulb is the key input: measure it at the return with a psychrometer, or work it out from dry bulb and humidity with the wet bulb calculator. In Celsius, the calculator converts your readings to Fahrenheit, applies the formula, and converts the target back, so a 17°C wet bulb and 29°C outdoor gives a target of about 6.6°C.
Superheat and subcooling chart readings in the field
Good readings need a steady system and clean measurements. Insulate the line thermometer clamp, place it on a clean copper section a few inches from the service valve, and give the system time to settle after each charge adjustment. Saturation temperatures come from the pressure temperature chart for the exact refrigerant, R-410A, R-22, R-32, or R-454B, and digital gauges show them directly. Blends with glide, such as R-407C, use the dew point for superheat and the bubble point for subcooling. Before you blame the charge, rule out a dirty filter, a dirty coil, or a slow blower; poor airflow mimics an overcharge on the low side. Check the temperature drop across the coil with the delta T calculator to confirm airflow is in range.
Target superheat chart
Target superheat in °F for fixed orifice systems from (3 × WB − 80 − DB) ÷ 2. n/a means the target falls below 5°F, outside the charging range:
| Indoor WB | Outdoor 75°F | Outdoor 85°F | Outdoor 95°F | Outdoor 105°F |
|---|---|---|---|---|
| 57°F | 8 | n/a | n/a | n/a |
| 61°F | 14 | 9 | n/a | n/a |
| 65°F | 20 | 15 | 10 | 5 |
| 69°F | 26 | 21 | 16 | 11 |
Superheat and Subcooling Calculator FAQ
How do you calculate superheat?
Subtract the suction saturation temperature from the suction line temperature. If the line reads 57°F and the saturation is 45°F, superheat is 12°F.
How do you calculate subcooling?
Subtract the liquid line temperature from the liquid saturation temperature. A 115°F saturation and a 104°F line give 11°F of subcooling.
What is a normal target superheat?
It depends on indoor wet bulb and outdoor temperature. Hot outdoor air and dry indoor air lower the target; the chart shows typical values from 5°F to 26°F.
Do I charge a TXV system by superheat?
No. A TXV holds its own superheat, so charge TXV and EEV systems by subcooling, usually 8 to 12°F, using the target on the unit data plate.