Subcooling Calculator
By BRIX Editorial Team · Updated
Subcooling is the refrigerant's saturation (bubble) temperature at liquid line pressure minus the liquid line temperature. For example, R-454B at 340 psig has a bubble point of 108.1°F, so a 100°F liquid line means 8.1°F of subcooling. Judge it against the target from the unit's nameplate or installation instructions.
- Saturation (bubble) temperature at liquid pressure
- 108.1°F
- Liquid line temperature
- 100.0°F
- Your target
- 10.0°F ± 3°F
- Measured minus target
- −1.9°F
- If you'd read the dew column (wrong)
- 10.8°F
Subcooling is within your tolerance of the target. Confirm airflow and that the system has stabilized before you call it done. Rule of thumb only: follow the manufacturer's charging chart for the target and the method.
Key takeaways
- Subcooling = bubble-point saturation temperature at liquid pressure − liquid line temperature.
- On blends (R-454B, R-410A) use the bubble (liquid) column. On R-32 and R-22 there's one column.
- Low subcooling usually means an undercharge; high subcooling means liquid is stacking up in the condenser.
- TXV and EEV systems are usually charged by subcooling, to the manufacturer's target. This tool is a rule-of-thumb check.
How do you calculate subcooling?
Read the liquid line pressure at the service valve, convert it to the saturation (bubble-point) temperature on the refrigerant's PT chart, then subtract the liquid line temperature measured at the same spot. The answer is how many degrees the liquid has cooled below the point where it finished condensing.
Subcooling (°F) = Saturation (bubble) temperature at liquid pressure − Liquid line temperature
- Saturation (bubble) temperature
- from the PT chart at the liquid line gauge pressure; bubble column for blends
- Liquid line temperature
- measured with a clamp probe near the liquid service valve
- 1
Run the system until it settles.
Both units running, normal airflow, long enough for pressures and temperatures to stop drifting.
- 2
Read liquid line pressure.
Connect the high-side gauge at the liquid line service valve and note the psig.
- 3
Measure the liquid line.
Clamp an insulated probe on clean copper next to the service valve.
- 4
Convert pressure to saturation.
Look up the bubble-point temperature for that pressure on the right refrigerant's chart, or use the calculator.
- 5
Subtract and compare.
Saturation temperature minus line temperature is subcooling. Compare it with the manufacturer's target.
Same reading, different refrigerant
| Refrigerant | Saturation (bubble) temperature | Subcooling |
|---|---|---|
| R-410A | 100.3°F | 10.3°F |
| R-454B | 103.8°F | 13.8°F |
| R-32 | 98.8°F | 8.8°F |
What do high and low subcooling mean?
| Reading | Subcooling | Superheat | Often points to |
|---|---|---|---|
| Undercharge | Low | High | A leak. Find and repair it, then weigh in or charge to target. |
| Overcharge | High | Low | Too much refrigerant backing up in the condenser. |
| Restriction (metering device, drier) | High | High | Liquid trapped before the restriction; check temperature drop across the drier. |
| Dirty condenser or low outdoor airflow | Varies | Varies | High head pressure. Clean the coil before judging the charge. |
When to charge by subcooling
Systems with a thermostatic or electronic expansion valve are usually charged by subcooling, because the valve keeps superheat roughly constant on its own. The target is often printed on the outdoor unit's nameplate or in the installation instructions. Fixed-orifice systems are usually charged by superheat. Whatever the system, the manufacturer's instructions set the method, the target and the conditions to measure under.
Common subcooling mistakes
- Using the dew column on a blend. Subcooling uses the bubble (liquid) column. On R-454B the error is more than 2°F.
- Measuring line temperature far from the pressure tap. Read both at the liquid service valve.
- Charging with a dirty condenser. High head pressure changes subcooling. Clean the coil and confirm outdoor airflow first.
- Not letting it settle. Subcooling drifts for several minutes after a charge change. Wait before adjusting again.
- Charging at altitude without correcting gauges. Use the refrigerant PT calculator altitude setting.
Frequently asked questions
What is a good subcooling reading?
The one on the unit's nameplate or in its installation instructions. Many TXV systems list a subcooling target there, and the manufacturer also gives the tolerance and conditions. There isn't one correct subcooling number for every system.
Do I use the bubble or dew temperature for subcooling?
Bubble. Subcooling is measured on the liquid line, where the refrigerant has just finished condensing to liquid, so the bubble-point saturation temperature applies. On R-32 and R-22 the two columns are the same.
What causes low subcooling?
Most often an undercharge, usually from a leak. Not enough liquid backs up in the condenser to cool below saturation. Find and fix the leak before adding refrigerant, and check superheat to confirm.
What causes high subcooling?
Too much liquid held in the condenser: an overcharge, or a restriction such as a plugged filter drier or a stuck metering device. If superheat is also high, suspect a restriction rather than an overcharge.
Is subcooling or superheat better for charging?
Neither is better in general; it depends on the metering device. TXV and EEV systems are usually charged by subcooling, fixed-orifice systems by superheat. Always follow the method in the manufacturer's installation instructions.
Does this calculator handle R-454B glide?
Yes. For blends it uses the bubble column of our CoolProp PT table, and it shows what you'd get from the dew column so you can see the size of the mistake. It also covers R-410A, R-32 and R-22.
Related
- Superheat CalculatorFree superheat calculator: enter suction pressure and suction line temperature for R-410A, R-454B, R-32 or R-22 and compare with your target superheat.
- Refrigerant PT Calculator (Pressure ↔ Temperature)Convert refrigerant pressure to saturation temperature and back for R-410A, R-454B, R-32 and R-22, with bubble and dew points and an altitude correction.
- Refrigerant PT Charts (Pressure-Temperature)Refrigerant pressure-temperature charts for R-410A, R-454B, R-32 and R-22, computed with CoolProp, plus a side-by-side comparison and how to read them.
- R-454B PT Chart (Pressure-Temperature)R-454B pressure-temperature chart from −40°F to 150°F with bubble (liquid) and dew (vapor) psig, a quick lookup, °F/°C toggle and how to read glide.
- R-32 PT Chart (Pressure-Temperature)R-32 pressure-temperature chart, −40°F to 150°F in psig, one column because R-32 is a pure refrigerant. A2L safety notes, GWP 675 and a PT lookup.
- Superheat and Subcooling, Explained with Worked ExamplesSuperheat and subcooling explained with worked R-454B and R-32 examples: which PT column to use, the formulas, common mistakes and five check questions.
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Sources
- CoolProp 8.0.0: thermophysical property library, used to compute the PT tables (checked October 24, 2026)
- Daikin Comfort Technologies: A1 vs. A2L Pressure Temperature Chart, USA (PM-A2LPTC-USA_04-24), used to check the tables (checked October 24, 2026)
- Arkema Forane: Pressure Temperature Chart, R-454B (PPC/SDC 05-2025), used to check the R-454B table (checked October 24, 2026)