# Subcooling Calculator

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.

## 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.

> **Rule of thumb: follow the manufacturer's charging chart**
>
> This tool does the arithmetic from our CoolProp PT tables. The target subcooling, tolerance and charging method come from the unit's nameplate, installation instructions and charging chart, which always take precedence. Readings are assumed at sea level.

## 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**

```
Subcooling (°F) = Saturation (bubble) temperature at liquid pressure − Liquid line temperature
```

Where:
- 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.

> **Worked example (illustrative readings)**
>
> An R-454B system with a TXV reads 340 psig at the liquid valve and the liquid line is 100°F. The nameplate target is 10°F, and say the instructions allow ± 3°F.
>
> 340 psig on the R-454B bubble column is 108.1°F. Subcooling = 108.1 − 100 = **8.1°F**, 1.9°F under target and inside the tolerance.
>
> The trap: read the dew column by mistake and you'd get 110.8°F and 10.8°F of subcooling. R-454B's glide at this pressure is about 2.6°F, enough to make a slightly short charge look perfect.

## Same reading, different refrigerant

**Subcooling from 320 psig and a 90°F liquid line**

| 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 |

_Computed with the calculator above. A 5.0°F spread from one gauge reading. R-22 runs far lower head pressure: on an R-22 system, 230 psig with a 100°F liquid line is 111.1°F saturated and 11.1°F of subcooling._

## What do high and low subcooling mean?

**Common patterns (confirm with superheat and airflow before acting)**

| 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. |

_General diagnostic patterns taught in HVAC programs, not rules. The manufacturer's troubleshooting chart takes precedence._

## 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](https://www.brix.so/tools/superheat-calculator). 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](https://www.brix.so/tools/refrigerant-pt-calculator) altitude setting.

Between service calls, BRIX answers the phone, books the next appointment and sends the invoice when the job's done, so you can stay on the gauges.

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## 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 Calculator](https://www.brix.so/tools/superheat-calculator)
- [Refrigerant PT Calculator (Pressure ↔ Temperature)](https://www.brix.so/tools/refrigerant-pt-calculator)
- [Refrigerant PT Charts (Pressure-Temperature)](https://www.brix.so/reference/refrigerant-pt-chart)
- [R-454B PT Chart (Pressure-Temperature)](https://www.brix.so/reference/refrigerant-pt-chart/r-454b)
- [R-32 PT Chart (Pressure-Temperature)](https://www.brix.so/reference/refrigerant-pt-chart/r-32)
- [Superheat and Subcooling, Explained with Worked Examples](https://www.brix.so/academy/hvac/refrigeration/superheat-subcooling)
- [Maintenance Agreement Pricing Calculator](https://www.brix.so/tools/maintenance-agreement-calculator)
- [HVAC Software That Answers the Phone, Books the Tech and Keeps the Books](https://www.brix.so/for/hvac)
- [BTU Calculator](https://www.brix.so/tools/btu-calculator)
- [AC Tonnage Calculator](https://www.brix.so/tools/hvac-tonnage-calculator)

## Sources

- [CoolProp 8.0.0: thermophysical property library, used to compute the PT tables](https://coolprop.org/coolprop/HighLevelAPI.html) (checked 2026-10-24)
- [Daikin Comfort Technologies: A1 vs. A2L Pressure Temperature Chart, USA (PM-A2LPTC-USA_04-24), used to check the tables](https://www.chadwellsupply.com/globalassets/articles-pk/a2l/a2lptc-usa_04-24.pdf) (checked 2026-10-24)
- [Arkema Forane: Pressure Temperature Chart, R-454B (PPC/SDC 05-2025), used to check the R-454B table](https://forane.arkema.com/files/live/sites/shared_arkema/files/downloads/fluorochemicals/Forane%20Ref%20PT%20Regular%20Chart%20EN) (checked 2026-10-24)

Talk to BRIX: call or text (518) 588-1122, email eeharris2004@gmail.com, or book a 15-minute call at https://cal.com/ethanharris/15min.

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Written by BRIX Editorial Team. Last updated 2026-10-24.
Canonical: https://www.brix.so/tools/subcooling-calculator