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How to calculate subcooling

Last updated: October 17, 2026

Subcooling tells you how far liquid refrigerant has cooled below its saturation temperature. Here is the calculation, step by step.

What subcooling is

Subcooling is the number of degrees the liquid refrigerant is below the temperature at which it would condense at the same pressure. It is measured on the high side, on the liquid line, and it shows whether the metering device is being fed solid liquid.

The formula

Subcooling = saturation temperature at liquid pressure − liquid line temperature

For a blend with temperature glide, use the bubble point (saturated liquid) column for the saturation temperature. See bubble point versus dew point.

Step by step

  1. Let the system run and stabilize, and confirm airflow is normal.
  2. Read the liquid pressure, in gauge pressure, at the point your manufacturer’s procedure specifies.
  3. Look up the saturation temperature for that pressure for the refrigerant in the system, using the bubble-point value.
  4. Measure the liquid line temperature with a pipe clamp or probe, with good contact and insulation over the probe.
  5. Subtract: saturation temperature minus line temperature is the subcooling.

Worked example

R-410A, liquid pressure 400 psig. The bubble-point saturation temperature at 400 psig is about 116.4 °F in the P-T data bundled in HVAC Charge Pro. The measured liquid line temperature is 106.0 °F.

116.4 °F − 106.0 °F = 10.4 °F subcooling

The inputs are illustrative. Whether 10.4 °F is right depends on the equipment’s target.

What the number is compared with

Subcooling is usually compared with the manufacturer’s target on the data plate, in the installation instructions or in the service literature. TXV and EEV systems are commonly charged and verified this way. Published rules of thumb vary by source and system, so treat them only as rough checks, never as a replacement for the manufacturer’s figure.

HVAC Charge Pro calculates the actual subcooling from your readings. It does not supply a target.

Before you trust a reading

  • Airflow and load conditions are normal and the system is stable.
  • Gauges and probes are accurate and the selected refrigerant matches the system.
  • Gauge pressure and a consistent temperature unit are used throughout.

HVAC Charge Pro and this guide are calculation and reference aids. A superheat or subcooling value alone does not decide whether to add or remove refrigerant. Follow the equipment manufacturer’s instructions, approved service procedures and applicable safety requirements.

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