Calculate HVAC subcooling or reverse-solve saturation and liquid-line temperatures in Fahrenheit, Celsius, or Kelvin, with an optional equipment-target comparison.

Find the difference between saturation and measured liquid-line temperature.

Changing units reinterprets entered numbers; convert your inputs first. Differences use the selected scale without an absolute-temperature offset.

Equipment target (optional)

Blank omits comparison. Use the equipment’s specified target; no target is assumed.

Use the applicable refrigerant’s bubble-point temperature from P/T data at the same liquid pressure and measurement location as the liquid-line temperature. This arithmetic does not determine refrigerant charge.

Subcooling Formula

The following formula calculates subcooling as a temperature difference.

SCT = SLT - LLT
  • Where SCT is the subcooling temperature difference
  • SLT is the applicable bubble-point saturation temperature at the measured liquid pressure
  • LLT is the liquid line temperature

Subtract the measured liquid-line temperature from the applicable bubble-point saturation temperature. Obtain that saturation temperature from refrigerant P/T data using the same liquid pressure and measurement location.

Use matching absolute-temperature units for saturation and liquid-line inputs. Their difference is subcooling: a °F difference converts to a °C or K difference by multiplying by 5/9, without an absolute-temperature offset.

Subcooling Definition

What is subcooling?

Subcooling describes a liquid below its applicable saturation temperature at the actual pressure. For refrigerant subcooling, use the applicable saturated-liquid or bubble-point reference, rather than a normal boiling point at standard pressure.

Subcooling is often used in refrigeration-cycle analysis. Its value alone does not determine refrigerant charge or diagnose system condition.

Example Problem

How to calculate subcooling temperature?

  1. First, determine the saturated liquid temperature.

    For this example problem, the saturated liquid temperature is 121 °F.

  2. Next, determine the liquid line temperature.

    In this example, the liquid line temperature is measured to be 104 °F.

  3. Finally, calculate the subcooling temperature of the refrigeration device.

    Using the formula above, the subcooling temperature is calculated as follows:
    SCT = SLT - LLT
    SCT = 121 °F - 104 °F
    SCT = 17 °F difference