How do you calculate subcooling?

Subcooling = saturation (bubble point) temperature at the measured liquid line pressure − actual liquid line temperature. Example: R-410A at 365 psig saturates at about 105 °F; a 95 °F liquid line gives 105 − 95 = 10 °F of subcooling.

What is normal subcooling?

Most TXV systems target 8–12 °F of subcooling at the condenser outlet; always follow the data plate when it specifies a value.

What does low subcooling indicate?

Low or zero subcooling generally means undercharge or a liquid line restriction; the condenser is not fully filling with liquid.

What does high subcooling indicate?

High subcooling usually means overcharge, a restricted metering device, non-condensables, or a dirty or airflow-restricted condenser.

What does high subcooling with low superheat mean?

High subcooling together with low superheat points to an overcharge or an overfeeding metering device — liquid is stacked in the condenser and the evaporator is flooding at the same time.

Do I charge by subcooling or superheat?

Charge by subcooling on TXV and EEV systems, and by superheat on fixed-orifice (piston or capillary) systems. Using the wrong method will leave the charge off even when the number looks good.

Where do I measure liquid line temperature?

At the condenser outlet, on a clean section of the liquid line near the service valve, with the sensor insulated so ambient air does not pull the reading.

Subcooling Calculator — How to Calculate Subcooling

All calculators

Formula

Subcool = Bubble-point Temp (from P/T) − Measured Liquid Line Temp

Pick the refrigerant; the bubble point (liquid side) is looked up from its PT chart. Target subcool is on the equipment data plate (typ. 8–12°F).

Pressure basis

Rounding

Refrigerant

Liquid line pressure

psig

Measured liquid line temp

°F

Target subcool (nameplate)

°F

Saturation temp

83.6°F

R-410A @ 250.0 psig (250.0 psig)

Actual subcool

-11.4°F

Target 10.0°F · -21.4°F vs. target

Undercharged — low subcool means refrigerant flashing in liquid line. Find leak before adding. Negative subcool = no liquid at metering device. Check charge immediately.

How to calculate subcooling

Subcooling is how many degrees the liquid leaving the condenser sits below its condensing temperature. It tells you how much liquid refrigerant is stacked in the condenser, which is why it is the charging number on TXV and EEV systems.

Subcooling (°F) = Saturation (bubble point) temperature − Liquid line temperature

Run the system 10–15 minutes so pressures settle.
Read liquid line (head) pressure at the liquid service valve.
Convert it to saturation temperature on the refrigerant's P/T chart — use the bubble point column on glide blends.
Clamp an insulated thermometer on the liquid line at the condenser outlet.
Subtract the liquid line temperature from the saturation temperature.

Worked example (R-410A)

Liquid line pressure: 365 psig → saturation temperature ≈ 105 °F
Liquid line temperature: 95 °F
Subcooling = 105 − 95 = 10 °F

With a 10 °F nameplate target, that charge is correct. Always use the value on the data plate when the manufacturer gives one — it beats any rule of thumb.

Target subcooling ranges

System	Typical subcooling	Notes
Residential TXV split	8–12 °F	Data plate wins
Heat pump, cooling mode	8–12 °F	Heating mode differs
Fixed orifice / piston	Not used for charging	Charge by superheat
Receiver systems	1–5 °F at receiver outlet	Receiver absorbs charge

Reading subcooling with superheat

Subcooling	Superheat	Most likely cause
Low	High	Undercharge or leak
High	High	Restriction between condenser and coil (drier, TXV, kink)
High	Low	Overcharge or overfeeding metering device
Normal	Low	Low evaporator airflow — dirty filter or coil
High	Normal	Dirty or airflow-restricted condenser, or non-condensables

Superheat calculator →Refrigerant P/T chart →Evaporator delta-T →

Common mistakes

Using the dew point instead of the bubble point on glide blends — subcooling is a liquid-side reading.
Measuring liquid line temperature in direct sun or without insulating the clamp.
Charging by subcooling on a fixed-orifice system, where the number moves with load instead of charge.
Adding refrigerant to fix low subcooling without first finding the leak.
Ignoring a dirty condenser — it raises head pressure and skews subcooling before you touch the charge.

How to use this calculator

Determine liquid line subcooling from head pressure and liquid line temperature.

Select the refrigerant and enter liquid line pressure with the correct psig/psia basis.
Enter the liquid line temperature at the condenser outlet.
Read subcooling as saturation (bubble point) temperature minus liquid temperature.
Compare against the manufacturer's target, typically 8–12 °F.

Frequently asked questions

How do you calculate subcooling?: Subcooling = saturation (bubble point) temperature at the measured liquid line pressure − actual liquid line temperature. Example: R-410A at 365 psig saturates at about 105 °F; a 95 °F liquid line gives 105 − 95 = 10 °F of subcooling.
What is normal subcooling?: Most TXV systems target 8–12 °F of subcooling at the condenser outlet; always follow the data plate when it specifies a value.
What does low subcooling indicate?: Low or zero subcooling generally means undercharge or a liquid line restriction; the condenser is not fully filling with liquid.
What does high subcooling indicate?: High subcooling usually means overcharge, a restricted metering device, non-condensables, or a dirty or airflow-restricted condenser.
What does high subcooling with low superheat mean?: High subcooling together with low superheat points to an overcharge or an overfeeding metering device — liquid is stacked in the condenser and the evaporator is flooding at the same time.
Do I charge by subcooling or superheat?: Charge by subcooling on TXV and EEV systems, and by superheat on fixed-orifice (piston or capillary) systems. Using the wrong method will leave the charge off even when the number looks good.
Where do I measure liquid line temperature?: At the condenser outlet, on a clean section of the liquid line near the service valve, with the sensor insulated so ambient air does not pull the reading.