Subcooling is the number of degrees a liquid refrigerant is cooled below its condensing (saturation) temperature at the measured pressure. It confirms the refrigerant has fully condensed and has extra cooling margin before the metering device.

What is a normal subcooling range?

Most residential split systems target 10–20°F of subcooling. Some manufacturers specify a tighter range (e.g. 10–15°F). Always check the unit's data plate or installation manual for the manufacturer's target subcooling value.

What does low subcooling (under 10°F) indicate?

Low subcooling typically means undercharge (not enough refrigerant), a refrigerant restriction upstream of the measurement point, or excessive heat load on the condenser. Check for restrictions, verify airflow across the condenser, and add refrigerant if other causes are ruled out.

What does high subcooling (over 20°F) indicate?

High subcooling usually means overcharge (too much refrigerant) or a liquid-line restriction (e.g. a partially closed service valve or blocked filter-drier). Recover refrigerant if overcharged; check for restrictions if charge weight is correct.

Where do I measure the liquid line temperature?

Clamp a contact thermometer or thermocouple directly on the liquid line as close to the condenser outlet as possible, before any significant line run. Insulate the sensor with putty or a wrap for an accurate reading.

Does this tool work for heat pump systems?

Yes. In cooling mode, the outdoor unit is the condenser — use outdoor high-side pressure and the liquid line leaving the outdoor unit. In heating mode, the indoor coil is the condenser — use the indoor high-side pressure if you can measure it, or consult the manufacturer's heating-mode subcooling spec.

Subcooling Calculator

Enter the refrigerant type, condenser pressure, and liquid line temperature to calculate subcooling degrees and check system charge status.

System Inputs

Refrigerant Type

Pressure Unit

High-Side Pressure (PSIG)

Temperature Unit

Liquid Line Temperature (°F)

Subcooling Result

Enter values and press Calculate.

Typical Subcooling Targets

Range	Indication
< 5°F	Likely undercharged
5 – 10°F	Low — check charge
10 – 20°F	Normal range
21 – 30°F	High — possible overcharge
> 30°F	Likely overcharged / restriction

Always verify with the manufacturer's installation manual for your specific equipment model.

Summary

Subcooling is the number of degrees a refrigerant liquid is cooled below its saturation (condensing) temperature at a given pressure. Proper subcooling — typically 10–20°F for most residential systems — confirms adequate refrigerant charge and efficient heat transfer in the condenser. Too little subcooling indicates undercharge or a restriction; too much may signal overcharge. This tool looks up the saturation temperature for the selected refrigerant at the measured high-side pressure and subtracts the actual liquid line temperature to give you subcooling degrees.

How it works

Select your refrigerant type (R-410A, R-22, R-134a, R-32, or R-404A).
Enter the high-side (condenser outlet / liquid line) pressure in PSIG as read from your manifold gauges.
Enter the actual liquid line temperature in °F measured with a contact thermometer just downstream of the condenser.
The tool looks up the saturation temperature for that refrigerant at the entered pressure.
Subcooling = Saturation Temperature − Liquid Line Temperature.
A color-coded result shows whether subcooling is low, normal (10–20°F), or high, with a diagnostic note.

Use cases

Verify refrigerant charge on a residential split-system AC or heat pump during commissioning.
Diagnose undercharge or overcharge conditions on a service call.
Cross-check manufacturer target subcooling against field measurements.
Train HVAC apprentices on proper charging procedures using real pressure/temperature values.
Quick field calculation without carrying a PT chart booklet.
Validate a new refrigerant charge after recovering and recharging a system.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu