Subcooling Calculator

Enter the liquid line pressure and temperature to get subcooling. For blends like R-454B and R-407C it uses the bubble point, the way refrigerant makers say to.

What is subcooling?

Subcooling is how many degrees the liquid refrigerant has cooled below its saturation temperature. In the condenser, hot vapor condenses at the saturation temperature for its pressure. Once it has all turned to liquid, it keeps giving off heat, and every degree it drops below that point is a degree of subcooling.

Measured at the liquid line, it shows how much liquid is backed up in the condenser, which is why it's the usual way to check the charge on TXV and EEV systems. Any subcooling at all also tells you the refrigerant is solid liquid where you measured it. A long line set, a tall vertical rise or a clogged filter drier can still flash some of it to gas before it gets to the metering device. High subcooling usually points to an overcharge or a restriction, and low subcooling to an undercharge.

How it's calculated

Absolute pressure = gauge pressure + 14.696 psi (101.325 kPa, one standard atmosphere)
Subcooling = bubble point at the liquid line pressure − liquid line temperature
Glide = dew point − bubble point at the same pressure (blends only)
Saturation data: CoolProp 8.0.0, interpolated from tables every 0.5 °C; R-448A from Arkema's Forane PT chart and R-449A from Chemours' Opteon XP40 tables, every 0.5 °F

Sources

If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.

Checked by Paul Williams · Updated October 31, 2026

Questions

How do you calculate subcooling?

Find the saturation temperature for the liquid line pressure on a PT chart, then subtract the liquid line temperature from it. For example, R-410A at 318.5 psig saturates at 100.0°F, so a liquid line at 90°F has 10.0°F of subcooling.

Why does subcooling use the bubble point?

Subcooling is how far the liquid has cooled below the point where the last bubble of vapor condensed, and that point is the bubble point. For a blend with a lot of glide the difference is big: R-407C at 225.5 psig has a bubble point of 100.0°F but a dew point of about 109°F. Chemours' PT guide says to use saturated liquid pressures for subcooling.

Where do I take the readings?

Read pressure at the liquid line service valve port and clamp the temperature probe on the liquid line near that valve, where it leaves the outdoor unit, insulated from the outside air. Let the system run first: manufacturers ask for 10 to 20 minutes.

What should my subcooling be?

Use the target on the unit's nameplate or in its installation manual. It differs between brands and models, and some manufacturers adjust it for long line sets. Carrier's 38CMC instructions allow 3°F either side of the target, and so does the installer check in California's Title 24 (an ECC-Rater allows ±6°F). Manufacturers also set a lowest outdoor temperature for charging by subcooling, such as 55°F for Trane's R-454B units and 65°F for Lennox's; below that, weigh in the charge.

What does negative subcooling mean?

A liquid line with flash gas in it sits at its saturation temperature, so on a single-component refrigerant it reads about 0, not below. On a blend it can read below zero by up to the glide (about 9°F on R-407C, 2.7°F on R-454B). Anything lower than that is a reading problem: check the refrigerant choice, the pressure port and that the probe is on the liquid line and insulated, before adding refrigerant.