Subcooling Calculator — Free Online Calculator
Subcooling tells you how much the liquid refrigerant has cooled below its saturation temperature in the condenser — the primary charge check on TXV systems. Enter the liquid line saturation temperature and the liquid line temperature, and this tool gives your measured subcooling, whether it's in the healthy band, and the most likely causes when it isn't.
Enter liquid-side temperatures
From liquid gauge converted on a PT chart
Pipe clamp thermometer on the liquid line
Measured subcooling
Reading
Low-normal
Ideal target
8–12°F
Likely causes
Below TXV target — undercharge, dirty condenser, or high ambient reducing condenser capacity. Clean the coil and verify charge.
See the breakdown
Subcooling is the primary TXV charging method. Confirm superheat afterward and follow the manufacturer spec for your refrigerant type.
What your reading means (TXV systems)
Measured subcooling at the liquid line. Piston systems typically target 10–15°F.
| Subcooling | Reading | Most likely cause |
|---|---|---|
| Under 4°F | Very low | Undercharge or poor condenser heat rejection |
| 4–8°F | Low | Undercharge, dirty condenser, or high ambient |
| 8–12°F | Healthy | Proper TXV charge — fully condensed liquid to the valve |
| 12–18°F | High | Overcharge, restricted metering, or cool ambient |
| Over 18°F | Very high | Severe overcharge — recover excess refrigerant |
Sources & standards: HVAC School — Subcooling charging fundamentals · EPA Section 608 — refrigerant handling · Manufacturer TXV charging procedures.
The formula, explained in plain English
Subcooling is how many degrees the liquid refrigerant has cooled past its boiling point at condenser pressure. It confirms the condenser is delivering fully condensed liquid to the metering device.
What subcooling tells you
Whether the condenser has fully condensed the refrigerant and stacked enough liquid before it reaches the metering device — the liquid-side charge check.
Where to measure
Liquid line 6–12 inches before the metering device with a pipe clamp thermometer. Saturation temp comes from the liquid (high-side) gauge on a PT chart.
TXV vs piston
TXV systems are charged by subcooling because the valve adjusts superheat. Piston systems use superheat as primary and subcooling as a cross-check — target 10–15°F.
Ambient affects the reading
Cool outdoor temps add subcooling even with correct charge. Always charge at stable, moderate ambient and compare to the manufacturer chart for your refrigerant.
Worked examples
Three readings showing slightly low TXV subcooling, healthy charge, and overcharge from excess refrigerant.
Slightly low TXV — 95°F sat, 88°F liquid line
Result: just below the 8–12°F TXV band — check for undercharge or a dirty condenser before adding refrigerant.
Healthy TXV — 102°F sat, 92°F liquid line
Result: 10°F sits in the 8–12°F TXV band — the condenser is delivering fully condensed liquid to the expansion valve.
Overcharge — 108°F sat, 88°F liquid line
Result: excess refrigerant stacked in the condenser — recover to manufacturer subcooling target before returning to service.
Frequently asked questions
Common questions about measuring and interpreting HVAC subcooling.
What should subcooling be on my AC?
For a TXV system, target 8–12°F of subcooling at the liquid line. For a piston (fixed-orifice) system, the typical band is 10–15°F. Always confirm against the manufacturer's charging chart for your refrigerant and conditions.
How do I calculate subcooling?
Read the liquid line pressure and convert it to saturation temperature on a PT chart, clamp a thermometer on the liquid line, then subtract: subcooling = liquid line saturation temp − liquid line temp.
What does low subcooling mean?
Low subcooling means the liquid line is too close to saturation — usually undercharge, a dirty condenser, or poor heat rejection. The condenser isn't stacking enough liquid before it reaches the metering device.
What does high subcooling mean?
High subcooling means excess refrigerant is cooled below saturation in the liquid line — typically overcharge, a restricted metering device, or unusually cool ambient conditions adding extra subcooling.
Do I use subcooling or superheat to charge a TXV system?
Charge a TXV system by subcooling as the primary method — the expansion valve controls superheat automatically. Check superheat afterward as a cross-check that the evaporator is fed properly.
Where on the liquid line should I measure?
Clamp the thermometer on the liquid line 6–12 inches before the metering device, away from the condenser outlet and service valve so you read stable liquid temperature, not mixed gas.
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