Superheat & Subcooling Calculator
Enter your saturation and line temperatures. Get superheat, subcooling, how each compares to target, and a plain-English charge diagnosis — undercharge, overcharge, or restriction.
Readings
Tip: read saturation temp straight off your gauge's P-T scale or a digital manifold for the refrigerant you're working with (R-410A, R-22, R-32, R-454B, R-407C). On blends with glide, use dew point for superheat and bubble point for subcooling.
Results
Charge looks correct.
Guidance only. Always verify against the equipment manufacturer's charging chart and target subcooling on the data plate, and confirm airflow before adjusting charge.
How superheat and subcooling are calculated
- Superheat = suction line temp − evaporator saturation temp. It tells you how much the vapor has warmed past boiling on the low side. Too little superheat means liquid can reach the compressor; too much means the evaporator is being starved.
- Subcooling = condenser saturation temp − liquid line temp. It tells you how much liquid is backed up in the condenser. It's the most reliable charge indicator on a TXV system.
Reading the two numbers together
| Superheat | Subcooling | Most likely cause |
|---|---|---|
| High | Low | Undercharge or leak |
| Low | High | Overcharge |
| High | High | Liquid-line or metering restriction |
| Low | Low | Metering device overfeeding / low load — floodback risk |
| Normal | Normal | Charge is in range |
On a fixed-orifice (piston) system, superheat isn't fixed — it swings with indoor load and outdoor temperature, so you charge to a target superheat from the manufacturer's chart rather than a single number. On a TXV/EEV system the valve holds superheat steady, so subcooling is your primary charging target.
Frequently asked questions
How do you calculate superheat?
Superheat = suction line temperature minus the evaporator saturation temperature. Read the low-side saturation temperature off your gauge's P-T scale (or a digital manifold), measure the actual suction line temperature near the compressor, and subtract. A typical target on a TXV system is 8 to 12°F.
How do you calculate subcooling?
Subcooling = condenser (high-side) saturation temperature minus the liquid line temperature. Read the high-side saturation temperature off your gauge, measure the liquid line temperature at the condenser outlet, and subtract. A typical target is 8 to 12°F, but always confirm the manufacturer's spec on the data plate.
What do high superheat and low subcooling mean?
High superheat with low subcooling usually means the system is undercharged or has a refrigerant leak. The evaporator is starved and there is not enough liquid backing up in the condenser. Confirm airflow and metering first, then weigh in charge or find the leak.
Log it where it counts
CrewConductor keeps superheat, subcooling, and refrigerant added/recovered on the equipment record and the EPA 608 refrigerant log — so the next tech sees the last reading, and your compliance paperwork builds itself. Start a free 14-day trial.
More free tools
Built for HVAC, not generic field service.
Equipment history, refrigerant logging, and EPA 608 compliance — included on every plan.
Start Your Free Trial14 days free. No credit card required.
Disclaimer: This free calculator is provided for general informational and educational purposes only, "as is" and without warranty of any kind, and you use it at your own risk. It is not engineering, compliance, or professional advice, and it does not replace the equipment manufacturer's charging chart, data-plate targets, or installation instructions. Verify every result before relying on it. See our Terms of Service.