Free Calculator
Subcooling Calculator
Enter high-side pressure and liquid line temperature for R-410A, R-22, or R-134a to get condensing temperature, actual subcooling, and a quick refrigerant-charge diagnosis in the field.
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Check Subcooling and Refrigerant Charge
Pick your refrigerant, enter your high-side gauge and liquid-line readings, and set the manufacturer's target to see how your actual subcooling compares.
Used for the pressure-to-saturation-temperature lookup.
From the data plate or charging chart. Default 11°F.
Read from the high-side manifold gauge.
Measured with a clamp thermometer on the liquid line.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on interpolated pressure-temperature data and should be used as a field reference only. They are not a substitute for the equipment manufacturer's charging specifications, calibrated instruments, or verified airflow. Actual subcooling and the correct charge will vary based on your gauges, thermometers, system condition, and operating conditions. Always confirm against manufacturer data and a stabilized system before adding or recovering refrigerant.
How to Read and Use Subcooling
What subcooling actually measures
Subcooling is the number of degrees the refrigerant liquid sits below its condensing (saturation) temperature at the measured high-side pressure. You find it by converting the liquid-line pressure to a saturation temperature, then subtracting the actual liquid line temperature from it:
| Reading | What it usually means |
|---|---|
| Subcooling well below target | Undercharge — not enough liquid stacked in the condenser |
| Subcooling near target | Charge is in a healthy range for the conditions |
| Subcooling well above target | Overcharge, or a liquid-line restriction backing liquid up |
| Near-zero subcooling | Severe undercharge or non-condensables — stop and diagnose |
Subcooling is one data point. Pair it with superheat, delta-T, and airflow before concluding anything about the charge.
Why subcooling is the charging measurement on TXV systems
A thermostatic or electronic expansion valve (TXV/EEV) actively modulates refrigerant flow to hold a nearly constant superheat across a wide range of loads. Because superheat barely moves, it can't tell you much about the charge on these systems — but subcooling responds directly to how much refrigerant is in the system. Add charge and subcooling rises; remove charge and it falls. That's why you charge TXV/EEV equipment to the manufacturer's target subcooling and use superheat only as a cross-check. Fixed-orifice systems are the opposite: no valve to hold superheat, so you charge those to superheat instead.
The subcooling formula
Subcooling = condensing (saturation) temperature − liquid line temperature. Read the high-side pressure, convert it to a saturation temperature for your refrigerant, and subtract the measured liquid-line temperature. This calculator does the pressure-to-temperature lookup for R-410A, R-22, and R-134a and compares the result to the target you enter from the equipment's data plate.
Accurate readings depend on your instruments and airflow
A subcooling number is only as good as the inputs behind it. Use calibrated gauges and a good clamp thermometer on a clean section of liquid line near the condenser, let the system run and stabilize for 10–15 minutes, and confirm condenser airflow and a clean coil before trusting the diagnosis. A dirty condenser coil can raise head pressure and mimic an overcharge. This tool is a field aid, not a replacement for proper procedure. If you manage a team of techs, standardized job workflows in HVAC Contractor Software help make sure every diagnosis and charge is documented the same way.
Before you add or recover refrigerant
- Confirm the correct refrigerant and use the matching pressure-temperature data
- Use the manufacturer's target subcooling from the data plate — not a generic number
- Verify condenser airflow and a clean coil before blaming the charge
- Let the system stabilize before taking readings
- Always follow the manufacturer's charging chart and EPA refrigerant-handling requirements
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Book a Free DemoFrequently Asked Questions
What is subcooling and why does it matter?
Subcooling is the number of degrees a refrigerant liquid has been cooled below its saturation (condensing) temperature at the measured high-side pressure. It's found at the liquid line by subtracting the actual liquid line temperature from the saturation temperature. Subcooling is the primary charging measurement on TXV and EEV systems because it directly reflects how much liquid refrigerant is stacked in the condenser — low subcooling points to an undercharge, while high subcooling points to an overcharge or a liquid-line restriction.
How do I calculate subcooling?
Measure the high-side (liquid) pressure with a manifold gauge and convert it to a saturation temperature using a pressure-temperature chart for your refrigerant. Then measure the actual liquid line temperature near the condenser outlet with a clamp thermometer. Subtract the measured liquid line temperature from the saturation temperature. The difference is your actual subcooling in degrees Fahrenheit. This calculator does the pressure-to-temperature lookup for R-410A, R-22, and R-134a automatically.
What is a good subcooling reading?
Most residential TXV/EEV systems target a subcooling of about 10–12°F, but the correct number is always the value stamped on the unit's data plate or listed in the manufacturer's charging instructions. Some systems call for as little as 8°F or as much as 15°F. Charge to the manufacturer's target, then cross-check superheat. This tool lets you enter your own target so you can compare your actual reading against the spec for the specific equipment.
Should I charge to subcooling or superheat?
Charge TXV and EEV systems to subcooling — the valve holds superheat nearly constant, so subcooling is what actually reflects the charge. Charge fixed-orifice (piston) systems to superheat, because they have no valve to modulate flow and subcooling is less meaningful. On a TXV system, use superheat only as a sanity check after setting subcooling to the manufacturer's target.
Can I rely on this subcooling calculator to charge a system?
No — treat it as a field aid, not a substitute for proper procedure. The pressure-to-temperature values here are interpolated from a small anchor table and are approximate. Real charging depends on accurate, calibrated gauges and thermometers, correct airflow, a stabilized system, and the equipment manufacturer's target subcooling. Always verify against manufacturer charging data before adding or removing refrigerant.
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Get a Free DemoAll results are approximate field-reference figures based on interpolated pressure-temperature data and should be verified against the equipment manufacturer's charging specifications and calibrated instruments before adding or recovering refrigerant.