Superheat & Subcooling Calculator

• •Use the Superheat & Subcooling Calculator to check your hand calculations on homework or exam problems — comparing its result with your own working helps identify arithmetic errors before they propagate.
• •Use it during the early design phase to rapidly iterate on parameters and narrow down feasible configurations before committing time to detailed finite element simulations or full design packages.
• •Use it when reviewing a colleague's calculation or checking a vendor's data sheet for plausibility — a quick sanity check can prevent costly downstream errors.
• •Use it to generate reference data for a technical report or presentation without manual computation, ensuring consistent, reproducible numbers throughout the document.
• •Use it in the field when a quick estimate is needed and a full engineering software package is not available.

The Superheat & Subcooling Calculator is a precision engineering calculation tool designed for students, engineers, and technical professionals. Superheat and subcooling from each line's pressure (gauge or absolute) and temperature, for R-410A (dew and bubble lines), R-134a and ammonia, using their published equations of state All calculations are performed using established engineering formulas from the relevant scientific literature and standards. Follow the units printed beside each field. Where a unit selector is provided, the selected unit is converted internally. Results are computed in your browser without sending data to a server, which keeps your inputs private. This calculator is intended as a supplementary tool for learning and design exploration; always verify results against authoritative references for safety-critical applications.

Superheat and subcooling say how far the refrigerant in a line is from saturation at that line's own pressure. Superheat is measured on the suction line: SH = T_line − T_dew(P), the amount by which the vapor is warmer than the temperature at which it would start to condense. Subcooling is measured on the liquid line: SC = T_bubble(P) − T_line, the amount by which the liquid is colder than the temperature at which it would start to boil. Both need the saturation temperature at the measured pressure, which a pressure–temperature chart or an equation of state supplies, and that pressure must be absolute. A gauge reads the pressure above the surrounding air, so P_abs = P_gauge + P_baro, with the standard atmosphere at 101.325 kPa (14.696 psi, 1.01325 bar) and lower values at altitude. Entering a gauge reading as absolute lowers the saturation temperature and inflates the superheat. For a pure refrigerant such as R-134a or ammonia, the dew and bubble temperatures at one pressure are the same temperature. R-410A is a blend of R-32 and R-125 whose dew temperature lies slightly above its bubble temperature at the same pressure; in Lemmon's pseudo-pure model the difference, called the glide, is 0.095 K at 500 kPa and 0.116 K at 3000 kPa absolute. Superheat is therefore taken from the dew temperature and subcooling from the bubble temperature, and using the other line shifts the answer by the glide. The calculator takes both temperatures from each refrigerant's equation of state: Tillner-Roth and Baehr (1994) for R-134a, Lemmon's (2003) pseudo-pure equation for R-410A, whose bubble and dew lines are a property-model convention for the blend rather than a mixture-equilibrium calculation, and Gao et al. (2023) for ammonia. It keeps the sign of the difference. A negative superheat means the suction line is colder than its dew temperature, and a negative subcooling means the liquid line is warmer than its bubble temperature; neither is a small superheat or subcooling. Each is a reading on the wrong side of saturation, which points to liquid or vapor where it should not be, or to a wrong reading, refrigerant or pressure basis. The tool gives no saturation temperature below the lowest pressure each model covers, at or above the critical pressure (for R-410A, the nominal critical pressure of its bubble and dew lines, 4901.20 kPa), within a few pascals below a pure fluid's critical pressure, or in a narrow band just below R-410A's nominal critical point where the model has no bubble point.

• •Charging and commissioning split air conditioners and heat pumps: equipment makers publish target superheat or subcooling values for their units, and the value measured from a suction or liquid-line reading is what gets compared with them. For R-410A the tool shows the dew and bubble temperatures side by side and uses the right one for each line.
• •Automotive and small R-134a systems: a low-side reading in psig and a line temperature in °F are entered as read; the barometric pressure is added before the saturation temperature is found, and the conversion is shown with the result.
• •Industrial ammonia refrigeration: suction superheat ahead of a compressor shows whether the vapor there is warmer than saturation, and liquid-line subcooling ahead of an expansion valve shows whether the liquid is colder than its bubble temperature. Readings in bar gauge and °C are handled the same way.
• •Checking a suspicious reading: a negative result is reported as a reading on the wrong side of saturation instead of being hidden behind an absolute value, so a wrong gauge or absolute setting, the wrong refrigerant or a misplaced probe shows up.
• •Teaching vapor-compression cycles: the tool links a refrigerant's pressure–temperature relation to the superheat at the evaporator outlet and the subcooling at the condenser outlet, and shows why a blend's dew and bubble temperatures differ.

• !Entering a gauge reading as absolute pressure, or an absolute reading as gauge. Saturation temperature depends on absolute pressure, so a gauge reading needs the barometric pressure added; the calculator shows the conversion it used.
• !Taking R-410A superheat from the bubble temperature or subcooling from the dew temperature. The two lines differ by about 0.1 K, so the error is small but always in the same direction; the calculator uses the dew temperature for superheat and the bubble temperature for subcooling.
• !Converting a temperature difference as if it were a temperature: a 5 K superheat is 9 °F, not 41 °F. A difference scales by 1.8 with no 32 °F offset.
• !Leaving the barometric pressure at the sea-level standard at altitude. Where the local barometric pressure is lower than 101.325 kPa, enter it, or the absolute pressure and the saturation temperature come out too high.
• !Measuring the line temperature far from the pressure tap or with poor probe contact. Superheat and subcooling compare one pressure with one temperature, so both should describe the same point in the line.
• !Reporting a negative result as a small positive one. A negative superheat or subcooling is a reading on the wrong side of saturation that needs an explanation, not an absolute value.