How to Calculate Superheat and Subcooling (R-410A, R-32, R-454B Step by Step)

Superheat is the suction line temperature minus the saturation temperature at the suction pressure. Subcooling is the saturation temperature at the liquid line pressure minus the liquid line temperature. Both are single subtractions, and the subtraction itself is rarely where techs go wrong. The mistake happens one step earlier, at the saturation temperature you plug in. On a blended refrigerant like R-454B, there are two different saturation temperatures at any one gauge pressure, and picking the wrong one is baked into how most charts are read.

What Superheat and Subcooling Actually Measure

Superheat tells you how much heat has been added to a refrigerant vapor above the point where it finished boiling. Subcooling tells you how much heat has been removed from a refrigerant liquid below the point where it finished condensing. Neither number describes a temperature by itself. Both describe a gap between an actual measured temperature and a saturation temperature that depends on pressure.

That's why a superheat or subcooling reading always needs two instruments at once: a clamp-on thermometer on the line, and a manifold gauge reading pressure, which you then convert to a saturation temperature using a pressure-temperature (P-T) chart for the specific refrigerant in the system. Skip the chart and use the wrong refrigerant's numbers, and every reading downstream is wrong by a fixed, silent offset.

The Superheat Formula

Superheat = suction line temperature − suction saturation temperature.

Take the temperature reading off the suction line near the compressor with a clamp-on thermometer, and take the suction saturation temperature from the low-side gauge pressure on the P-T chart. The suction line is entirely vapor by the time it reaches this measurement point, so the saturation temperature you look up is the dew point: the temperature at which that vapor would just start condensing back to liquid at the pressure on your gauge.

The Subcooling Formula

Subcooling = liquid saturation temperature − liquid line temperature.

This one runs the other direction on purpose. You're checking how much colder the liquid is than the point where it would start to boil, so the saturation lookup uses the high-side gauge pressure and the bubble point: the temperature at which the last trace of liquid would still be liquid at that pressure. Measure the liquid line temperature near the condenser outlet or the metering device inlet with the clamp-on thermometer, then subtract.

Bubble Point vs. Dew Point: The Step Most Charts Skip

On a single-component refrigerant, bubble point and dew point are the same number, so this distinction is invisible and the shortcut of using "the" saturation temperature never causes a problem. R-22 and R-32 are both single compounds, so there's no glide: one saturation temperature per pressure, and that's it.

Blended refrigerants don't work that way. A zeotropic blend boils across a temperature range instead of at a single point, because its components don't change phase together. The bubble point is where the last bit of liquid is still liquid; the dew point is where the last bit of vapor has just finished condensing. The gap between those two temperatures, at the same pressure, is called glide.

This is where a one-column P-T chart quietly produces a wrong reading. R-410A is a near-azeotropic blend, and its bubble and dew columns differ by only 0.1 to 1.0 psig across the working range, about 0.2°F, small enough that most techs never notice a chart that only prints one column. R-454B is a genuine zeotrope of R-32 and R-1234yf, and it doesn't get that pass. Chemours states its glide at roughly 1.1 K (about 2.0°R), and the published pressure-temperature data works out to close to 2.3°F of separation at both 40°F and 100°F saturation. Since 2025, R-454B is the refrigerant shipping in most new residential equipment sold in the US, which means the chart-reading habit that was harmless on R-410A now costs a working technician about 2°F on every superheat and subcooling reading, before any measurement error of their own. Superheat comes out roughly 2°F high, subcooling roughly 2°F low, if the dew-point number gets used for both.

Worked Example: A Complete Superheat and Subcooling Check

Say a technician is checking a TXV-equipped system. The suction line temperature reads 52°F. Cross-referencing the low-side gauge pressure against the refrigerant's dew-point column gives a suction saturation temperature of 44°F.

Superheat = 52°F − 44°F = 8°F.

On the liquid side, the line temperature reads 96°F. Cross-referencing the high-side gauge pressure against the bubble-point column gives a liquid saturation temperature of 105°F.

Subcooling = 105°F − 96°F = 9°F.

Both numbers land in a normal working range for a TXV system, and this is the part that matters: they were read off two different columns on purpose. Using the dew-point number for the subcooling side of that same calculation would have shifted the second answer by whatever the glide happens to be for that refrigerant, without the tech ever seeing an error.

Target Superheat: The Formula for Fixed-Orifice Systems

A fixed orifice (a piston or a capillary tube) has no valve regulating superheat, so the correct value moves with the weather instead of holding at one number. The widely published approximation for the target is:

Target Superheat = [(3 × indoor wet bulb) − 80 − outdoor dry bulb] / 2

Run it at 64°F indoor wet bulb and 96°F outdoor dry bulb: (3 × 64) − 80 − 96 = 16, divided by 2 gives a target of 8°F. Run the same formula on a hotter, more humid day (71°F wet bulb indoors, 93°F dry bulb outside) and the target becomes 20°F, on the identical system with no refrigerant added or removed. The formula isn't measuring a fixed property of the equipment. It's tracking how hard a fixed orifice has to work under the day's conditions.

Treat this as an approximation to the manufacturer's own charging chart, because that's what it is: a widely published rule of thumb, not a substitute for the equipment's nameplate data, and one that's only reliable roughly within a 5°F to 30°F superheat band. Outside that range, it's run past where the approximation holds up, and the manufacturer's chart governs.

TXV vs. Fixed Orifice: Which Number Actually Moves

This is the distinction that determines which of the two readings to trust for charge adjustment. A thermostatic expansion valve actively regulates the flow of refrigerant into the evaporator to hold superheat close to a fixed setpoint, so on a properly functioning TXV system, superheat stays roughly steady regardless of charge level, and charge errors show up in subcooling instead. On a fixed orifice, nothing is regulating anything, so superheat is the number that tracks charge, while subcooling drifts as a secondary indicator. Checking superheat against a target-superheat formula on a TXV system, or checking subcooling as your primary number on a fixed-orifice system, means measuring the wrong variable for that metering device.

Diagnosing With Both Numbers Together

Neither superheat nor subcooling means much read alone. High superheat says the evaporator isn't getting enough refrigerant, but that's equally consistent with a system that's short on charge and a system with a restriction stopping refrigerant from arriving. The two look identical on the low side; reading the high side is what separates them:

• High superheat + low subcooling → undercharge. The system doesn't have enough refrigerant, full stop.
• Low superheat + high subcooling → overcharge. Too much refrigerant is backing up in the system.
• High superheat + high subcooling → a restriction between the condenser and the evaporator: a plugged filter drier, a kinked or crushed liquid line, or a metering device that's underfeeding. This pairing looks contradictory at first (the evaporator is starved and the condenser is backed up at the same time), but that's exactly the signature of refrigerant that's present in the system and not making it through. It's worth knowing this combination specifically, because the starved-evaporator half of it looks identical to an undercharge on its own, and adding refrigerant to a restricted system raises head pressure without fixing the actual problem.
• Low superheat + low subcooling → a metering device overfeeding, letting too much refrigerant flood through on both sides at once.

One confound sits underneath all four of these and has nothing to do with refrigerant charge: airflow. On a fixed-orifice system, a coil starved of air (a dirty filter, a failing blower, a coil packed with debris) pushes superheat down for the same reason low charge on a restricted system pushes it up: it's a symptom of the evaporator not getting what it needs, just from the other side of the equation. A low-airflow coil reads exactly like an overcharge. Confirm airflow before you touch the charge. Refrigerant added to fix an airflow problem doesn't fix the airflow problem; it just adds a second one.

Refrigerant Charge on a Longer Line Set

Superheat and subcooling aren't only for verifying an existing charge. They're also how you finish a charge after adding refrigerant for line-set length. Most residential outdoor units ship factory-charged for 15 feet of line set. Beyond that, the widely published rate for a 3/8" liquid line is 0.6 ounces per foot, a figure that shows up consistently in Goodman's TP-107 long-line-set guidance and in Trane's installer literature. The catch is which length you multiply by 0.6, because the two manufacturers don't agree.

Goodman's method subtracts the 15 feet the unit already left the factory charged for, and multiplies only the length beyond that. Trane's weigh-in worksheet multiplies the entire run, on the assumption that the system gets evacuated first and the whole charge goes in fresh. Run both methods on a 50-foot line set with a 3/8" liquid line:

Goodman method: (50 ft − 15 ft) × 0.6 oz/ft = 35 × 0.6 = 21 ounces.

Trane method: 50 ft × 0.6 oz/ft = 30 ounces.

That's a 9-ounce gap between two manufacturer-published methods on the identical line set, which is exactly why it matters which manual you're reading before you weigh in a charge. Either number is only a starting point regardless. Both manuals require a final adjustment against superheat on a fixed orifice, or against subcooling on a TXV, once the system is running.

How Trustworthy Is a Generic P-T Chart?

More trustworthy than techs sometimes assume, for the older refrigerants, and specifically less so for R-454B. Across three independently published sources (Chemours' Pressure-Temperature Guide for A/C, Arkema's Forane chart, and a Daikin-branded A2L chart) the values for R-22, R-32, and R-410A, including both of R-410A's columns, agree to the printed 0.1 psig at every row. R-454B doesn't get the same agreement: that Daikin chart, whose own footer credits its R-454B data to Weitron, runs 0.4 to 0.8 psig above Chemours on the dew line and up to 4.6 psig above it on the bubble line, and its columns imply about 2.6°F of glide against the roughly 2.0°R Chemours states. Chemours is the company that manufactures R-454B and publishes its data working from NIST REFPROP, which is the reference source used here rather than splitting the difference between competing charts. One more thing every chart shares regardless of refrigerant: the values are PSIG at sea level. At altitude, the real saturation temperature runs lower than a sea-level chart returns for the same gauge reading.

Skip the Lookup Table

Every step above is one lookup and one subtraction, but doing it by hand means keeping the right P-T chart on hand, remembering which column applies to which measurement, and re-deriving target superheat for the day's weather. The superheat and subcooling calculator runs R-410A, R-32, R-454B and R-22 with the bubble-point and dew-point columns kept separate, so a liquid-line reading can't accidentally land on the wrong saturation temperature. Enter a suction reading and a liquid reading and it names the combination (undercharge, overcharge, restriction, or an overfeeding metering device) and prints the full chart for the truck. If the job starts with figuring out what's actually installed on an older system before you get to any of this, an AC tonnage calculator decodes the size straight off the data plate's model number.

Whatever any chart or formula here says, the installed equipment's own charging chart and nameplate data take priority over a general approximation every time.