SizingKitSuction side
Superheat calculator
Superheat is the suction line temperature minus the temperature its own pressure says the refrigerant boils at, and this page returns that difference in °F and °C for R-410A, R-22 and R-134a, free and with no signup. It also prints the evaporating temperature it worked from, corrected for your site elevation, which is a column no paper pressure-temperature card carries. Outside the 0-140 °F anchored range it tells you the table stops rather than continuing the curve into a guess.
- 100% free
- No signup
- 3 refrigerants
- 0-140 °F anchors
- Elevation-corrected
What the blue gauge and the clamp read
Both instruments on the same suction service valve, with the system run long enough to settle. The metering device decides what the answer means, so it is asked first.
Near-azeotropic blend, 50% R-32 / 50% R-125 by mass
Nothing but the charge controls superheat here, which is what makes it the charging measurement.
Gauge pressure, not absolute — the number on the dial, before anything is added for the atmosphere.
Clamped to bare copper, insulated over the clamp, six inches back from the valve.
Deliberately empty: no figure that is not the manufacturer's belongs here.
Default 0 ft — Sea level, where the barometer is 14.696 psia — the condition every printed pressure-temperature card assumes. Your own elevation. At 5,280 ft the barometer is 12.10 psia, and a 118 psig R-410A suction reading then saturates at 38.7 °F instead of the 39.9 °F a sea-level card gives.
Default 5 °F — The floor the trade works to for total superheat measured at the compressor suction, below which liquid refrigerant is reaching the compressor. Manufacturer-dependent, and some scroll compressors are specified higher. Liquid floodback destroys a compressor slowly rather than immediately, so a system running below this reads normal until it fails.
Superheat and evaporating temperature
SUPERHEAT °F
12.1 °F
SUPERHEAT °C
6.7 °C
- Evaporating temperature
- 39.9 °F / 4.4 °C
- Barometer used
- 14.7 psia / 101.3 kPa
- Table covers
- 47.6-540 psig
No target can be supplied here, and that is the honest answer. The authority for a charge is the charging chart printed on the equipment — on the outdoor unit's service panel, or in its installation instructions. It is written for that specific coil, metering device and line set, and it overrides every general figure on this page. On a fixed-orifice system there is no single target superheat at all: the target is read off a chart against the indoor wet-bulb and outdoor dry-bulb temperatures at the moment of charging, and it moves by more than twenty degrees across the range of conditions that chart covers.
Where the saturation figure comes from: Bubble-point saturation pressures for R-410A read from the published saturation table at 20 °F intervals, psig at sea level; psia adds one standard atmosphere. Checked for internal consistency against the Clausius–Clapeyron form, which reproduces each anchor from its neighbors to within about 1 psi. Between those anchors it is interpolated in ln P against 1/T, the shape the Clausius-Clapeyron relation gives a vapor pressure curve. Checked against printed chart values the anchors do not contain, R-22 at 45, 50, 70 and 110 °F returns 75.99, 83.99, 121.38 and 226.39 psig where the card prints 76.0, 84.0, 121.4 and 226.4; the worst disagreement found anywhere was 0.65 psi, on R-410A at 45 °F. Down here a psi is worth roughly 0.6 °F of saturation temperature between 60 and 70 psig, so that is two tenths of a degree — under what a manifold can be read to, and the reason the last digit will not always match your card. R-410A pressures are bubble point, and its 0.2 °F glide is inside reading error, which is why that blend is charged like a single compound and R-407C is not.
Low-side card: what a suction gauge means at 0 ft
Saturation temperature against gauge pressure across the band a suction gauge sits in, recomputed for the elevation above — which is the one column a printed card cannot carry. Every cell is °F then °C.
| psig | bar (gauge) | R-410A | R-22 | R-134a |
|---|---|---|---|---|
| 40 | 2.76 | — | 17.2 °F / -8.2 °C | 45.0 °F / 7.2 °C |
| 50 | 3.45 | 1.8 °F / -16.8 °C | 26.1 °F / -3.3 °C | 54.0 °F / 12.2 °C |
| 60 | 4.14 | 8.8 °F / -12.9 °C | 33.9 °F / 1.1 °C | 62.0 °F / 16.6 °C |
| 70 | 4.83 | 15.1 °F / -9.4 °C | 41.0 °F / 5.0 °C | 69.3 °F / 20.7 °C |
| 80 | 5.52 | 20.9 °F / -6.2 °C | 47.5 °F / 8.6 °C | 76.0 °F / 24.4 °C |
| 90 | 6.21 | 26.4 °F / -3.1 °C | 53.6 °F / 12.0 °C | 82.1 °F / 27.9 °C |
| 100 | 6.89 | 31.5 °F / -0.3 °C | 59.1 °F / 15.1 °C | 87.8 °F / 31.0 °C |
| 110 | 7.58 | 36.3 °F / 2.4 °C | 64.4 °F / 18.0 °C | 93.0 °F / 33.9 °C |
| 120 | 8.27 | 40.7 °F / 4.9 °C | 69.3 °F / 20.7 °C | 98.0 °F / 36.7 °C |
| 130 | 8.96 | 45.0 °F / 7.2 °C | 74.0 °F / 23.3 °C | 102.7 °F / 39.3 °C |
| 140 | 9.65 | 49.1 °F / 9.5 °C | 78.5 °F / 25.8 °C | 107.2 °F / 41.8 °C |
| 150 | 10.34 | 52.9 °F / 11.6 °C | 82.7 °F / 28.2 °C | 111.5 °F / 44.2 °C |
| 160 | 11.03 | 56.6 °F / 13.7 °C | 86.7 °F / 30.4 °C | 115.6 °F / 46.5 °C |
An em dash is a pressure below the bottom of that refrigerant’s anchor table at this elevation: R-410A starts at 47.6 psig, R-22 starts at 24 psig, R-134a starts at 6.5 psig.
How to take a superheat reading that means something
Three things decide whether the number is worth acting on, and only the last of them is arithmetic.
Let the system settle before you read anything
Hook up, open the manifold to the suction service valve and wait until the pressure stops drifting. A machine that has just started is still moving refrigerant around between the condenser and the accumulator, and superheat taken during that is a snapshot of the transient rather than of the charge. Check the indoor airflow first as well: a dirty filter or a blocked return starves the coil and raises superheat exactly the way an undercharge does, so a reading taken over a restricted coil sends you to the wrong repair.
Read the pressure and the line temperature at the same place
The gauge goes on the suction service valve; the clamp goes on bare copper about six inches back from it, on the underside of the tube, with insulation put back over the clamp so the outdoor air is not being measured alongside the line. Both numbers have to describe the same point in the system — a pressure read at the valve against a temperature read at the evaporator outlet includes the pressure drop of the whole suction line and reports a superheat that is not there.
Set the metering device, then compare against the right target
Choose the piston or the valve at the top of the tool, because it decides what the answer is. On a fixed orifice, type the target the manufacturer's superheat charging chart gives for today's indoor wet bulb and outdoor dry bulb, and the page reports how far off you are. On a TXV it falls back to the 8-12 °F band the valve is set to hold, and a reading outside that band is a question about the valve, its sensing bulb or a restriction rather than about how much refrigerant is in the machine.
Technical specifications
| Refrigerants | R-410A, R-22 and R-134a, each anchored at 20 °F intervals from 0 to 140 °F. R-410A pressures are bubble point, which is legitimate because its glide is 0.2 °F. |
|---|---|
| Suction pressure accepted | 47.6 to 540.0 psig for R-410A, 24.0 to 337.2 for R-22 and 6.5 to 229.0 for R-134a at sea level. Past either end the page reports that the table stops. |
| Interpolation error | 0.65 psi at worst, on R-410A at 45 °F. Checked against printed values the anchors do not contain, R-22 at 45, 50, 70 and 110 °F returns 75.99, 83.99, 121.38 and 226.39 psig where the card prints 76.0, 84.0, 121.4 and 226.4. |
| Low-side sensitivity | One psi moves the R-410A saturation temperature about 0.6 °F between 60 and 70 psig, so a manifold you can read to two psi is worth a degree of superheat before the thermometer is considered. |
| Elevation correction | 0 to 15,000 ft. At 5,280 ft the barometer is 12.10 psia and a 118 psig R-410A reading saturates at 38.7 °F rather than 39.9 °F; the shift grows as the pressure falls, so it is worst on this side of the system. |
| Target superheat | No default is offered for a fixed orifice, because no figure that is not the manufacturer's exists. A TXV falls back to the 8-12 °F band the valve holds, labeled as a valve check rather than a charge check. |
| Compressor superheat floor | 5 °F prefilled and editable. It is written for the compressor inlet, so a reading at the outdoor service valve is that figure plus whatever the suction line gains on the way in. |
| Where it runs | In this browser tab. The pressures, temperatures and elevation you type are never uploaded, logged or kept. |
Frequently asked questions
Why will this page not give me a target superheat for a piston system?
Because no such number exists outside the manufacturer's own chart, and printing one would be inventing it. On a fixed orifice the target is read against the indoor wet bulb and the outdoor dry bulb at the moment of charging, and across the range those charts cover it moves by more than twenty degrees — high in cool humid weather, single digits in hot dry weather. A calculator that answers with one figure is right on one day in ten. Read yours off the chart on the service panel or in the installation instructions and type it into the target field, and the page will do the comparison.
Where exactly do the gauge and the clamp go?
Both at the suction service valve on the outdoor unit, and the clamp on bare copper with the insulation replaced over it. Paint, dirt and an unclamped probe all read low by a degree or two because they are partly measuring the outdoor air. If the line is buried in foam, cut a small window rather than reading through it. A magnetic probe on a fitting rather than on the tube is another degree, which on a target of ten is ten percent of the whole measurement.
My R-410A suction pressure is 40 psig and I get no answer. Why?
40 psig is below the bottom of the R-410A table, which starts at 47.6 psig at sea level, so the page refuses rather than extending the curve past its last anchor. That refusal is also information: 47.6 psig is the pressure of a 0 °F evaporator, so 40 psig is a coil running below freezing, which on an air conditioning system in cooling means it is icing or about to. The cause is almost always airflow or a serious undercharge, and neither is diagnosed by a superheat figure.
Does the elevation setting really change anything?
Yes, and more here than anywhere else on the refrigerant side. A gauge is a differential instrument: it reads against the atmosphere outside the manifold, and the atmosphere at 5,280 ft is 12.10 psia rather than 14.696. Reading a sea-level card in Denver hands back a saturation temperature about a degree too high on the low side, and the error grows as the pressure falls. The printed card below the tool is regenerated at whatever elevation you set, which is the one thing a card in a van cannot do.
Why is your saturation temperature a couple of tenths off my PT card?
Because this page interpolates between anchors 20 °F apart rather than storing a full chart, and it says so instead of hiding it. The interpolation runs in ln P against 1/T, the shape the Clausius-Clapeyron relation gives a vapor pressure curve, and it lands within a fraction of a psi of published values it does not contain. In saturation temperature that is a couple of tenths of a degree, which is well inside what a manifold and a clamp thermometer can resolve between them.
Superheat is right where the chart wants it but the house is not getting cool.
On a TXV system that is exactly what you would expect whatever the charge is, which is why superheat is not the charging measurement there. The valve modulates to hold evaporator superheat steady, so it stays in band while the machine is undercharged, correctly charged or somewhat overcharged. Move to the liquid side: the subcooling calculator reads the measurement that does track charge on a valve system. On a fixed orifice a correct superheat with poor capacity points instead at airflow, at a dirty condenser, or at a compressor that is no longer pumping.
Can I use this for R-407C, R-454B or R-32?
No, and substituting a nearby refrigerant would produce a plausible wrong answer rather than an obvious one. Only R-410A, R-22 and R-134a are anchored here. R-407C in particular cannot be charged from a single column at all: its glide is around 10 °F, so its bubble point and dew point are far apart and superheat has to be worked from the dew-point column while subcooling uses the bubble point. Take those from the manufacturer's own chart for the blend in the system.
About superheat, and why a fixed orifice is charged with it
Refrigerant enters the evaporator as a cold mixture of liquid and vapor and boils along the coil at a temperature its pressure sets. Once the last of the liquid has gone, the remaining length of tube stops boiling anything and simply warms the vapor up, and the degrees it gains from that point on are the superheat. That is why the measurement exists at all: a compressor pumps vapor and is wrecked by liquid, so a positive superheat at the suction line is the proof that the boiling finished inside the coil rather than somewhere in the line set. It is also, on a system with no valve to interfere, a direct report on how much refrigerant is in the machine — too little and the liquid runs out early, leaving a long dry stretch of coil and a high reading; too much and it runs out late, or not at all.
That last sentence is what makes superheat the charging measurement for a fixed orifice, a piston or a capillary tube, and it is also why the target is not a constant. The correct superheat for a piston system depends on how much moisture the coil is being asked to remove and how hard the condenser is having to work, which is to say on the indoor wet bulb and the outdoor dry bulb — the two numbers every manufacturer's charging chart is indexed by. If you are taking wet bulb with a sling psychrometer and want the rest of the air's state along with it, the psychrometric calculator works from dry bulb and relative humidity at a stated altitude. Get the airflow right before any of this: at the nominal 400 CFM per ton a coil behaves the way the chart assumes, and the AC tonnage calculator is where that nominal comes from.
Three things go wrong on other superheat pages often enough to be worth naming. The first is the unit conversion: superheat is a difference between two temperatures, not a temperature, so twelve degrees Fahrenheit of it is 6.7 °C — a page that runs it through the ordinary Fahrenheit-to-Celsius conversion reports −11 °C and tells a technician to add refrigerant to a system that was already right. The second is extrapolation: a saturation table has ends, and a calculator that keeps returning numbers past them is guessing in a way that looks identical to knowing. The third is offering a target superheat table at all. What this page does instead is take yours and do the comparison honestly, and then send you to the subcooling calculator if the system turns out to have a valve, because on a TXV the two measurements swap jobs entirely.
Where the gauge readings go
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The printable low-side card under the tool is generated in the page each time you change the elevation, not fetched, so it works on a roof with no signal and leaves no record of which site you were standing on.