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Superheat & Subcooling Calculator

Verify refrigerant charge from your gauge and probe readings. Get the target, the actual, and a clear add / recover verdict — with the saturation math shown.

Glide: ~0.2 °F (negligible)

Metering device
psig
°F

Probe on the line ~6″ from the service port.

°F
°F
Advanced assumptions
ft

Corrects gauge → absolute pressure. 0 at sea level.

Superheat charging
Charge is on target
15°F actual
Actual SH
15°F
Target SH
13.5°F
Difference
+1.5°F
Sat temp
40°F
ƒShow the math
  1. 1.Saturation temperature (dew point)per refrigerant PT data

    Look up 118 psig on the R-410A dew line.

    = 40 °F
  2. 2.Actual superheat
    suction line temp − saturation temp
    55 − 40
    = 15 °F
  3. 3.Target superheatper standard field charging formula
    (3 × indoor WB − 80 − outdoor DB) / 2
    (3 × 64 − 80 − 85) / 2
    = 13.5 °F
  4. 4.Difference vs target
    actual − target
    15 − 13.5
    = +1.5 °F → Charge is on target
Before you rely on this
  • Verify airflow BEFORE charging — target ~350–400 CFM/ton. Low airflow mimics an undercharge and will send you chasing refrigerant you don't need.
  • Allow 10–15 minutes for the system to stabilize between adjustments before re-reading.

Pressure–temperature (PT) charts

Printable, interactive PT charts with a built-in pressure ↔ temperature converter for each refrigerant.

R-410A PT chartR-454B PT chartR-32 PT chartR-22 PT chartR-134a PT chartR-407C PT chartR-404A PT chart

How the charge check works

Every charge check compares a measured line temperature to the refrigerant's saturation temperature at the pressure you read. The gap is superheat (on the suction / vapor side) or subcooling (on the liquid side).

  • Superheat = suction line temp − saturation temp (read on the dew line for blends). High superheat means the evaporator is starved — undercharged.
  • Subcooling = saturation temp (read on the bubble line) − liquid line temp. Low subcooling means not enough refrigerant is backing up in the condenser — undercharged.
  • Target superheat comes from the indoor and outdoor conditions, so it moves with the weather. Subcooling target comes from the equipment nameplate.

Saturation temperatures come from published refrigerant PT data. Blend PT data here is interpolated from manufacturer anchor points; regenerate with CoolProp for the full authoritative curve before you rely on it for final charging.

Frequently asked

What is the target superheat formula?
The standard field formula is target superheat = (3 × indoor wet-bulb − 80 − outdoor dry-bulb) ÷ 2. With a 64 °F return wet-bulb and an 85 °F outdoor dry-bulb, that is (192 − 80 − 85) ÷ 2 = 13.5 °F. If the result is below 5 °F, conditions are unsuitable for superheat charging and you should charge by weight.
Do I use superheat or subcooling to charge?
Use superheat on fixed-orifice (piston / cap-tube) systems and subcooling on systems with a TXV or EXV. A TXV holds superheat roughly constant, so subcooling is the reliable indicator of charge on those systems.
What pressure should R-410A run at?
It depends on temperature, not a fixed number. On the R-410A dew line, about 118 psig corresponds to a 40 °F saturation temperature; 40 °F is a common low-side target on a mild day. Read your saturation temperature from the pressure, then compare line temperature to it. See the R-410A PT chart for the full curve.
Why check airflow before charging?
Low airflow across the evaporator lowers suction pressure and raises superheat, mimicking an undercharge. If you add refrigerant to "fix" high superheat that is really an airflow problem, you overcharge the system. Confirm roughly 350–400 CFM per ton first.
Can I vent or purge the excess refrigerant?
No. Venting refrigerant is illegal under EPA Section 608. Recover excess charge into a recovery cylinder — never release it to atmosphere.

Related tools

HVAC
CFM per Ton Calculator

Confirm airflow (350–400 CFM/ton) before you touch the charge.

HVAC
Target ΔT Calculator

Cross-check with evaporator temperature split.

HVAC
Duct Size Calculator

Fix the airflow that mimics a charge problem.

Related guides

Sources & references

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a licensed HVAC contractor. See our methodology, sources, and code editions.