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Refrigerant Charge Calculator (Superheat / Subcooling)

Diagnose R-410A refrigerant charge balance using superheat for fixed-orifice systems or subcooling for TXV metering. Live PT saturation lookups, dynamic target superheat, and phase cycle stability badges guide field charging decisions.

Refrigerant Charge Inputs

Target Evaluation Mode

Diagnostic Method

Superheat Structural Inputs

Live Results

Active Diagnostic Value

Calculated Superheat: 11.1°F

Metric Operational Profile Targets

System Design Target: 12.5°F

System Condition

SYSTEM CHARGE PROFILE: OVERCHARGED

Marginal Off-Target Shift — System charge exhibits a 10-20% slight operational variance. Performance efficiency will track low. Verify air filter clean state and coil cleanliness before adding mass.

R-410A Saturation Temp

38.9°F

PT Lookup Pressure

120 PSI

How to Use This Refrigerant Charge Calculator

  1. Select superheat or subcooling diagnostic mode. Choose Superheat Mode (Fixed Orifice) for piston or capillary-tube metering devices, or Subcooling Mode (TXV Valve) for thermostatic expansion valve systems. The input fields transition smoothly based on your selection.
  2. Enter gauge pressures and line temperatures. For superheat, input low-side suction pressure (100–200 PSI), suction line thermistor temperature, indoor wet bulb, and outdoor dry bulb. For subcooling, input high-side liquid pressure (250–500 PSI), liquid line temperature, and manufacturer target subcooling.
  3. Review calculated superheat or subcooling. The Live Results panel displays the calculated diagnostic value at 1-decimal precision using the R-410A PT lookup table. Saturation temperature is resolved from the closest indexed pressure key.
  4. Compare against system design target. Superheat target is dynamically calculated from outdoor dry bulb and indoor wet bulb conditions. Subcooling target uses the manufacturer frame value (default 12°F). Deviation and charge status are derived from measured vs. target comparison.
  5. Evaluate charge status and severity badge. Review the SYSTEM CHARGE PROFILE (UNDERCHARGED or OVERCHARGED) and the RED/YELLOW/GREEN phase cycle stability badge indicating whether deviation exceeds 10% or 20% of the design target.

Formulas & Example

Superheat and subcooling diagnostics compare measured line temperatures against R-410A saturation temperatures from the PT lookup index. Charge status and severity badges derive from deviation against design targets.

PT Lookup: satTemp = pt410a[closestPressureKey] || 40.0

Superheat Mode:
  Calculated Superheat = Suction Line Temp − satTemp (at suction pressure)
  Target Superheat = 5 + (Outdoor Dry Bulb − 70) × 0.1 + (Indoor Wet Bulb − 50) × 0.5
  Deviation = Calculated − Target
  Charge Status: Calculated > Target → UNDERCHARGED, else OVERCHARGED

Subcooling Mode:
  Calculated Subcooling = satTemp (at liquid pressure) − Liquid Line Temp
  Target Subcooling = Manufacturer Input (default 12.0°F)
  Deviation = Calculated − Target
  Charge Status: Calculated < Target → UNDERCHARGED, else OVERCHARGED

Severity:
  percentError = |deviation| ÷ target
  RED (>20%) | YELLOW (10–20%) | GREEN (≤10%)

Worked Example — Superheat Mode

Suction pressure 120 PSI, suction line 50°F, outdoor 85°F dry bulb, indoor 62°F wet bulb:

PT Lookup: 120 PSI → satTemp = 38.9°F
Calculated Superheat = 50.0 − 38.9 = 11.1°F
Target Superheat = 5 + (85 − 70) × 0.1 + (62 − 50) × 0.5 = 12.5°F
Deviation = 11.1 − 12.5 = −1.4°F
Charge Status: 11.1 ≤ 12.5 → OVERCHARGED
percentError = 1.4 ÷ 12.5 = 11.2% → YELLOW badge

Worked Example — Subcooling Mode

Liquid pressure 325 PSI, liquid line 90°F, target subcooling 12°F:

PT Lookup: 325 PSI → satTemp = 102.1°F
Calculated Subcooling = 102.1 − 90.0 = 12.1°F
Target Subcooling = 12.0°F
Deviation = 12.1 − 12.0 = +0.1°F
Charge Status: 12.1 ≥ 12.0 → OVERCHARGED (within tolerance)
percentError = 0.1 ÷ 12.0 = 0.8% → GREEN badge

Frequently Asked Questions

What is superheat and when should I use it?▾
Superheat is the temperature rise of refrigerant vapor above its saturation temperature at the suction pressure. It is measured on the suction line near the evaporator outlet. Use the superheat method for fixed-orifice metering devices (piston, capillary tube) where subcooling targets are not manufacturer-specified. Formula: Superheat = Suction Line Temp − Saturation Temp at Suction Pressure.
What is subcooling and when should I use it?▾
Subcooling is the temperature drop of liquid refrigerant below its saturation temperature at the liquid-line pressure. It is measured on the liquid line near the condenser outlet. Use subcooling for TXV (thermostatic expansion valve) systems where the manufacturer publishes a target subcooling value. Formula: Subcooling = Saturation Temp at Liquid Pressure − Liquid Line Temp.
How does the R-410A PT lookup table work?▾
The calculator uses an indexed R-410A pressure-temperature table spanning 100–200 PSI (low-side) and 250–500 PSI (high-side). Your entered gauge pressure is matched to the closest available integer key, and the corresponding saturation temperature is used for superheat or subcooling calculation. If no key matches, a safe fallback of 40.0°F prevents calculation errors.
How is the dynamic superheat target calculated?▾
Target superheat adjusts for ambient and indoor conditions: Target = 5 + (Outdoor Dry Bulb − 70) × 0.1 + (Indoor Wet Bulb − 50) × 0.5. Example: 85°F outdoor and 62°F indoor wet bulb yields Target = 5 + 1.5 + 6.0 = 12.5°F. Higher indoor humidity and warmer outdoor air increase the acceptable superheat envelope.
What do the RED, YELLOW, and GREEN severity badges mean?▾
Severity is based on percent deviation from target: Green (≤10%) means charge is within standard trade safety boundaries. Yellow (10–20%) indicates marginal off-target shift — verify air filter and coil cleanliness before adjusting charge. Red (>20%) signals critical charge deviation requiring refrigerant mass correction. Undercharged systems show high superheat or low subcooling; overcharged systems show the inverse.

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