Subcooling Calculator

Enter your liquid-line pressure and temperature for any refrigerant; get subcooling plus diagnostic context. Bubble-curve math so high-glide blends (R-407C, R-454C, R-455A) don't read 9–18°F high at a 100°F bubble point.

Updated October 26, 2026

Refrigerant

Temp

Pressure

Liquid-line pressure (PSIG)

Liquid-line temperature (°F)

Subcooling Enter your liquid-line pressure and temperature, then click Calculate.

Chart range for R-410A: −40°F to 150°F.

Pipe-clamp thermometer on the liquid line at the outdoor service valve; manifold gauge on the liquid port. Subcooling = saturation temperature at the measured liquid pressure minus liquid-line temperature.

What subcooling is and why it matters

Subcooling is the temperature of liquid refrigerant below its saturation temperature at the same pressure. At any discharge pressure, the saturation temperature is the boundary above which any refrigerant is vapor; once the refrigerant has fully condensed and continues to lose heat to the condenser airstream, every degree below that saturation reading is one degree of subcooling.

On a working system, subcooling serves two purposes. First, it guarantees liquid (not flash gas) is reaching the metering device — a TXV or fixed-orifice device fed with two-phase refrigerant loses massive capacity because the orifice meters by volume, and vapor takes up most of the volume with little cooling effect. Second, subcooling indirectly measures refrigerant charge: more refrigerant in the system means more liquid backed up in the condenser, which means more subcooling.

Schematic of a residential split-system liquid line, showing the smaller uninsulated copper line, the probe location at the outdoor service valve, and the high-side manifold pressure port. Source: Carrier / Trane / Lennox residential service literature.

Subcooling is the TXV charging signal

For TXV / EEV residential AC and most commercial systems, subcooling is the primary charging metric — superheat is determined by the valve, but subcooling is determined by how much refrigerant you have. Match the SC target, and the system is charged.

When to use SC vs SH for charging — the metering-device rule

The reason TXV systems are charged by subcooling and fixed-orifice systems by superheat traces to what each device controls. A TXV regulates superheat to its setpoint by modulating refrigerant flow; charge changes do not directly alter superheat on a TXV system (the valve compensates). But charge changes do alter subcooling: more refrigerant means more liquid in the condenser, raising SC.

Fixed-orifice devices (pistons, capillary tubes, accurators) have no feedback control. Superheat varies directly with charge, ambient temperature, and indoor load. Subcooling on a fixed-orifice system is informational — it depends on too many factors to give a clean charge reading.

Metering device → charging metric

Device	Charge by
TXV	Subcooling
EEV	Subcooling
Fixed orifice (piston, captube)	Superheat

California Title 24 charges fixed metering devices by superheat (RA3.2.2.6.1), and TXV and EXV systems by subcooling (RA3.2.2.6.2).^[1]

Five common subcooling measurement mistakes

Wrong curve on zeotropes. Using dew pressure for saturation temperature on R-407C / R-454C / R-455A / R-449A overestimates subcooling by the glide value (8–18°F at a 100°F bubble point). This calculator uses the bubble curve automatically — verify any paper PT chart shows both columns and use the bubble column for SC.
Probing the wrong line. The liquid line is the smaller, uninsulated copper line at the outdoor unit service valve. The suction line is larger and foam-insulated. Probing the suction line gives you a superheat measurement, not subcooling.
High SC interpreted as "extra capacity". High SC actually reduces capacity — excess refrigerant in the condenser raises condensing pressure (more compressor work) and reduces effective condenser area for vapor condensation. Always investigate high SC, never celebrate it.
Ignoring line-set length on mini-splits. Long line sets (>50 ft) require higher SC at the outdoor unit to deliver adequate SC at the indoor TXV. Manufacturers publish line-length correction tables — use them.
Reading before steady state. Subcooling stabilizes 10-20 minutes after compressor start. Brief transient values after defrost or cycle changes aren't charge-decision data — wait for steady state.

When to use this calculator vs the others

Subcooling Calculator (this page) — liquid-line measurement. Primary charging signal for TXV / EEV systems. Diagnose condenser-side issues.
Superheat Calculator — suction-line measurement. Charge fixed-orifice systems; verify TXV operation; diagnose evaporator-side issues (undercharge, restriction, flooding). Always pair with SC.
Combined SH / SC / PT — both sides plus pattern-matching diagnostic banner. Use for full system commissioning or comprehensive diagnostic.
PT Calculator — raw saturation lookup, no measurement input. Reference tool for cross-checking or comparing refrigerants.
System Pressure Diagnostic — high-low pressure × high-low SH / SC pattern matcher. Use when you have all four values and want a quick fingerprint identification.
High head pressure causes — companion guide when SC and head pressure are both high. Decision tree for condenser-side troubleshooting.

Primary sources behind the calculator and content

CoolProp 7.2.0 (Bell, Wronski, Quoilin, Lemort 2014, doi:10.1021/ie4033999) — REFPROP-compatible Helmholtz EOS for all saturation temperatures.
ASHRAE Handbook—Refrigeration (2022) — “Equipment and System Dehydrating, Charging, and Testing” (service procedures; target subcooling by application for commercial refrigeration).
ASHRAE Handbook—HVAC Systems and Equipment (2024) — “Liquid-Chilling Systems” (centrifugal chiller subcooling targets) and “Condensers” (condenser approach).
EPA Section 608 (40 CFR Part 82 Subpart F) — refrigerant handling certification; size-based leak-repair duties (40 CFR 82.157; 40 CFR 84.106 from January 1, 2026), not a blanket rule against adding refrigerant to a leaking system.
SAE J2912 / J639 — mobile AC service procedures (R-1234yf, R-134a SC targets).
OEM service literature — Carrier, Trane, Lennox, Daikin, Mitsubishi, Goodman, LG, Fujitsu charging procedures, target SC ranges per model, and line-set length correction tables.

[1]
California Energy Commission, Reference Appendices for the 2025 Building Energy Efficiency Standards (Reference Appendix RA3.2), CEC-400-2025-010-AP. California Energy Commission. July 2025 (in effect January 1, 2026). https://www.energy.ca.gov/sites/default/files/2025-07/CEC-400-2025-010-AP.pdf (accessed October 19, 2026)
[2]
Measure Guideline: Air Conditioner Diagnostics, Maintenance, and Replacement (D. Springer, B. Dakin), NREL/SR-5500-56283. U.S. DOE Building America. March 2013. https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_air_cond_diagnostics.pdf (accessed October 20, 2026). Section 3.4 pp. 32–33, Table 3 p. 32, Figures 17–18 p. 33 — order-of-checks diagnostic flowcharts for fixed-orifice and TXV systems.

How to read your results

Subcooling (°F or °C): The headline number: how far the liquid sits below its saturation temperature, computed as saturation temperature minus measured liquid-line temperature. Compare it with the manufacturer's target from the charging label or installation instructions; California's charge test passes when it is within 3°F of that target.^[1] A negative value usually means a reading error. Switching the temperature unit to °C shows the value as a temperature difference, not an absolute temperature.
Saturation temperature at your liquid-line pressure: The bubble-point temperature the tool looks up from the PT chart at the pressure you entered — the boundary the liquid line must fall below to be subcooled. Subtracting your measured line temperature from it yields the subcooling. It is echoed back so you can sanity-check the pressure-to-temperature conversion.
Diagnostic banner (negative / low / positive / high): A color-coded reading of the number. Negative (red) means the liquid is warmer than its saturation temperature, which is physically impossible — usually a reading error, the wrong refrigerant, or pressure and temperature taken at different points, so check the reading first. Below 3°F (amber, “Low subcooling”) and 3–15°F (green, “Positive subcooling”) both tell you to compare the value against the manufacturer's target for the unit; California's charge test for TXV and EXV systems passes an installer's reading within 3°F of that target^[1]. Above 15°F (amber, “High subcooling”) says the same, and adds that low superheat together with high subcooling points to an overcharge^[2].
Bubble-curve note (zeotropic blends only): Appears only for high-glide blends (R-407C, R-454C, R-455A, R-449A) to confirm the saturation lookup used the bubble curve rather than the dew curve. Reading subcooling off the dew curve would overstate it by the refrigerant's glide — roughly 9°F for R-407C and up to about 18°F for R-455A — so this note flags that the math is correct for the liquid line.
Chart range for the selected refrigerant: The lowest-to-highest temperature the refrigerant's PT data covers. If your liquid pressure converts to a saturation temperature outside this span, the calculator reports that the pressure is out of range instead of a number; transcritical CO2 (R-744) above its critical point has no saturation state and returns no subcooling.

How to use this calculator

Pick the refrigerant. Defaults to R-410A.
Read the high-side (discharge / liquid-line) pressure from the manifold gauge.
Clamp a contact temperature probe on the liquid line at the outdoor unit's service valve — the smaller, uninsulated copper line. Insulate from ambient.
Allow 10-20 minutes after compressor start for steady-state. Enter both values.
Compare against your equipment's target subcooling (TXV target typically 8-12°F; check the OEM nameplate or service manual for the specific equipment).

Common errors

Probing the wrong line. The LIQUID line is the smaller, uninsulated line at the outdoor unit; the suction line is larger and foam-insulated.
Confusing high subcooling for 'extra capacity' — it usually means overcharge or condenser fouling, both of which reduce capacity.
On zeotropic blends, using the dew curve at the discharge pressure — overestimates subcooling by the glide value (9°F for R-407C). This calculator uses the bubble curve automatically.
Forgetting line set length adjustments — long mini-split line sets require higher SC at the outdoor unit to deliver adequate SC at the indoor TXV.

Underlying math

Formula

Subcooling (°F) = T_sat(P_liquid) − T_liquid_line T_sat is read off the BUBBLE curve at the measured liquid pressure for zeotropic blends. For pure refrigerants and azeotropes, bubble ≡ dew, so the curve choice is moot.

Source

Saturation temperatures from CoolProp 7.2.0 (Bell, Wronski, Quoilin, Lemort 2014, doi:10.1021/ie4033999), REFPROP-compatible Helmholtz EOS. Target subcooling per equipment manufacturer service literature (Carrier, Trane, Lennox, Daikin, Goodman), the ASHRAE Handbook—Refrigeration (2022), “Equipment and System Dehydrating, Charging, and Testing”, and the ASHRAE Handbook—HVAC Systems and Equipment (2024), “Liquid-Chilling Systems”.

Worked example

R-410A residential AC TXV system, 95°F outdoor: Liquid pressure: 380 PSIG Liquid-line temperature: 100°F Saturation temperature at 380 PSIG: 112.7°F (CoolProp 7.2.0) Subcooling = 112.7 − 100 = 12.7°F At the top of the typical 8-12°F TXV target range. TXV systems are charged BY subcooling — adjust refrigerant in 1-2 oz increments until SC lands on target (usually 10°F).

Related tools

Superheat Calculator

Suction-line companion. Together they pin down a system's charge state.

Combined PT / SH / SC

All three measurements on one form with pattern-matching diagnostic banner.

PT Calculator

Raw saturation lookup for any refrigerant.

High head pressure causes

High SC often signals a condenser-side condition. Diagnostic decision tree.

System Diagnostic

Pattern matcher for high/low pressure × high/low SH/SC fingerprints.

Overcharged AC Symptoms

8 signs of too much refrigerant — high SC is the definitive fingerprint.

R-410A Charging Chart

TXV subcooling target + fixed-orifice superheat matrix on one page.

High Suction Low Head

Diagnostic tree for the specific converging-pressure pattern.

Frequently asked

›What is subcooling?

Subcooling is the temperature of liquid refrigerant below its saturation temperature at the same pressure. It is measured on the liquid line leaving the condenser: condenser saturation temperature minus measured liquid-line temperature equals subcooling. Positive subcooling confirms fully-liquid refrigerant entering the metering device; zero or negative subcooling means vapor bubbles (flash gas) are present, starving the metering device and reducing capacity.

›What is the target subcooling for an HVAC system?

TXV / EEV residential AC: 8-12°F at the condenser outlet (per Carrier, Trane, Lennox, Daikin OEM service literature). Heat pumps in cooling mode: 8-15°F; in heating mode the indoor coil becomes the condenser and target is similar. Walk-in commercial refrigeration: 5-15°F depending on line run length. Centrifugal chillers: 2-5°F at the condenser exit. Fixed-orifice residential systems are charged by superheat — subcooling is informational only. Always cross-check the equipment label and OEM service literature.

›How do I measure subcooling in the field?

Read the high-side (discharge / liquid-line) pressure from the manifold gauge in PSIG. Clamp a contact temperature probe on the liquid line at the outdoor unit's service valve — the smaller, uninsulated copper line. Make solid metal-to-metal contact, insulate from ambient air, and let the reading stabilize (10-20 minutes after compressor start). Convert the liquid pressure to saturation temperature using a PT chart for your refrigerant — use the bubble curve for zeotropic blends. Subtract: subcooling = T_sat − T_line. This calculator handles the conversion and bubble-curve selection automatically.

›What does low subcooling indicate?

Low subcooling (under 3°F on a TXV system) usually means undercharge — the compressor can't condense enough vapor to fill the condenser with a liquid column, so refrigerant leaves the condenser still partly vapor. Negative subcooling means flash gas reaching the metering device. Cross-check superheat: high SH + low SC is the textbook undercharge fingerprint. Look for leaks before adding refrigerant; federal leak-repair duties are size-based (40 CFR 82.157 and 40 CFR 84.106 from January 1, 2026), not a blanket rule against topping off a leaking system. Less commonly, low SC can indicate a stuck-open bypass valve or sensor malfunction on commercial equipment.

›What does high subcooling indicate?

High subcooling (over 15°F on a residential system) usually means overcharge — excess refrigerant backs up in the condenser, taking up space normally used by condensing vapor. Less commonly: a dirty condenser coil (heat-transfer fouling raises condenser saturation temperature for the same heat rejection load), restricted condenser airflow, recirculation of hot discharge air over the coil, or non-condensable gases trapped in the system. Cross-check superheat: low SH + high SC is the overcharge fingerprint. Always verify condenser airflow and coil cleanliness before adjusting charge — fouling looks like overcharge.

›Why does subcooling math differ for zeotropic blends?

Zeotropic blends condense across a temperature range at constant pressure. On the liquid line the refrigerant has fully condensed — the relevant saturation boundary is the bubble temperature (below which everything is liquid), not the dew temperature. This calculator uses the bubble curve automatically for zeotropic blends. Using the dew curve for R-407C would overestimate subcooling by approximately 11°F; for R-455A by approximately 22°F.

›Why is TXV charged by subcooling and fixed-orifice by superheat?

A TXV / EEV regulates superheat to its setpoint regardless of how much refrigerant is in the system. So superheat on a TXV system tells you about valve operation, not charge. Subcooling, by contrast, measures how much liquid is backed up in the condenser — directly proportional to charge. Fixed-orifice devices have no feedback control, so superheat varies directly with charge and ambient; it is the right signal to charge against. Standard charging practice formalizes this: TXV = subcooling, fixed orifice = superheat.

›How does subcooling differ from condenser approach?

Subcooling is T_sat (at discharge pressure) − T_liquid_line, measured on the air side at the condenser exit. Condenser approach is T_sat − T_air_off_condenser (air-cooled) or T_sat − T_leaving_condenser_water (water-cooled), measured on the heat-rejection medium side. Approach tells you how efficiently the condenser is transferring heat; subcooling tells you how much liquid is sitting in the condenser. They're related but separate metrics. A high condenser approach with normal subcooling indicates condenser fouling without overcharge; high subcooling with normal approach indicates overcharge without fouling.

›Why does long line-set installation affect subcooling?

Long liquid line sets — common on mini-splits and multi-zone installations — introduce pressure drop and heat pickup that change subcooling between the outdoor unit and the indoor metering device. Manufacturers commonly recommend higher subcooling at the condenser outlet (e.g., 12-15°F instead of 8-10°F) for line sets over 50 feet to ensure sufficient liquid column reaches the indoor TXV. Always check the manufacturer's line set length adjustment table when commissioning long-line installations.