Superheat & Subcooling Calculator

It turns your manifold gauge readings into the two numbers that describe a system’s charge. Pick the refrigerant, enter the low-side pressure (psi) and suction line temperature for superheat, and the high-side pressure and liquid line temperature for subcooling — the calculator looks up the saturation temperatures from a built-in PT chart and does the subtraction, in °F (°C).

Superheat is how far the suction vapour has warmed above its boiling (saturation) temperature at the measured low-side pressure — proof that all the liquid boiled off before leaving the evaporator. Measure the suction line temperature near the outdoor unit, read the low-side gauge in psi, and superheat = line temperature − saturation temperature, in °F (°C).

Subcooling is how far the liquid leaving the condenser has cooled below its condensing (saturation) temperature at the measured high-side pressure — proof there’s a solid column of liquid at the metering device. Measure the liquid line temperature at the condenser outlet, read the high-side gauge in psi, and subcooling = saturation temperature − line temperature, in °F (°C).

Always use the manufacturer’s figures when you have them. As broad rules: systems with a TXV/EEV hold superheat around 9–22°F (5–12°C) and are charged to a subcooling target, typically 7–14°F (4–8°C). These are temperature differences, not absolute readings. Fixed-orifice systems are charged to a superheat target instead, which varies with indoor and outdoor conditions — look it up on a charging chart rather than assuming a number.

High superheat with low subcooling usually means undercharge — the evaporator is being starved. Very low superheat means liquid may be returning to the compressor (floodback). High subcooling points to overcharge or a liquid-line restriction, and low subcooling to undercharge. Diagnose with airflow, filters and coils checked first; charge problems and airflow problems mimic each other.

Saturation temperatures are interpolated from standard pressure–temperature tables and assume sea-level atmospheric pressure when converting your gauge reading. R-410A and R-32 have negligible temperature glide, so one curve serves both sides; the values land within a fraction of a degree of published charts. For other blends with real glide (like R-407C), or for legal record work, use the manufacturer’s tables.