HVAC Superheat & Subcooling Calculator

How it works

Superheat = suction line temp − saturation temp at suction pressure; subcooling = saturation temp at liquid pressure − liquid line temp; fixed-orifice target superheat ≈ (3 × indoor WB − 80 − outdoor DB) ÷ 2

Two temperatures tell a technician whether an air-conditioning system holds the right amount of refrigerant. Superheat is how far the vapour leaving the evaporator has warmed beyond its boiling point at that pressure — too little means liquid may reach the compressor, too much means a starved coil — and on fixed-orifice systems it is the charging target, varying with indoor humidity and outdoor temperature. Subcooling is how far the liquid leaving the condenser has cooled below its condensing temperature; on systems with a thermostatic or electronic expansion valve the valve holds superheat steady, so subcooling becomes the charge indicator, typically 8–12 °F. Saturation temperatures come from the refrigerant’s pressure-temperature chart; this tool interpolates those charts, accurate to about a degree over the service range. Airflow problems mimic charge problems, so check filters and coils first.

Worked example

R-410A with 118 psig suction and a 52 °F suction line: saturation is about 40 °F, so superheat is 12 °F. Liquid at 346 psig and 95 °F: condensing about 105 °F, subcooling 10 °F — a TXV system charged correctly.

Frequently asked questions