About this tool
The Superheat & Subcooling Calculator is a free, in-browser HVAC tool that converts measured line temperatures and gauge pressures into the actual superheat and subcooling of a running system and compares them to targets to judge the refrigerant charge. It uses built-in pressure-temperature (P-T) tables for R-410A, R-22, R-32, and R-134a: the low-side (suction) pressure gives the evaporator saturation temperature, and the high-side (liquid) pressure gives the condenser saturation temperature.
Everything is computed locally in your browser and nothing you enter is transmitted. Actual superheat is the suction line temperature minus the suction saturation temperature — the degrees the vapour has been heated past boiling, protecting the compressor from liquid. Actual subcooling is the liquid saturation temperature minus the liquid line temperature — the degrees the liquid has been cooled below condensing, confirming a full column of liquid to the metering device. For a TXV or EEV system, subcooling is the primary charge indicator, with a manufacturer default of about 8–12°F; for a fixed-orifice (piston) system, superheat is primary, and the target is estimated from the standard charging-chart formula (3 × indoor wet-bulb − 80 − outdoor dry-bulb) ÷ 2, clamped to at least 5°F.
The verdict follows the classic rules: low superheat or high subcooling indicates an overcharge (or restricted airflow), while high superheat or low subcooling indicates an undercharge or a liquid-line restriction. This is an informational technician's aid, not a substitute for the equipment manufacturer's charging instructions — always confirm the target values and conditions on the unit's data plate or charging chart before adjusting charge.