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Ethylene glycol (EG) is a colligative antifreeze, it depresses the freezing point of water in proportion to the number of dissolved molecules. The relationship is non-linear: protection improves rapidly up to about 60% V/V (−52.8 °C / −63 °F), beyond which adding more glycol actually raises the freezing point.
The freezing point (ASTM D 1177) is the temperature at which the first ice crystals form. At this point the mixture is still a pumpable slush, so it is a conservative but widely used design threshold.
A 30% V/V EG solution protects to approximately −13.7 °C (7.3 °F); 50% V/V extends protection to −36.8 °C (−34.2 °F). For most HVAC and process cooling applications, concentrations between 25% and 50% V/V are used, balancing freeze protection with heat transfer efficiency.
Note: Higher glycol concentrations reduce heat transfer performance. Avoid over-concentrating beyond what frost protection requires.