Coolant Freeze Point Calculator

Coolant (antifreeze) serves two purposes: it prevents the coolant from freezing in cold weather and prevents it from boiling in hot weather. A 50/50 mix of ethylene glycol antifreeze and distilled water is the standard recommendation for most climates.

How Ethylene Glycol Works

Pure water freezes at 0°C (32°F) and boils at 100°C (212°F) at sea level. Adding ethylene glycol lowers the freezing point and raises the boiling point through a process called freezing point depression. The relationship is not linear, and past roughly 70% it reverses: adding more antifreeze from there makes the freeze protection worse, not better.

Freeze and Boil Protection vs. Concentration

Every row below is what this calculator returns for that concentration with a 1.0 bar (15 psi) cap selected. If you change the cap pressure the boil column moves with it, about 21°C per bar.

That last row is the one that surprises people. Straight concentrate out of the jug freezes at about -12°C, which is worse than a 25% mix. Ethylene glycol needs water present to depress the freezing point, so pouring in neat antifreeze because it is “stronger” leaves you with less cold protection than a proper 50/50, not more. The calculator caps its input at 70% for the same reason.

Recommended Concentrations by Climate

• Mild climate (rarely below -10°C / 14°F): 33 to 40% antifreeze
• Standard climate (down to -25°C / -13°F): 50% antifreeze (standard recommendation)
• Severe cold climate (down to -45°C / -49°F): 60% antifreeze
• Extreme climate (below -45°C / -49°F): 65 to 70% antifreeze (do not exceed 70%)

Leave yourself margin. A mix that freezes at exactly the coldest night you expect is not protected at that temperature, because glycol solutions do not snap solid at one point. They start throwing ice crystals several degrees above the stated freeze point and turn to slush, and slush will not circulate through a radiator. Aim about 10°C below your real worst case.

Pre-Mixed vs. Concentrate

Most antifreeze is sold either as a concentrate (100% glycol, needing dilution) or pre-mixed at 50/50 and ready to use. Always use distilled water, not tap water, to prevent mineral deposits in the cooling system.

When to Change Coolant

Most modern coolants are rated for 5 years or 150,000 km (100,000 miles). Check your owner’s manual. Old coolant becomes acidic and corrodes cooling system components.