Coolant Freeze Point Calculator
Calculate the freeze and boil protection of your coolant mixture.
Find the right antifreeze concentration for your climate.
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 — there is an optimal concentration beyond which adding more antifreeze actually makes the protection worse.
Freeze and Boil Protection vs. Concentration
| Antifreeze Concentration | Freeze Point | Boil Point (with 1 bar / 15 psi cap) |
|---|---|---|
| 25% antifreeze / 75% water | -8°C (18°F) | 110°C (230°F) |
| 33% antifreeze / 67% water | -17°C (1°F) | 113°C (235°F) |
| 50% antifreeze / 50% water | -37°C (-34°F) | 127°C (260°F) |
| 60% antifreeze / 40% water | -52°C (-62°F) | 133°C (271°F) |
| 70% antifreeze / 30% water | -55°C (-67°F) | 138°C (280°F) |
| 100% antifreeze | -12°C (10°F) | Reduced protection! |
Note: Pure antifreeze provides worse freeze protection than a 70% mix — it must be diluted to function properly.
Recommended Concentrations by Climate
- Mild climate (rarely below -10°C / 14°F): 33–40% 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): 65–70% antifreeze (do not exceed 70%)
Pre-Mixed vs. Concentrate
Most antifreeze is sold either as a concentrate (100% glycol — requires dilution) or pre-mixed (typically 50/50, 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.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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