Coolant Mixture Calculator
Calculate the right antifreeze-to-water ratio for your target freeze point. Shows how much coolant and water to mix for any cooling system capacity.
Or enter a custom target below.
Enter a negative number for the lowest temperature you expect.
Total cooling system capacity (check owner's manual).
Why Coolant Ratio Matters
Antifreeze (ethylene glycol) does more than prevent freezing — it raises the boiling point, prevents corrosion, and lubricates the water pump. But more isn't always better. Pure antifreeze actually freezes at a higher temperature than a 50/50 mix and transfers heat poorly.
The 50/50 Rule
A 50% antifreeze / 50% water mix is the standard recommendation for most climates. It protects to about −37°C (−34°F) and provides boiling protection to about 129°C (265°F) with a 15 PSI radiator cap. This ratio balances freeze protection, boil protection, corrosion inhibition, and heat transfer.
When to Go Higher
In extreme cold climates, you can increase antifreeze concentration up to 60–70% for additional freeze protection. However, above 70% antifreeze, freeze protection actually decreases and cooling performance drops significantly. Never run above 70%.
The Freeze Point Curve
The relationship between antifreeze concentration and freeze point is not linear. Protection improves rapidly from 0% to 60%, peaks around 60–70%, then reverses above 70%. This is because ethylene glycol and water form a eutectic mixture — the lowest freezing point occurs at a specific ratio, not at 100% concentration.
Water Quality
Always use distilled or deionized water. Tap water contains minerals that form scale deposits, reduce heat transfer, and accelerate corrosion. This is especially important in aluminum engines and radiators, which are more susceptible to electrolysis and galvanic corrosion.
Frequently Asked Questions
What is the best coolant-to-water ratio?
A 50/50 mix of antifreeze and water is the standard recommendation. It protects to about -37°C (-34°F) and provides boiling protection to about 129°C (265°F) with a 15 PSI cap. Going richer than 70% antifreeze actually reduces freeze protection and heat transfer. Never run straight water (no freeze or corrosion protection) or straight antifreeze (poor heat transfer, higher freeze point than a 50/50 mix).
Can I use distilled water instead of tap water?
Distilled or deionized water is recommended. Tap water contains minerals (calcium, magnesium) that can form deposits in the cooling system, reduce heat transfer, and accelerate corrosion. If distilled water isn't available, soft tap water is acceptable. Never use hard water or well water.
How often should coolant be replaced?
Conventional green coolant (IAT): every 2 years or 30,000 miles. Extended-life coolant (OAT/HOAT, usually orange or pink): every 5 years or 150,000 miles. Always follow your vehicle manufacturer's recommendation. Old coolant becomes acidic and loses corrosion protection, which can damage head gaskets, radiators, and water pumps.
Does coolant concentration affect cooling performance?
Yes. Water transfers heat better than antifreeze — a 50/50 mix transfers about 80% as well as pure water. Higher antifreeze concentrations (60-70%) transfer even less heat and can cause overheating in extreme conditions. However, the corrosion protection and freeze/boil protection of a proper mix outweigh the small reduction in heat transfer for street use.
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