What is the correct coolant to water ratio?

For most UK conditions, a 50:50 mix of coolant (antifreeze) to water is recommended. This provides freeze protection down to about -37°C and boil-over protection up to about 112°C. In very cold areas, you can go up to 60:40 coolant to water. Never exceed 70% coolant as it actually reduces performance.

Can I use tap water or should I use distilled?

Distilled or deionised water is recommended. Tap water contains minerals that can cause scale buildup and corrosion inside the cooling system over time. If distilled water is not available, soft tap water is acceptable. Never use hard water as the mineral deposits can clog the system.

How often should I change coolant?

Most manufacturers recommend changing coolant every 2-5 years or 30,000-50,000 miles. Long-life coolants (OAT type) can last up to 5 years. Check your owner's manual. Old coolant loses its corrosion inhibitors, which can lead to internal corrosion and head gasket issues.

What type of coolant should I use?

Always use the type specified by your manufacturer. The main types are: IAT (Inorganic Additive Technology) — typically green, lasts 2 years; OAT (Organic Acid Technology) — typically orange/pink, lasts 5 years; HOAT (Hybrid OAT) — a mix, typically yellow. Never mix different types as they can gel and block the system.

Can I just add pure antifreeze?

No. Pure antifreeze is less effective than a mixed solution. It has a higher freeze point than a 50:50 mix and transfers heat less efficiently. Coolant needs water to work properly. Always mix coolant with water before adding it to the system, or buy pre-mixed coolant.

CarCalcs

Coolant Mix Calculator — Antifreeze Ratio

Calculate the correct ratio of coolant (antifreeze) to water for your cooling system and climate.

Cooling System Capacity

Check your owner's manual for capacity

Desired Coolant RatioDesired Coolant Ratio

UK Climate ZoneUk climate zone

How We Calculate This

How The Mix Is Calculated

Coolant amount = System capacity x Desired percentage / 100

Water amount = System capacity - Coolant amount

Protection Levels by Concentration

30%: Freeze to -15°C, Boil to 104°C
40%: Freeze to -25°C, Boil to 108°C
50%: Freeze to -37°C, Boil to 112°C (recommended)
60%: Freeze to -52°C, Boil to 116°C (extreme cold)

Above 70% concentration, freeze protection actually decreases and cooling efficiency drops.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Actual figures may vary based on driving conditions, vehicle specifications, and other factors.