Antifreeze Mix Ratio Calculator

Antifreeze (also called engine coolant) is a mixture of ethylene glycol (or propylene glycol) and water. It does two jobs: it lowers the freezing point of the coolant below the coldest temperatures in your climate, and it raises the boiling point above 100°C (212°F) to prevent overheating.

Why Not Use Pure Antifreeze?

Counterintuitively, pure (100%) antifreeze freezes at about −12°C (10°F), which is much warmer than a 50/50 mix at −37°C (−35°F). Glycol needs water present to depress the freezing point at all. Water also carries heat far better than glycol does, so the mix beats either component on both counts. The useful range is 30 to 60%, with 50/50 the standard answer.

Freeze and Boil-Over Protection by Concentration

Two things worth pulling out of that table. The boil column people usually quote is the pressurised one, because no cooling system runs at atmospheric pressure. A radiator cap is worth about 21°C (38°F) per bar, which is why a cap that has stopped holding pressure shows up as an overheat long before anything else goes wrong.

And past 60% the published figures stop agreeing with each other, some quoting −55°C at 70% and others −64°C, because the freezing curve flattens out near its minimum somewhere in that region. Nothing useful is gained by chasing it, so this calculator stops recommending more than 60% and treats 70% as a ceiling rather than a target. These are the same numbers the Coolant Freeze Point calculator and the Coolant Mix Ratio calculator use.

Always Use Distilled Water

Tap water contains minerals that can deposit on engine surfaces and corrode aluminum components. Always use distilled water when mixing coolant. Pre-mixed coolant already uses distilled water.

Coolant System Capacity

Before mixing, know your vehicle’s coolant system capacity (found in the owner’s manual). Typical capacities:

• Small car: 6–8 litres (1.5–2 gallons)
• Sedan/SUV: 8–12 litres (2–3 gallons)
• Truck: 12–18 litres (3–4.5 gallons)

Types of Antifreeze

Never mix different types. When in doubt, flush the system completely and refill with the correct type.