Coolant Mix Ratio Calculator

Antifreeze/coolant mixture ratio determines the freeze protection and boil-over protection your engine cooling system provides. The ratio of ethylene glycol (or propylene glycol) to distilled water must be matched to your climate and vehicle specifications.

Freeze point vs. mixture concentration:

These are the same figures the Coolant Freeze Point calculator and the Antifreeze Mix Ratio calculator use, so the three pages cannot drift apart. A radiator cap is worth roughly 21°C (38°F) of extra boiling headroom per bar, which is why a failing cap shows up as an overheat long before anything else does.

Above 60% the numbers get vague, and that is honest. Published tables disagree past that point, some quoting −55°C at 70% and others −64°C, because the glycol and water freezing curve flattens out near its minimum somewhere in the 60 to 70% region. Nothing is gained by chasing it. Treat 70% as the manufacturer’s hard ceiling rather than a target.

The counterintuitive fact: pure antifreeze freezes at about −12°C, far warmer than a 50/50 mix at −37°C. Glycol needs water present to depress the freezing point at all, so filling the system with neat concentrate because it seems stronger leaves you with less cold protection than a proper 50/50, not more.

Recommended ratio by climate:

• Mild climate (rarely below −15°C): 30 to 40% antifreeze
• Standard recommendation: 50/50, protecting to −37°C, and the right answer for most of the world
• Extreme cold (below −29°C): 60% antifreeze
• Maximum allowable: 70%. Beyond that the freeze protection stops improving, heat transfer drops off, and water pump cavitation becomes a risk

Mixing formula, changing concentration: To increase concentration (refill with pure antifreeze): Drain = Volume × (Target% − Current%) ÷ (100% − Current%) To decrease concentration (refill with distilled water): Drain = Volume × (Current% − Target%) ÷ Current%

The two directions use different divisors because what you drain is the current mix, but what you add back is either pure antifreeze or pure water.

One practical warning about draining. The arithmetic above assumes you can remove exactly the volume it names. In a real car you usually cannot. Opening the radiator petcock gets you somewhere around 40 to 60% of the system on most cars, because the block and the heater core hold the rest and it does not come out without pulling a block drain plug or a hose. If the calculator asks for more than about 40% of your capacity, do it in two passes: drain what you can, top up, run the engine to operating temperature with the heater on full so everything mixes, then measure again with a refractometer and repeat.

Always use distilled water, never tap water: Tap water contains minerals (calcium, magnesium, iron) that deposit scale inside coolant passages, reduce heat transfer, and can cause corrosion. A gallon of distilled water costs about a dollar and protects an engine worth several thousand.

Worked example: Your cooling system holds 8 liters. Current mixture: 40% antifreeze. You want 50%.

• Volume to drain: 8 × |( 40 − 50) ÷ (100 − 40)| = 8 × (10 ÷ 60) = 8 × 0.167 = 1.33 liters
• Drain 1.33 liters of current mix
• Refill with 1.33 liters of pure antifreeze concentrate
• New mixture: (0.40 × 8 − 0.40 × 1.33 + 1.33) ÷ 8 = (3.20 − 0.53 + 1.33) ÷ 8 = 4.0 ÷ 8 = 50% ✓

What that buys: freeze protection goes from −25°C to −37°C, and boiling headroom under a 15 psi cap from 127°C to 129°C. The freeze side is where the gain is. Going the other way, from 50% down to 40%, is a real downgrade in a cold climate and worth doing only if you are in a hot one and want the better heat transfer that more water gives you.