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Coolant mix and freeze protection calculator

How cold your current coolant will protect to, and how much to drain and replace with straight antifreeze to reach the mix you want.

Enter your numbers
Your cooling system
Check with a coolant tester or refractometer.

Drain and replace

–
Quarts of coolant to drain, then refill with straight concentrate.
Protects to now
–
Protects to after
–
For a fresh 50/50 fill
–

Stronger isn't better past 60%

Protection improves up to roughly 60 to 70% antifreeze and then gets worse. Straight concentrate also transfers heat poorly, so don't run it undiluted.

How much to drain

Quarts to drain = capacity × (target % − current %) ÷ (100 − current %)

You drain some of the existing mix and replace it with 100% concentrate. The formula finds the amount that lands exactly on your target. In practice, an engine block doesn't drain completely from the radiator petcock, so test again after the engine has run and mixed the coolant.

Freeze points

Freeze points are approximate values for ethylene glycol, the most common antifreeze: about −34 °F at 50% and −62 °F at 60%. Propylene glycol coolant protects a little differently, so check its label. Use distilled water for mixing to avoid mineral deposits.

Frequently asked questions

Why not just run 100% antifreeze for maximum protection?

Straight antifreeze actually has worse heat transfer than a proper water/antifreeze mix and can run hotter in normal operation — the commonly recommended 50/50 mix balances freeze and boil protection with the best actual cooling performance.

How do I know my coolant's current concentration?

A coolant refractometer or a simple floating hydrometer (sold cheaply at most auto parts stores) reads the freeze-protection percentage directly from a sample, which is the number this calculator needs to compute how much to drain and replace.

Does this work for the newer long-life coolant colors too?

The dilution math is the same regardless of coolant color or formulation (green, orange, pink, etc.) — what matters is never mixing incompatible coolant types in the same system, so check your vehicle's specified coolant type before topping off with a different color.

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