Coolant Freeze Point Calculator
Estimate how cold your engine coolant can get before it freezes, from the glycol concentration of the mix. Enter the percentage of ethylene glycol by volume and the tool interpolates a standard reference chart to give the freeze protection temperature, plus a rough boil point for a pressurised cooling system.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the glycol concentration of your coolant as a percentage by volume (0–70%). A typical long-life mix is 50%.
- Read the estimated freeze protection point in °C — the emphasised result — and the rough pressurised boil point below it.
- Confirm your actual coolant strength with a refractometer, and compare against the coolant maker's own chart before trusting the number for winter or alpine driving.
How it works
The calculator uses a standard ethylene-glycol/water reference table (0%→0 °C, 10%→−4, 20%→−9, 30%→−16, 40%→−24, 50%→−37, 60%→−52, 70%→−55 °C) and linearly interpolates between the two nearest points, clamping to the ends outside 0–70%. Freeze protection deepens with concentration up to around 60–70%, then reverses, which is why the table peaks. A separate small table (0%→100, 50%→108, 60%→111 °C) gives an approximate boil point assuming a ~15 psi radiator cap.
Worked example
Worked example. A 50/50 mix (50% glycol) reads directly from the table as −37 °C freeze protection and about 108 °C boil point under a 15 psi cap. A weaker 33% mix falls between the 30% (−16 °C) and 40% (−24 °C) rows: −16 + (3/10)×(−8) = −18.4 °C.
Common mistakes
- Assuming more glycol is always better — above roughly 70% the freeze protection actually gets worse, and heat transfer suffers, so 50–60% is the usual sweet spot.
- Confusing percentage by volume with a simple 'ratio' — a 50/50 pour is 50% by volume, but topping up with pre-mixed coolant or plain water changes the real concentration.
- Using ethylene-glycol figures for a propylene-glycol coolant; the two have different freeze curves, so always check the specific product's chart.
Frequently asked questions
Does this work for propylene glycol coolant?
The table is for ethylene glycol, the most common automotive type. Propylene glycol freezes at slightly different temperatures for the same concentration, so treat this as a guide only and use the propylene-glycol product's own chart.
How do I measure my actual coolant concentration?
Use a coolant refractometer (most accurate) or a float-type antifreeze hydrometer/tester on a sample of the coolant. Relying on how much concentrate you poured in is unreliable once the system has been topped up over time.
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