Freeze and boil point at any glycol concentration
Move the slider to see the freeze point and boiling point of an ethylene glycol mix, then add your radiator cap pressure to see the real boil-over margin the system has.
System pressure is what actually buys boil-over margin. A cap that no longer holds its rating removes this protection silently.
Follow the vehicle manufacturer's specified concentration where one is given.
Show the full concentration table
| Glycol % | Freeze point | Boil point (atmospheric) | Boil point (15 psi cap) |
|---|---|---|---|
| 0 % | 0 °C | 100 °C | 125.5 °C |
| 10 % | -4 °C | 101 °C | 126.5 °C |
| 20 % | -9 °C | 102 °C | 127.5 °C |
| 30 % | -15 °C | 104 °C | 129.5 °C |
| 40 % | -24 °C | 106 °C | 131.5 °C |
| 50 % | -37 °C | 107 °C | 132.5 °C |
| 55 % | -43 °C | 109 °C | 134.5 °C |
| 60 % | -52 °C | 111 °C | 136.5 °C |
| 65 % | -54 °C | 113 °C | 138.5 °C |
| 70 % | -55 °C | 115 °C | 140.5 °C |
| 80 % | -48 °C | 120 °C | 145.5 °C |
| 90 % | -29 °C | 140 °C | 165.5 °C |
| 100 % | -13 °C | 197 °C | 222.5 °C |
Two things this chart shows that marketers do not
More glycol is not more protection past about 70 %
Freeze point improves as glycol concentration rises, but only up to roughly 70 % by volume. Past that point the relationship reverses, and neat ethylene glycol freezes near −13 °C — worse than a 50 % mix. Pure glycol also has a much higher viscosity, which reduces flow through the radiator and heater matrix. If you have ever topped up with neat concentrate on the assumption that it protects better, the chart shows why that is a mistake.
The pressure cap is doing more work than the coolant
At 50 % glycol the atmospheric boiling point is around 107 °C. With a typical 15 psi cap the system will not boil until roughly 132 °C. That 25 °C of margin is not coming from the chemistry — it is coming from pressure. It is also why a cap that has lost its rating causes unexplained summer boil-over on a vehicle whose coolant tests perfectly.
What to do with the number
- 1Find your market's lowest expected ambient temperature.
- 2Read off the glycol concentration that protects to that temperature — or use the presets in the tool above.
- 3Check the resulting atmospheric boil point is adequate for your summer conditions.
- 4Confirm the pressure cap holds its rated value. Replace it on the maker's interval.
- 5Record the freeze point at every service with a refractometer, and watch for upward drift, which means someone is topping up with water.
Related reading
Our coolant grades
- Engine Coolant — 0 °C, 101 °C atmospheric boil point
Tropical and subtropical markets.
- Antifreeze Coolant — −45 °C, ≥109 °C boil point
Cold-climate and severe-winter markets.