Antifreeze concentration is a trade-off. Glycol both lowers the freeze point and raises the boil point, which sounds like free protection — but the relationship is not linear, and pushing concentration too high makes freezing more likely, not less.
Freeze point and boil point by glycol concentration
| Ethylene glycol by volume | Freeze point | Atmospheric boil point |
|---|---|---|
| 0 % (plain water) | 0 °C | 100 °C |
| 20 % | − 9 °C | 102 °C |
| 30 % | − 15 °C | 104 °C |
| 40 % | − 24 °C | 106 °C |
| 50 % | − 37 °C | 107 °C |
| 60 % | − 52 °C | 111 °C |
| 70 % | − 55 °C | 115 °C |
| 80 % | − 48 °C | 120 °C |
| 100 % (neat glycol) | − 13 °C | 197 °C |
What this means in practice
- 50 % glycol, roughly − 37 °C, is the standard fill for most temperate and cold-climate passenger cars. It also lifts the atmospheric boil point to about 107 °C, which becomes roughly 125 – 135 °C once the system is pressurised.
- 60 % glycol, roughly − 52 °C, suits severe winters and cold-chain or refrigerated transport.
- Around 33 % glycol, roughly − 18 °C, is enough for mild-winter markets and still gives a useful boil margin. This is also close to the 'premixed' coolant sold in many markets.
- Below about 30 % glycol you are no longer buying freeze protection; you are buying a corrosion-inhibited coolant. That is a legitimate product — much of Southeast Asia, Africa and South Asia needs exactly this — but it must be specified honestly as such.
The pressure cap changes everything
Every 1 psi of system pressure raises the boiling point of a 50 % glycol mix by roughly 1.7 °C. A typical 15 psi cap therefore adds about 25 °C, which is why an engine with a 107 °C atmospheric boil point will not boil until roughly 132 °C. This is the real mechanism behind 'boil-over protection' — the coolant and the cap work together.
It also means a failed cap silently destroys boil-over protection. A cap that no longer holds pressure can let a system boil at 110 °C instead of 132 °C, which is exactly the condition that boils over in traffic on a hot day.
Regional guidance
| Market | Typical winter low | Sensible grade |
|---|---|---|
| Southeast Asia | Above + 10 °C | 0 °C coolant, boil-over focused, 30 % or less glycol |
| South Asia (plains) | Above 0 °C | 0 °C to − 10 °C coolant, boil-over focused |
| South Asia (northern hills) | − 5 to − 15 °C | − 25 °C grade |
| Sub-Saharan Africa (tropical) | Above + 5 °C | 0 °C coolant, boil-over focused |
| Southern Africa (inland winter) | − 5 to − 12 °C | − 25 °C grade |
| North Africa / Middle East | Above 0 °C | 0 °C to − 10 °C, high boil margin |
| Central & Eastern Europe | − 20 to − 30 °C | − 37 °C grade |
| Northern Europe / Canada | − 30 to − 45 °C | − 45 °C grade |
Open the mix calculator
Enter your lowest expected temperature and get the glycol ratio, freeze point and boil point you need.
How to check what you actually have
Use a refractometer, not a float-type hydrometer. A refractometer reads the freeze point directly from a few drops and is not thrown off by the dye or by dissolved corrosion products the way a float gauge is. Check at every service, and record the reading — a freeze point that creeps upward between services means someone is topping up with water.