Engine coolant is a blend of antifreeze (ethylene glycol) and water. The ratio sets how cold it can get before freezing and how hot before boiling. A 50/50 mix is the all-around standard; richer mixes buy more cold protection up to a limit.
| Antifreeze : Water | Freeze protection | Boil protection (15 psi cap) |
|---|---|---|
| 40 : 60 | -12 °F | 259 °F |
| 50 : 50 | -34 °F | 265 °F |
| 60 : 40 | -62 °F | 270 °F |
| 70 : 30 | -84 °F | 276 °F |
Stay between 40% and 70% antifreeze. Below 40% loses freeze and corrosion protection; above 70% actually protects less and transfers heat worse. Use distilled water to avoid mineral deposits.
Use the coolant type your manufacturer specifies (IAT, OAT, HOAT) — mixing chemistries can gel. Pre-mixed coolant is already 50/50.
Mix a batch with the Antifreeze Dilution calculator in the automotive calculators.