Coolant Mix Ratio Calculator
Calculate the antifreeze-to-water mix ratio for your climate.
Returns freeze protection to -34°F and boil point to 265°F for 50/50, 60/40, and 70/30 blends.
Antifreeze/coolant mixture ratio determines your cooling system’s freeze protection, boil-over protection, and corrosion inhibitor effectiveness. Most vehicles use a 50/50 mixture of antifreeze concentrate and distilled water — but colder climates may call for up to 70% antifreeze.
Mixing formula: Antifreeze Volume = Total Volume × (Target % ÷ 100) Water Volume = Total Volume − Antifreeze Volume
Adjusting an existing mixture: Additional Antifreeze = (Target% − Current%) × System Capacity ÷ (100% − Target%)
Freeze and boil protection by mixture ratio:
| Antifreeze % | Freeze Point | Boil Point (pressurized 15 psi cap) |
|---|---|---|
| 30% | −15°F (−26°C) | 259°F (126°C) |
| 40% | −24°F (−31°C) | 261°F (127°C) |
| 50% | −34°F (−37°C) | 265°F (129°C) |
| 60% | −62°F (−52°C) | 268°F (131°C) |
| 70% | −84°F (−64°C) | 270°F (132°C) |
| 100% | +12°F (−11°C) | 330°F (166°C) |
Important: Pure antifreeze is NOT the best protection! It freezes at a higher temperature than a 50/50 mix. Always mix with distilled water (not tap water — minerals cause corrosion).
What each variable means:
- Antifreeze concentrate — ethylene glycol (most common) or propylene glycol (less toxic, used in RVs and food equipment).
- Distilled water — mineral-free water prevents scale deposits and corrosion inside the cooling system.
- System Capacity — total volume of coolant the system holds (radiator + engine block passages + heater core). Typically 10–15 liters for passenger vehicles.
Worked example: Cooling system capacity: 12 liters. Current mixture: 30% antifreeze. You need 50% for winter. Additional antifreeze = (50% − 30%) × 12 ÷ (100% − 50%) = 20% × 12 ÷ 50% = 2.4 ÷ 0.5 = 4.8 liters to add Drain 4.8 liters of the old mixture first, then add 4.8 liters of pure antifreeze concentrate.