Coolant Mix Ratio Calculator
Calculate how much coolant to drain and add to achieve your desired antifreeze-to-water ratio.
Enter system capacity, current ratio, and target ratio.
Antifreeze/coolant mixture ratio determines the freeze protection and boil-over protection your engine cooling system provides. The ratio of ethylene glycol (or propylene glycol) to distilled water must be matched to your climate and vehicle specifications.
Freeze point vs. mixture concentration:
| Antifreeze % | Water % | Freeze Point | Boiling Point (pressurized) |
|---|---|---|---|
| 0% (water only) | 100% | 0°C (32°F) | 112°C (234°F) |
| 30% | 70% | −15°C (5°F) | 115°C (239°F) |
| 50% | 50% | −37°C (−34°F) | 129°C (265°F) |
| 70% | 30% | −54°C (−65°F) | 140°C (284°F) |
| 100% (pure antifreeze) | 0% | −13°C (9°F) | Higher, but WORSE freeze protection |
The counterintuitive fact: Pure antifreeze freezes at −13°C — far warmer than a 50/50 mix (−37°C). The mixture is always more effective than either component alone.
Recommended ratio by climate:
- Mild climate (rarely below −15°C): 30–40% antifreeze
- Standard recommendation: 50/50 (protects to −37°C) — most popular year-round
- Extreme cold (below −29°C): 60–70% antifreeze
- Maximum allowable: 70% antifreeze (beyond this, protection decreases and pump cavitation risk increases)
Mixing formula — changing concentration: Volume to Drain = Total System Volume × |(Current% − Target%) ÷ (100% − Current%)|
After draining that volume, refill with pure antifreeze (to increase concentration) or distilled water (to decrease concentration).
Always use distilled water — never tap water: Tap water contains minerals (calcium, magnesium, iron) that deposit scale inside coolant passages, reduce heat transfer, and can cause corrosion. The $1 cost of a gallon of distilled water protects a $10,000+ engine.
Worked example: Your cooling system holds 8 liters. Current mixture: 40% antifreeze. You want 50%.
- Volume to drain: 8 × |( 40 − 50) ÷ (100 − 40)| = 8 × (10 ÷ 60) = 8 × 0.167 = 1.33 liters
- Drain 1.33 liters of current mix
- Refill with 1.33 liters of pure antifreeze concentrate
- New mixture: (0.40 × 8 − 0.40 × 1.33 + 1.33) ÷ 8 = (3.20 − 0.53 + 1.33) ÷ 8 = 4.0 ÷ 8 = 50% ✓