Coolant Mix Ratio Calculator
Engine coolant is a mix of concentrated antifreeze (usually ethylene glycol or propylene glycol) and water. The ratio sets the freeze point, boil point, and corrosion protection. When you buy concentrate rather than premix, you have to split your cooling system's total capacity into the right amount of concentrate and the right amount of water. This calculator takes your system capacity and your target antifreeze percentage, then tells you exactly how much concentrate and how much water to add. The ratio and capacity are user-editable so you can match your vehicle manufacturer's specification and your local climate.
Coolant mix formula
Concentrate = capacity * (antifreeze % / 100)
Water = capacity - concentrate
Gallons = quarts / 4
Ratio = antifreeze % : (100 - antifreeze %)
The mix is by volume. The concentrate fraction equals your target antifreeze percentage, and water makes up the remainder of the total system capacity.
Coolant mixing notes
- 50/50 is the standard all-season ratio for most passenger vehicles.
- Keep antifreeze at or below about 70 percent; pure concentrate protects worse than a mix.
- Use distilled or deionized water to limit scale and corrosion.
- Do not mix different coolant chemistries (IAT, OAT, HOAT) unless the maker says it is safe.
- Premixed 50/50 coolant should be added straight, not diluted.
Coolant mix: frequently asked questions
What coolant mix ratio should I use?
A 50/50 mix of concentrated antifreeze to water is the most common all-season ratio. A 70/30 mix gives more freeze and boil protection for extreme cold but reduces heat transfer. Ratios are user-editable here because the correct value depends on your climate and your vehicle manufacturer's specification. Never run more than about 70 percent antifreeze.
Should I use distilled water?
Yes. Tap water contains minerals that can form scale and accelerate corrosion in the cooling system. Distilled or deionized water is recommended when mixing concentrate. This calculator only tells you the volumes; the water quality is your choice.
Can I mix premixed (50/50) coolant in this calculator?
This calculator is for diluting concentrate with water. Premixed coolant is already at 50/50 and should be added straight, not diluted further. If you need a higher concentration than your premix, you must start from concentrate.
How do I find my cooling system capacity?
The total coolant capacity is listed in your owner's manual or service manual, usually in quarts or liters. Enter that total here. The calculator splits it into the concentrate and water volumes for your chosen ratio.
Does a higher antifreeze ratio always protect better?
No. Protection improves as antifreeze rises toward about 60 to 70 percent, but pure antifreeze actually has a higher freezing point and worse heat transfer than a mix. That is why manufacturers specify a ratio rather than pure concentrate.
Official sources
- U.S. Environmental Protection Agency: vehicle fluids and disposal guidance.
- U.S. Department of Energy: vehicle thermal systems.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.