About the Coolant Mix Ratio Calculator
This coolant mix ratio calculator tells you how much antifreeze concentrate and how much distilled water to put in a cooling system to reach a target concentration, and estimates the freeze point of the result. If your system already contains a weaker mix, it also works out how much coolant to drain and replace with pure concentrate to bring it up to strength.
It is for car, truck, motorcycle and equipment owners refilling after a flush, topping up before winter, or checking whether a 50/50, 60/40 or 70/30 mix suits their climate. Enter the cooling system capacity from your owner’s manual in whatever unit you like — quarts, liters or gallons — and the answer comes back in the same unit.
Freeze points are for ethylene glycol (the base of most automotive antifreeze) by volume, from a manufacturer freeze-point chart. Protection is best between about 50% and 70%; above that, the freeze point rises again and heat transfer suffers, and pure concentrate freezes near +9°F. Propylene-glycol coolants protect somewhat less at the same concentration. Never mix coolant types your manufacturer does not approve, and confirm with a refractometer or hydrometer.
With the default inputs, the antifreeze concentrate (fresh fill) is 6. Change any value above to recalculate instantly.
How to use the coolant mix ratio calculator
- 1Find the cooling system capacity in your owner’s manual and pick its unit.
- 2Choose a target concentration — 50% suits most climates, up to 60–70% for severe cold.
- 3For a fresh fill, use the antifreeze and water amounts shown.
- 4To strengthen an existing mix, test its concentration and enter it to see how much to drain and replace.
- 5Run the engine with the heater on, let it cool, and recheck the level and concentration.
Formula and method
For a fresh fill the concentrate needed is simply the system capacity multiplied by the target percentage by volume, and the remainder is distilled water. To strengthen an existing weaker mix, you drain some coolant and replace it with pure concentrate; solving the mass balance (Capacity × Current + Drain × (100 − Current) = Capacity × Target) gives the drain-and-replace amount shown.
The freeze point is interpolated linearly from a published freeze-point chart for ethylene glycol in water by volume. Note that commercial antifreeze contains a few percent of inhibitors, so labels may quote slightly different figures (for example about −34°F for 50/50). The chart lists about 63–74% only as “below −55°F”, so the calculator reports that range as below −55°F (−48°C) and draws it at −55°F on the graph.
- Capacity
- Total cooling system volume, in any unit
- Target %
- Desired antifreeze share by volume
- Current %
- Antifreeze share already in the system
Worked examples
12-quart system, 50/50 mix
Half of 12 quarts is 6 quarts of concentrate and 6 of water, giving a freeze point of roughly −35°F. If the system currently holds a 30% mix, draining 12 × 20 ÷ 70 = 3.43 quarts and refilling with concentrate brings it to 50%.
9-liter system, 60% for a very cold climate
60% of 9 liters is 5.4 liters of antifreeze and 3.6 liters of water, protecting to about −55°F. Starting from plain water, you would drain 5.4 liters and replace it with concentrate.
Light 40% mix for a warm climate
A 3-gallon system at 40% takes 1.2 gallons of concentrate and 1.8 gallons of water, protecting to about −13°F. The system is already at 40%, so nothing needs draining.
Frequently asked questions
What is the best coolant to water ratio?+
A 50/50 mix of antifreeze and distilled water is the standard recommendation for most vehicles and protects to about −34°F. In very cold regions you can go up to 60% or 70%, but not higher, because protection gets worse above roughly 70%.
Why not use 100% antifreeze?+
Pure ethylene glycol freezes at about +9°F (−13°C), transfers heat worse than a water mix and can cause overheating. Water is essential for both freeze protection and cooling.
Should I use tap water or distilled water in coolant?+
Use distilled or deionized water. Tap water contains minerals that can form scale in the radiator and react with corrosion inhibitors, shortening the life of the coolant.
Can I buy premixed coolant instead?+
Yes. Premixed 50/50 coolant is ready to pour and avoids mixing errors. It cannot raise the concentration of a weak system, though; for that you need concentrate.
How do I test my coolant concentration?+
Use a coolant refractometer (most accurate) or a floating-ball hydrometer. Refractometers read glycol percentage and freeze point directly; test strips also check pH and inhibitor condition.