Antifreeze Mix Calculator

How it works

A fresh fill is a straight split of the system capacity: the target percentage of it is antifreeze concentrate and the rest is water. Twelve quarts at 50% is 6 quarts of each.

Freezing points come from Dow's published table of typical freezing points of ethylene glycol and water, read by percentage of glycol by volume and interpolated between rows. The freezing point does not keep falling as glycol is added: Dow's table puts mixes from 62.8% to 73.6% below −60 °F without a figure, then shows the freezing point rising again, to −52.2 °F at 78.9% and −3 °F at 95%. Pure glycol freezes at 11.8 °F. Between 57.8% and 78.9% the result reads "below −52 °F" because the table gives no single value there.

The boiling point uses the vapour-pressure (Antoine) constants MEGlobal publishes for glycol solutions, at sea-level atmospheric pressure plus the pressure cap's rating. A pressurised system boils hotter than an open pot: a 50% mix boils at about 226 °F at atmospheric pressure and about 266 °F under a 15 psi cap.

To strengthen a weak mix, some coolant is drained and replaced with the same volume of concentrate. Dow gives the volume as A = V (D − P) ÷ (100 − P), where V is the system volume, D the desired percentage and P the present one. To weaken a mix that is too strong, the drained volume is replaced with water instead: A = V (P − D) ÷ P.

Formula

concentrate      = capacity × target % ÷ 100
water            = capacity × (100 − target %) ÷ 100
to raise P → D   : drain A = capacity × (D − P) ÷ (100 − P), refill with concentrate
to lower P → D   : drain A = capacity × (P − D) ÷ P, refill with water
freezing point   = Dow Table 3, interpolated by vol % ethylene glycol
boiling point    : log10(P mm Hg) = A − B ÷ (T °C + C), MEGlobal constants by wt %
pressure         = 760 mm Hg (1 atm) + cap rating in psi

Example

A 12-quart cooling system filled fresh at 50% takes 6 quarts of concentrate and 6 quarts of water. By volume, 50% glycol lies between Dow's rows for 49.6% (−33.6 °F) and 50.6% (−36.2 °F), so the mix freezes at about −34.6 °F (−37.0 °C). Under a 15 psi cap it boils at about 266 °F (130 °C).

If the system now tests at 30% (freezing at about 3.7 °F), raising it to 50% means draining 12 × (50 − 30) ÷ (100 − 30) = 3.43 quarts and refilling with 3.43 quarts of concentrate.

Assumptions and limitations

• The concentrate is treated as pure ethylene glycol and the percentages are by volume. Commercial antifreeze concentrate contains inhibitors and a little water, and propylene glycol and premixed coolants have different freezing points: the protection printed on the product label is the figure for that product.
• Freezing points are Dow's typical values for ethylene glycol and water, interpolated in a straight line between table rows. The freezing point is where ice crystals first form; the mix turns to slush below it before it freezes solid.
• Boiling points assume sea-level atmospheric pressure (1 atm) plus the cap rating, and interpolate MEGlobal's vapour-pressure constants by glycol weight percentage. At high altitude the boiling point is lower. Against Dow's atmospheric boiling points the method agrees to within about 1.5 °F up to 60% glycol by weight, and reads up to about 8 °F lower above that.
• The drain-and-refill volumes assume the drained coolant has the same concentration as the whole system and that the new fluid mixes completely. Coolant left in the engine block and heater core, and any plain water left after a flush, change the result; only a hydrometer or refractometer reading shows the concentration actually reached.
• This is an estimate, not a substitute for the vehicle manufacturer's coolant specification or the product's instructions. Antifreeze is poisonous to people and animals.

Frequently asked questions