Glycol Freeze Point Calculator

Find the freeze and burst point of any propylene or ethylene glycol mix, the percent you need for your coldest day, and how much glycol and water to mix.

Glycol
20/8030/7040/6050/5060/40

A 30% propylene glycol mix freezes at 8.4 °F (-13.1 °C).

Percent glycol by volume, the way 30/70 and 40/60 mixes are sold. Values between Dow's table rows are interpolated.

Freeze point
8.4 °F (-13.1 °C)
Burst protection to
-19.7 °F
By weight
30.6% glycol
Boiling point
216 °F
Refractometer
nD 1.3682 · 22.5 Brix
  • Freeze point: ice crystals start to form and the fluid turns to slush that won't pump. Burst protection: still slush, but it won't expand enough to split pipes.

Reviewed by Andre Taki, Chief Commercial Officer, Alliance Chemical. Data: Dow DOWFROST and DOWTHERM SR-1 technical data. Typical values, not specifications.

Glycol freeze point charts

Dow's published numbers for propylene and ethylene glycol in water. Percent glycol by volume unless a column says otherwise.

Freeze and burst points of common glycol mixes

Glycol by volumePropylene glycol freezes atPG burst protection toEthylene glycol freezes atEG burst protection to
20%18.7 °F (-7.4 °C)9.9 °F16.0 °F (-8.8 °C)4.2 °F
25%14.0 °F (-10.0 °C)-2.4 °F10.2 °F (-12.1 °C)-7.4 °F
30%8.4 °F (-13.1 °C)-19.7 °F3.7 °F (-15.7 °C)-60 °F (end of Dow's chart)
35%1.4 °F (-16.9 °C)-60 °F (end of Dow's chart)-3.7 °F (-19.8 °C)-60 °F (end of Dow's chart)
40%-6.8 °F (-21.5 °C)-60 °F (end of Dow's chart)-12.6 °F (-24.8 °C)-60 °F (end of Dow's chart)
45%-16.8 °F (-27.1 °C)-60 °F (end of Dow's chart)-22.7 °F (-30.4 °C)-60 °F (end of Dow's chart)
50%-28.6 °F (-33.7 °C)-60 °F (end of Dow's chart)-34.6 °F (-37.0 °C)-60 °F (end of Dow's chart)
55%-42.8 °F (-41.6 °C)-60 °F (end of Dow's chart)-48.2 °F (-44.6 °C)-60 °F (end of Dow's chart)
60%-59.9 °F (-51.1 °C)-60 °F (end of Dow's chart)Colder than -54.9 °F (-48.3 °C)-60 °F (end of Dow's chart)
Interpolated from Dow's tables below. Burst protection: the fluid turns to slush but won't expand enough to split pipes. Source: Dow DOWFROST (propylene glycol) and DOWTHERM SR-1 (ethylene glycol) technical data. Typical values, not specifications.

Propylene glycol freeze point chart

Glycol by volumeBy weightFreeze °FFreeze °CRefractive index (22 °C)Brix
0.0%0.0%32.00.01.33280.0
4.8%5.0%29.1-1.61.33834.8
9.6%10.0%26.1-3.31.34388.4
14.5%15.0%22.9-5.11.349512.9
19.4%20.0%19.2-7.11.355515.4
24.4%25.0%14.7-9.61.361519.0
29.4%30.0%9.2-12.71.367522.0
34.4%35.0%2.4-16.41.373326.1
39.6%40.0%-6.0-21.11.379029.1
44.7%45.0%-16.1-26.71.384731.8
49.9%50.0%-28.3-33.51.390334.7
55.0%55.0%-42.8-41.61.395638.0
60.0%60.0%-59.9-51.11.400840.6
Source: Dow DOWFROST Heat Transfer Fluid Technical Data Sheet, Form 180-01314-1019. Typical values, not specifications. Above 60%, Dow lists freeze points only as below -60 °F.

Ethylene glycol freeze point chart

Glycol by volumeBy weightFreeze °FFreeze °CRefractive index (22 °C)Brix
0.0%0.0%32.00.01.33280.0
4.4%5.0%29.4-1.41.33783.8
8.9%10.0%26.2-3.21.34286.8
13.6%15.0%22.2-5.41.34789.9
18.1%20.0%17.9-7.81.353013.0
20.1%22.0%15.9-8.91.355114.3
22.9%25.0%12.7-10.71.358216.1
24.8%27.0%10.4-12.01.360317.3
27.7%30.0%6.7-14.11.363519.2
29.6%32.0%4.2-15.41.365620.4
32.6%35.0%-0.2-17.91.368822.3
34.5%37.0%-3.0-19.41.370923.5
37.5%40.0%-8.1-22.31.374125.3
39.5%42.0%-11.7-24.31.376326.5
42.5%45.0%-17.5-27.51.379628.3
44.5%47.0%-21.6-29.81.381729.4
47.6%50.0%-28.9-33.81.384931.2
49.6%52.0%-33.6-36.41.386932.3
52.7%55.0%-42.0-41.11.390033.9
54.7%57.0%-47.5-44.21.392135.0
57.8%60.0%-54.9-48.31.395236.6
Every 5% by weight plus the rows closest to 20, 25, 30, 35, 40, 45, 50 and 55% by volume. Source: Dow, DOWTHERM SR-1 and DOWTHERM 4000 Engineering and Operating Guide, Table 3. Typical values, not specifications.

Freeze and burst protection by temperature

Coldest temperaturePG for freeze protectionPG for burst protectionEG for freeze protectionEG for burst protection
20 °F (-7 °C)18.4%12.1%16.1%11.1%
10 °F (-12 °C)29.2%20.0%25.2%17.0%
0 °F (-18 °C)36.1%24.1%33.1%22.1%
-10 °F (-23 °C)42.2%28.1%39.1%26.1%
-20 °F (-29 °C)46.4%30.1%44.1%30.0%
-30 °F (-34 °C)50.5%33.1%48.1%30.0%
-40 °F (-40 °C)54.1%35.0%52.1%30.0%
-50 °F (-46 °C)57.2%35.0%56.1%30.0%
-60 °F (-51 °C)60.1%35.0%60.1%30.0%
Percent glycol by volume, no safety margin added (Dow suggests designing at least 5 °F, 3 °C, below your coldest day). Freeze columns: the higher of the freeze point charts above and Dow's freeze-protection figures. Burst columns: Dow's burst-protection figures, never more than the freeze column. Dow's figures are % Dow fluid, converted here to % glycol. Source: Dow DOWFROST (propylene glycol) and DOWTHERM SR-1 (ethylene glycol) technical data. Typical values, not specifications.

Questions

What is the difference between freeze protection and burst protection?
Freeze protection keeps the fluid liquid and pumpable down to that temperature. Burst protection only keeps pipes from splitting: below the freeze point the fluid turns to slush and can't circulate, but it won't expand enough to burst the pipe. Dow says burst protection is enough for a system that sits idle in the cold and has room for the slush to expand; any system that has to run in the cold needs freeze protection. At 0 °F, propylene glycol needs 36.1% by volume for freeze protection and 24.1% for burst protection.
Is the glycol percent by volume or by weight?
This calculator uses percent glycol by volume, the way premixes like 30/70 and 50/50 are named. Dow's charts list both: 50% propylene glycol by weight is 49.9% by volume, and 50% ethylene glycol by weight is 47.6% by volume. The freeze point charts on this page show both columns.
Should I use propylene glycol or ethylene glycol?
Ethylene glycol gives a little more freeze protection at the same strength: a 30% mix freezes at 3.7 °F, against 8.4 °F for propylene glycol. Ethylene glycol is toxic, so use propylene glycol, USP grade, wherever food, beverages or drinking water could be touched. Don't mix OAT and conventional inhibited fluids in one system.
What temperature does a 30/70, 40/60 or 50/50 glycol mix protect to?
Propylene glycol: 30/70 freezes at 8.4 °F with burst protection to -19.7 °F; 40/60 freezes at -6.8 °F; 50/50 freezes at -28.6 °F with burst protection to -60 °F (end of Dow's chart). Ethylene glycol: 30/70 freezes at 3.7 °F; 50/50 freezes at -34.6 °F; both give burst protection to -60 °F (end of Dow's chart). These are Dow's typical values, interpolated; design at least 5 °F below your coldest day.
How do I check the glycol in my system with a refractometer?
Read a sample at about 22 °C (72 °F) on a refractometer scale made for that glycol, then enter the reading under "Check with a refractometer". For example, a propylene glycol loop reading 1.3675 (22.0 Brix) is 29.4% glycol and freezes at 9.2 °F. If it's weak, add a target and the calculator tells you how much to drain and refill with concentrate.
Why not use more than 60% glycol?
Dow recommends no more than 60% glycol for efficient heat transfer. A 60% propylene glycol mix already freezes at -59.9 °F, and 57.8% ethylene glycol at -54.9 °F. Past about 75%, ethylene glycol's freeze point rises again: Dow lists 78.9% by volume freezing at -52.2 °F.