Glycol Mix Calculator
How much propylene or ethylene glycol you need for a freeze point, by volume and by weight. Rounded up to the next whole percent.
How to use ↓Type the freeze point you need to see the glycol percentage.
Vol % is pure glycol by volume. Inhibited concentrates (for example DOWFROST or DOWTHERM SR-1) need slightly more, so use the product’s own chart. Typical values for reference only. Inhibited glycols and brines vary by maker: always use the manufacturer’s data for the fluid you are using.
How to use
- Pick Propylene or Ethylene glycol.
- Type the freeze point you need. Most makers say to pick one at least 3 °C (5 °F) below the lowest temperature expected.
- Read the percentage. Mixing by volume is easiest on site; the weight figure matches a refractometer chart or a lab result.
About this calculator
The answer comes from the Dow DOWFROST (propylene glycol) and DOWTHERM SR-1 (ethylene glycol) freeze-point tables, cross-checked against an independent set of freeze-point data. Where the two differ, the calculator uses the one that needs more glycol, then rounds up to the next whole percent, so it never gives you less protection than the tables.
Freeze point is where ice crystals start to form. The mix turns to slush below it and only bursts pipes at a lower temperature, but for chillers, heat exchangers and coils plan on the freeze point.
Sources: Dow DOWFROST Engineering and Operating Guide (Table 3); Dow DOWTHERM SR-1 Engineering and Operating Guide (Table 3). Typical values for reference only. Inhibited glycols and brines vary by maker: always use the manufacturer’s data for the fluid you are using.
FAQ
What percent glycol do I need for −20 °F?
About 48% propylene glycol or 44% ethylene glycol by volume (48% and 47% by weight), rounded up. Check the maker’s chart for your fluid.
Volume or weight percent?
Volume is what you measure when you mix on site. Weight is what refractometers and lab reports usually give. For propylene glycol the two are almost the same; for ethylene glycol the volume figure is a few points lower.
Why not use more glycol than I need?
More glycol lowers heat transfer and raises pumping power, so use enough for the freeze point plus a margin. Too little (under about 25 to 30%) may not give enough corrosion protection.