Quick answer
Ethylene glycol (EG) is the usual base for car antifreeze. It gives slightly more freeze protection at a given concentration, but it is more toxic. Propylene glycol (PG) is less toxic and is preferred for food-adjacent and potable-adjacent systems, RV plumbing and data-center PG25 loops. Use whichever your system specifies, and don't mix them.
Key takeaways
- EG: the common car antifreeze base. Toxic if swallowed.
- PG: lower toxicity. Used in RV/marine plumbing antifreeze, food processing and data-center PG25.
- At 50% by volume: EG freezes ≈ −36.8 °C, PG ≈ −34 °C.
- Mixing EG and PG confuses refractometer readings and may mix incompatible inhibitors.
- Glycol lowers specific heat, so a glycol mix carries less heat than pure water.
EG vs PG side by side
| Ethylene glycol (EG) | Propylene glycol (PG) | |
|---|---|---|
| Typical uses | Most car coolants; industrial heat transfer | Food and potable-adjacent systems; RV/marine plumbing antifreeze; data-center PG25 |
| Toxicity | More toxic; avoid where leaks could reach potable water or food | Lower toxicity; preferred where incidental contact is possible |
| Freeze point at 50% by vol. | −36.8 °C (−34 °F) | −34 °C (−29 °F) |
| Boiling point at 50%, 1 atm | 107.2 °C (225 °F) | about 106 °C (222 °F) |
| Viscosity | Lower | Higher, especially when cold |
Freeze and boil data: Engineering ToolBox EG [1] and PG [2] tables.
Freeze points by concentration
| Glycol % | EG freezing point | PG freezing point |
|---|---|---|
| 10% | −3.4 °C | −3 °C |
| 20% | −7.9 °C | −8 °C (19%) |
| 30% | −13.7 °C | −14 °C (29%) |
| 40% | −23.5 °C | −22 °C |
| 50% | −36.8 °C | −34 °C |
| 60% | −52.8 °C | −48 °C |
Why PG is used in data centers
Direct-to-chip loops use PG25, which is roughly 25% propylene glycol in water with inhibitors. PG is used where low toxicity matters and freeze protection only needs to be modest. See what is PG25? and the list of qualified PG25 fluids.
Freeze & boil point calculator
Interpolated from published glycol/water tables at 1 atm [EG] [PG]. Commercial coolants and pressurised systems differ.
Heat transfer: the trade-off
Adding glycol lowers the specific heat of the fluid. At 50% EG, specific heat drops by roughly 20% compared with water at typical temperatures [1], so systems circulate more fluid to compensate. Glycol is a compromise between freeze protection and heat transfer.
Can you mix EG and PG coolant?
Avoid it. Refractometers have separate scales for EG and PG, so a mix reads wrongly, and the inhibitor packages may not be compatible. If they have been mixed, drain and refill to the specified type.
Toxicity and safety
Ethylene glycol is poisonous if swallowed [3]. Propylene glycol has lower toxicity but should still be handled carefully, and used coolant may carry metals and contaminants. See antifreeze poisoning and coolant disposal.
Common mistakes
- Using EG antifreeze in potable or RV water lines. Use PG plumbing antifreeze labelled for potable systems.
- Reading a PG mix on an EG scale. You'll get the wrong concentration.
- Assuming PG is "non-toxic". Lower toxicity doesn't mean safe to swallow.
Heat transfer differences
At the same concentration, ethylene glycol mixes carry heat a little better than propylene glycol mixes, and PG mixes are more viscous, especially when cold, so pumps work slightly harder. That's why most cars use EG, while PG is chosen where lower toxicity matters, such as some data-center loops, food processing and potable-adjacent systems. If you switch a vehicle to PG coolant, use one that meets your vehicle's spec, and remember that refractometers need a PG scale.
Can you mix EG and PG coolants?
The glycols themselves mix, but the coolant's inhibitor technology matters more. Mixing also makes concentration testing harder, since refractometer scales differ. If you're changing from EG to PG, flush thoroughly first.
Check your mix ratio's freeze protection
Open calculator →Toxicity in context
The ASPCA and the Merck Veterinary Manual both describe propylene glycol antifreeze as having a wider safety margin than ethylene glycol, though it's not harmless. Merck also notes that PG lacks the sweet taste of EG, so animals may be less attracted to it. Either way, store and dispose of coolant carefully.
Switching a car to PG coolant
- Check that a PG coolant meeting your vehicle's spec exists, and that your manual permits it.
- Flush thoroughly to remove the EG coolant.
- Fill with the PG coolant at the recommended ratio.
- Label the reservoir "PG coolant" so future top-ups match.
- Use the PG scale on refractometers.
Where each glycol is used
| Application | Usual glycol |
|---|---|
| Cars and light trucks | EG (most) |
| Heavy-duty engines | EG or PG (both covered by ASTM D6210) |
| RV/marine plumbing | PG (non-toxic products) |
| Data-center TCS | PG (e.g. PG25) |
| Food and beverage chillers | PG |
Testing PG vs EG
A refractometer with both scales lets you test either. If you don't know which glycol is in the system, the label or service record is the only reliable answer.
More questions answered
Is PG safe for pets?
Less toxic than EG, but not safe to drink.
Does PG cost more?
Pricing varies. We don't publish prices without a current source.
Reader questions
Which glycol is in my car?
Most car coolants are EG. Check the label.
Key point to remember
Use the glycol your vehicle or system specifies, and test it on the right scale.
In practice
In practice, most drivers never need to choose: the car's spec decides. The choice matters most for RV, marine, food-processing and data-center systems, where lower toxicity can outweigh the slight heat-transfer advantage of ethylene glycol. Label containers clearly so the two aren't mixed by accident.
Next step
Check your label's glycol type.
One more tip
Label converted systems.
Frequently asked questions
Is propylene glycol coolant better than ethylene glycol?
Neither is better overall. PG is less toxic, while EG gives slightly more freeze protection at the same concentration. Use what your system specifies.
Is ethylene glycol coolant toxic?
Yes. It is poisonous if swallowed. Keep it away from children and pets.
Can I mix ethylene and propylene glycol?
It's best avoided. It confuses concentration readings and may mix incompatible inhibitors.
What glycol is used in data-center cooling?
Usually propylene glycol, as PG25 (about 25% PG in water with inhibitors).
Related guides
Sources
- Engineering ToolBox: Ethylene glycol heat-transfer fluid properties
- Engineering ToolBox: Propylene glycol heat-transfer fluids
- PubChem: Ethylene glycol
- ASTM D3306: Glycol base engine coolant for automobile and light-duty service
- ASPCA: Dangers of antifreeze and your pets
- Merck Veterinary Manual: Ethylene glycol poisoning