Quick answer
Antifreeze is a glycol-based fluid, usually ethylene glycol, blended with corrosion inhibitors. Mixed with water, it becomes engine coolant. Coolant carries heat from the engine to the radiator, keeps the system from freezing in winter or boiling in summer, and protects the metals, seals and hoses from corrosion.
Key takeaways
- Antifreeze = glycol + corrosion inhibitors + anti-foam + dye.
- Coolant moves engine heat to the radiator and heater core.
- A 50% ethylene glycol mix freezes around −37 °C (−34 °F) and boils around 107 °C (225 °F) at 1 atm.
- Inhibitor chemistry (IAT, OAT, HOAT…) is what makes coolants differ.
- Ethylene glycol is toxic. Store and dispose of it carefully.
What antifreeze is made of
| Ingredient | Role |
|---|---|
| Glycol (ethylene or propylene) | Lowers the freezing point and raises the boiling point of water |
| Corrosion inhibitors | Protect aluminium, iron, copper, brass and solder; chemistry varies by coolant type |
| Buffers | Keep pH in a protective range |
| Anti-foam agents | Reduce foaming that hurts heat transfer |
| Dye | Identification and leak spotting (not a standard) |
| Bittering agent (some products) | Discourages accidental swallowing |
Ethylene glycol is the most common base for car coolant. Propylene glycol is less toxic and is used in some car coolants, RV plumbing antifreeze and data-center PG25 fluids. Compare them in ethylene vs propylene glycol.
What coolant does in a car
An engine turns only part of its fuel energy into motion. Much of the rest is heat, and it has to go somewhere. Coolant circulates through passages in the engine, picks up heat, and carries it to the radiator, where airflow removes it. A thermostat controls flow so the engine warms up quickly and then holds its operating temperature. A branch of the loop feeds the heater core that warms the cabin.
- Heat transfer: moves heat from the engine to the radiator.
- Freeze protection: stops the coolant freezing and cracking the block or radiator.
- Boil-over protection: together with the pressure cap, keeps coolant liquid at engine temperatures.
- Corrosion protection: inhibitors protect metals and seals.
- Lubrication of the pump seal: coolant additives help protect the water pump seal.
Freeze & boil point calculator
Interpolated from published glycol/water tables at 1 atm [EG] [PG]. Commercial coolants and pressurised systems differ.
How antifreeze lowers the freezing point
Dissolving glycol in water disrupts ice formation, so the mixture freezes at a lower temperature than either pure water or, over most of the range, pure glycol. Published tables for ethylene glycol show:
| EG % by volume | Freezing point |
|---|---|
| 30% | −13.7 °C (7 °F) |
| 40% | −23.5 °C (−10 °F) |
| 50% | −36.8 °C (−34 °F) |
| 60% | −52.8 °C (−63 °F) |
| 100% | −12.8 °C (9 °F) |
Source: Engineering ToolBox ethylene glycol tables [1].
Types of antifreeze
The glycol base is similar across products. The inhibitor package is where they differ:
| Type | Inhibitors |
|---|---|
| IAT | Inorganic (silicates, phosphates) |
| OAT | Organic acids |
| HOAT | Organic acids plus some silicate |
| P-HOAT | Organic acids plus phosphate |
| Si-OAT | Organic acids plus silicate |
Full details: OAT vs HOAT vs IAT. European "G" designations are covered in G11, G12, G13, G48 explained.
Safety: antifreeze is toxic
Ethylene glycol is poisonous if swallowed [2], and its sweet taste can attract children and pets. Store it sealed and out of reach, clean up spills, and dispose of used coolant properly. If someone may have swallowed it, see antifreeze poisoning for emergency steps.
Common misconceptions
- "Antifreeze is only for winter." It also prevents boiling and corrosion year-round.
- "More antifreeze is always better." Above roughly 60–70%, freeze protection gets worse.
- "Color tells you the type." It doesn't. Read the spec on the label.
Antifreeze beyond cars
Glycol antifreeze is also used in solar thermal collectors, HVAC chilled water loops, ice rinks and data-center liquid cooling. The Merck Veterinary Manual notes some of these heat-exchange fluids contain ethylene glycol, which matters for pet safety. Data-center loops typically use inhibited propylene glycol such as PG25. See the PG25 guide.
Bottom line
Antifreeze is glycol plus inhibitors. Choose it by your vehicle's specification.
Not sure which coolant your car takes?
Find your coolant →Antifreeze vs coolant on the shelf
In shops, "antifreeze" usually refers to concentrate and "coolant" to ready-to-use premix, but brands use the words interchangeably. The label tells you which it is: concentrate gives mixing instructions, premix says ready to use or 50/50.
Side-by-side: coolant vs antifreeze.
Why water is still the main ingredient
Water carries heat better than glycol. Glycol is added to lower the freeze point and raise the boiling point, and inhibitors protect the metals. That's why a 50/50 mix, not pure glycol, is the common target.
Common mistake
- Using pure antifreeze without water. Freeze protection worsens above about 60–70%.
More questions answered
Does antifreeze stop rust?
Its inhibitors help prevent corrosion while they last.
Reader questions
Is antifreeze the same in all countries?
Standards and naming vary by region. Check the label and spec.
Key point to remember
Glycol, water and inhibitors, matched to your vehicle's spec.
In practice
In practice, use premix for top-ups.
Antifreeze and the heater
Your cabin heater runs on hot coolant, so correct antifreeze strength and level also keep the heater working well.
When to see a mechanic
See a mechanic promptly if…
- coolant is low again soon after a top-up
- you smell sweet coolant in the cabin (see antifreeze smell)
- the engine runs hot
Specs referenced: ASTM D3306 (glycol-base engine coolant for automobile and light-duty service) and ASTM D6210 (fully formulated glycol-base coolant for heavy-duty engines). Your owner's manual and the OEM spec number take priority.
Frequently asked questions
What is antifreeze made of?
Mostly ethylene or propylene glycol, plus corrosion inhibitors, buffers, anti-foam additives and dye.
What does coolant do in a car?
It carries engine heat to the radiator, prevents freezing and boiling, and protects the system against corrosion.
Is antifreeze the same as coolant?
In everyday use, yes. Antifreeze usually means the concentrate, and coolant the diluted mix.
Is antifreeze poisonous?
Yes. Ethylene glycol is toxic if swallowed. Keep it away from children and pets.
New to cooling systems? Start with how a car cooling system works. Looking for a product? Search the coolant directory or match your manual's spec with Find my car coolant. See also coolant by car make and coolants by brand.