Quick answer
Enter your glycol type and concentration to estimate the freeze and boil points. For example, a 50% ethylene glycol mix freezes at about −37 °C and boils at about 107 °C at atmospheric pressure (higher under a pressure cap). Values are approximate, from published engineering tables. Product labels take priority.
Key takeaways
- 50/50 EG is the common automotive target.
- PG freezes at slightly warmer temperatures than EG at the same concentration.
- Above about 60–70% glycol, freeze protection gets worse, not better.
- Pressure caps raise the boiling point further.
- Measure your actual mix with a refractometer.
How to use the calculator
- Choose ethylene glycol (most car coolants) or propylene glycol.
- Enter the concentration by volume (a refractometer gives this).
- Read the estimated freeze and boil points.
- Compare with your coldest expected temperature.
Ethylene glycol reference table
| EG % | Freeze °C | Boil °C (1 atm) |
|---|---|---|
| 0 | 0 | 100.0 |
| 10 | -3.4 | 101.1 |
| 20 | -7.9 | 102.2 |
| 30 | -13.7 | 104.4 |
| 40 | -23.5 | 104.4 |
| 50 | -36.8 | 107.2 |
| 60 | -52.8 | 111.1 |
Propylene glycol reference table
| PG % | Freeze °C | Boil °C (1 atm) |
|---|---|---|
| 0 | 0 | 100.0 |
| 10 | -3 | 100.0 |
| 19 | -8 | 100.6 |
| 29 | -14 | 102.2 |
| 40 | -22 | 103.9 |
| 50 | -34 | 105.6 |
| 60 | -48 | 107.2 |
Freeze & boil point calculator
Interpolated from published glycol/water tables at 1 atm [EG] [PG]. Commercial coolants and pressurised systems differ.
Why not 100% antifreeze?
Pure glycol has a higher freeze point than a 60% mix, carries heat worse, and costs more. See premix vs concentrate.
Related
Freeze point vs burst point
The calculator reports the freeze (slush) point. Below it, ice crystals form. Glycol mixes don't freeze solid at that temperature, so the point at which pipes or blocks would burst is lower. Plumbing antifreezes market burst ratings for this reason. For engines, use the freeze point and keep a margin below your coldest expected temperature.
How pressure changes the boiling point
The boiling points shown are at atmospheric pressure. A cooling system's pressure cap raises the boiling point substantially. Ford, for example, quotes boil protection to 129 °C for its prediluted 50/50 coolant in a pressurized system. A weak cap or a leak lowers it.
Ethylene vs propylene glycol results
At equal concentrations, PG mixes in the tables freeze at slightly warmer temperatures than EG mixes. If you switch glycol types, re-check your target concentration and use the matching refractometer scale.
Worked example
- A refractometer reads 35% EG.
- The table shows about −13.7 °C at 30% and −23.5 °C at 40%, so roughly −18 °C.
- If winter lows reach −30 °C, raise the mix toward 50% (about −37 °C).
- Use the mixing checker and the premix guide to adjust.
Limitations
- Values are approximate engineering-table data
- Products vary, so label figures take priority
- Doesn't measure inhibitor condition
More tools and guides
- Coolant mixing checker
- DC coolant comparator
- Premix 50/50 vs concentrate
- Water instead of coolant?
- How often to change coolant
Choosing a target for your climate
| Coldest expected temperature | Common choice (check your manual) |
|---|---|
| Above about −10 °C | 50/50 is still typical, for corrosion and boil protection |
| Down to about −35 °C | 50/50 EG |
| Colder | Up to the maximum your coolant label allows (often about 60–70%) |
Most carmakers specify 50/50 regardless of climate because the inhibitor package is designed for it. Don't go below about 40% glycol unless your manual allows it.
About to top up with a different coolant?
Check compatibility →Data-center fluids
The calculator's PG table also helps when reading PG25 datasheets: suppliers such as Shell and Dow quote about −10 °C for PG25. Always use the supplier figure for the actual product.
Frequently overlooked
- Water left in the block after draining dilutes concentrate
- Top-ups with water slowly weaken the mix
- Hydrometers can misread PG coolant
Frequently confused terms
| Term | Meaning |
|---|---|
| Freeze point | Ice crystals start to form (slush) |
| Burst point | Lower; pipes or blocks at risk of rupture |
| Boil point (1 atm) | Boiling at sea-level pressure without a cap |
| Boil-over protection | With the system pressurized by its cap |
Using the result
- Note the estimate.
- Compare it with your coldest expected temperature minus a safety margin.
- If it's too weak, use the mixing guide to strengthen it.
- Re-test after driving, so the coolant is fully mixed.
Why 50/50 is the default
A 50/50 mix gives a good balance of freeze protection, boil-over margin, heat transfer and inhibitor concentration. ASTM D3306 notes concentrates are generally used at 40–60%. Going stronger trades away heat transfer for little benefit in most climates.
Altitude and boiling
At higher altitude, atmospheric pressure is lower, so open-vessel boiling points fall. A pressurized cooling system still adds its cap pressure, but the margin shrinks a little. If you drive in mountains in hot weather, keep the mix and cap in good condition.
Checking the calculator against a label
If your coolant label lists freeze and boil protection, those figures win. For example, Ford quotes −34 °F (−37 °C) freeze and 265 °F (129 °C) boil protection for its prediluted Motorcraft Orange, the latter in a pressurized system.
More questions answered
Is 70% glycol better than 50%?
Generally not. Freeze protection stops improving and heat transfer gets worse.
Does the calculator work for RV antifreeze?
No. Use the product's burst rating.
Why do EG and PG values differ?
They're different molecules with different mixing properties.
Can I trust a hydrometer reading here?
A refractometer is more reliable, especially for PG.
Reader questions
Does salt water change these figures?
Don't use salt water in cooling systems.
Key point to remember
Use the calculator with a measured concentration, and treat label figures as authoritative. Re-test after top-ups and before winter.
In practice
In practice, test, calculate and adjust, then test again after driving.
Next step
If your result is weaker than your target, read our premix vs concentrate guide to correct the mix. If it's stronger than about 60–70%, dilute with distilled water and retest.
Frequently asked questions
What is the freezing point of 50/50 coolant?
About −37 °C (−34 °F) for ethylene glycol. Check the product label.
Does more antifreeze mean better protection?
Only up to about 60–70%. Beyond that, the freeze point rises.
New to cooling systems? Start with how a car cooling system works.