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Need a tester for a real reading? A refractometer is more accurate than a float tester.
Check refractometer on Amazon Not sure which coolant you need?Quick answer
Use this coolant freeze and boil point calculator: 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.
Tool for this job: a coolant refractometer
A refractometer reads freeze protection from a single drop. Pick one with a scale for your glycol type (ethylene or propylene) and automatic temperature compensation.
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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.
On this page
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
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
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 |
Why not 100% antifreeze?
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
Related
How much coolant and water?
How much coolant and water?
Work out the concentrate and distilled water for a refill, or how much to drain to raise a weak mix, with the coolant mix calculator.
Freeze point vs burst point
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
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
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
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
Limitations
- Values are approximate engineering-table data
- Products vary, so label figures take priority
- Doesn't measure inhibitor condition
More tools and guides
More tools and guides
Related tool
About to top up with a different coolant?
Check compatibility →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.
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.
What to do next
Use the calculator with a measured concentration, and treat label figures as authoritative. Re-test after top-ups and before winter.
Test, calculate and adjust, then test again after driving.
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.
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.
Does salt water change these figures?
Don't use salt water in cooling systems.
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.
How we source this page
- Facts come from manufacturer pages, datasheets, owner's manuals and standards bodies, linked under Sources.
- We do not test products, and no brand pays to be listed or ranked. Advertising and affiliate links are labelled and never change the data.
- Anything we could not find is marked "not published" rather than guessed. Spotted an error? Tell us.