Quick answer
Premix (50/50) is ready to pour and best for top-ups. Concentrate must be diluted, usually 50/50 with distilled or deionized water, and suits full refills or cold climates where you want to adjust the ratio. Both work if they meet your car's spec. The difference is convenience, control and cost per mixed litre.
Key takeaways
- Premix is pre-diluted. Never add more water to it.
- Concentrate must be mixed. 50/50 is the most common ratio. Follow the label.
- Use distilled or deionized water for mixing, not tap water.
- A 50% ethylene glycol mix freezes at about −37 °C (−34 °F). More glycol helps only up to roughly 60%.
- Compare cost per mixed litre or gallon, not per bottle.
Premix vs concentrate at a glance
| Premix (50/50 or ready-to-use) | Concentrate | |
|---|---|---|
| What it is | Coolant already diluted with purified water | Glycol plus inhibitors, undiluted |
| Use | Pour straight in | Dilute before use (or as the label directs) |
| Best for | Top-ups, convenience, avoiding mistakes | Full refills, adjusting the ratio for very cold climates |
| Water quality | Handled by the maker | Your job: distilled or deionized water |
| Cost per mixed volume | Usually higher | Often lower, since you supply the water |
| Risk of error | Low | Wrong ratio if not measured |
Some coolants are sold in other premix ratios, such as 40/60, aimed at particular markets or OEM specs. The principle is the same: use the coolant at the ratio its label and your manual specify.
How to mix concentrate correctly
- Check the label and your manual for the recommended ratio. 50/50 by volume is common.
- Use a clean, marked container. Measure equal volumes of concentrate and distilled water for 50/50.
- Mix before adding to the vehicle. Don't pour concentrate and water in separately and hope they blend.
- Fill to the cold-full mark, then bleed any air if you've done a refill.
- Confirm the result with a refractometer or test strips.
Freeze and boil protection by ratio
For ethylene glycol and water at atmospheric pressure, published tables give these freezing points [1]:
| EG % by volume | Freezing point | Boiling point (1 atm) |
|---|---|---|
| 30% | −13.7 °C (7.3 °F) | 104.4 °C (220 °F) |
| 40% | −23.5 °C (−10.3 °F) | 104.4 °C (220 °F) |
| 50% | −36.8 °C (−34.2 °F) | 107.2 °C (225 °F) |
| 60% | −52.8 °C (−63 °F) | 111.1 °C (232 °F) |
| 100% (pure) | −12.8 °C (9 °F) | 197 °C (386 °F) |
Two things stand out. Pure glycol actually freezes at a higher temperature than a 50–60% mix, so never run straight concentrate. And a car's pressurized cooling system raises the real boiling point above these 1-atm figures. Try any ratio in the calculator.
Freeze & boil point calculator
Interpolated from published glycol/water tables at 1 atm [EG] [PG]. Commercial coolants and pressurised systems differ.
Why distilled or deionized water?
Tap water varies by region and can contain dissolved minerals and chlorine. Minerals can drop out as scale on hot surfaces, and chlorine is corrosive. Engineering references recommend distilled or deionized water for glycol solutions [1]. Distilled water is cheap and widely available. Use it whenever you dilute concentrate. Read more in can you use water as coolant?.
Cost per mixed litre: how to compare
Bottle prices mislead, because a bottle of concentrate makes roughly twice its volume at 50/50. To compare fairly:
- Concentrate: price ÷ (bottle volume × 2) at 50/50, plus the cost of distilled water.
- Premix: price ÷ bottle volume.
We don't publish typical prices because we have no independent, current source. See coolant price factors.
Check your mix ratio's freeze protection
Open calculator →Which should you buy?
- Topping up a little: premix of the correct spec.
- Full drain and refill: concentrate plus distilled water, or enough premix for your system capacity.
- Very cold climate: concentrate, so you can set the ratio within the manual's limits.
- No measuring kit or water to hand: premix.
Common mistakes
- Adding water to premix. It dilutes protection below the intended ratio.
- Running 100% concentrate. Pure glycol freezes around −13 °C and transfers heat poorly.
- Using tap water. Minerals and chlorine can cause scale and corrosion.
- Mixing in the radiator. Pre-mix in a container for an accurate ratio.
- Buying the wrong chemistry. Premix or concentrate, it still has to match your spec. See which coolant your car needs.
Buying a coolant (premix or concentrate): what to look for
- Meets your manual's spec
- Premix for top-ups; concentrate for refills
- Same technology as what's already in the system
Specific product recommendations are not published yet. We only list products we have checked against published specs.
Mixing concentrate: a worked example
- You need 6 litres of 50/50 for a refill.
- Measure 3 litres of concentrate and 3 litres of distilled water into a clean container.
- Mix, then pour.
- After running and cooling, test the strength. Water trapped in the engine may have diluted it.
- Correct it with a little more concentrate if needed.
Always follow the ratio on the label and in your manual. 50/50 is common but not universal.
Shelf life and storage
Unopened concentrate and premix keep well when sealed and stored away from heat. Check the label for any stated shelf life. Once opened, close tightly and store away from children and pets.
Short checklist
- Ratio known
- Distilled water
- Clean container
When to see a mechanic
See a mechanic promptly if…
- your coolant tests weak even after topping up with premix
- you see overheating, or coolant loss with no visible leak
- you're unsure what's in the system and need a proper flush
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
Is 50/50 coolant ready to use?
Yes. 50/50 premix is pre-diluted and goes straight in.
Can I add water to premix coolant?
Not as a habit. It dilutes freeze and corrosion protection. Use premix for top-ups.
What water should I mix with concentrated coolant?
Distilled or deionized water. Tap water can contain minerals and chlorine.
Is concentrate cheaper than premix?
Often per mixed litre, because you supply the water. Compare price per mixed volume.
Can I use 100% antifreeze for better protection?
No. Pure ethylene glycol freezes around −13 °C (9 °F), higher than a 50–60% mix, and transfers heat poorly.
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.