---
title: "Can you mix dot 3 brake fluid and dot 4 brake fluid?"
description: "DOT 3 brake fluid and dot 4 brake fluid: yes, this is fine. What actually happens, and what to do instead."
url: https://atoneplace.net/can-you-mix/dot3-and-dot4-brake-fluid/
site: "At One Place"
---

# Can you mix dot 3 brake fluid and dot 4 brake fluid?

**DOT 3 brake fluid and DOT 4 brake fluid: Yes, this is fine**

Yes. Both are glycol-based and fully miscible.

## What actually happens

DOT 3, DOT 4 and DOT 5.1 are all polyglycol fluids and mix freely. Topping DOT 4 with DOT 3 lowers the boiling point of the mixture towards the DOT 3 figure, which matters for hard braking and not for a supermarket run, so it is acceptable in an emergency and worth flushing afterwards. Never the reverse assumption: a system specified for DOT 4 should get DOT 4.

## What to do instead

Match the specification on the reservoir cap. Brake fluid is hygroscopic and should be changed on time regardless of what is in it.

## Vehicle fluids

| Combination | Verdict | Why |
| --- | --- | --- |
| One engine coolant and a different coolant type | Do not | No — not across chemistries. The additives react and can gel. |
| One engine oil and a different brand or grade | Fine | Yes. Any two oils meeting the same specification mix safely. |
| Regular petrol and premium petrol | Fine | Yes. They blend, and the tank ends up somewhere between the two octane ratings. |
| DOT 5 brake fluid and dot 4 brake fluid | Do not | No. DOT 5 is silicone and is not compatible with any glycol fluid. |

## Common questions

### Can you mix dot 3 brake fluid and dot 4 brake fluid?

Yes, this is fine. Yes. Both are glycol-based and fully miscible.

### What happens if you do?

DOT 3, DOT 4 and DOT 5.1 are all polyglycol fluids and mix freely. Topping DOT 4 with DOT 3 lowers the boiling point of the mixture towards the DOT 3 figure, which matters for hard braking and not for a supermarket run, so it is acceptable in an emergency and worth flushing afterwards. Never the reverse assumption: a system specified for DOT 4 should get DOT 4.

### What should I do instead?

Match the specification on the reservoir cap. Brake fluid is hygroscopic and should be changed on time regardless of what is in it.

## Related pages

- [One engine coolant and a different coolant type](https://atoneplace.net/can-you-mix/engine-coolant-types/)
- [One engine oil and a different brand or grade](https://atoneplace.net/can-you-mix/engine-oil-brands/)
- [Regular petrol and premium petrol](https://atoneplace.net/can-you-mix/petrol-grades/)
- [DOT 5 brake fluid and dot 4 brake fluid](https://atoneplace.net/can-you-mix/dot5-and-dot4-brake-fluid/)
- [Bleach and ammonia](https://atoneplace.net/can-you-mix/bleach-and-ammonia/)
- [Bleach and vinegar](https://atoneplace.net/can-you-mix/bleach-and-vinegar/)
- [Bleach and toilet cleaner](https://atoneplace.net/can-you-mix/bleach-and-toilet-cleaner/)
- [Bleach and rubbing alcohol](https://atoneplace.net/can-you-mix/bleach-and-rubbing-alcohol/)
- [Hazard symbols](https://atoneplace.net/symbol/)

## Sources

- Official vendor product documentation — At One Place (/about/)