Antifreeze and screenwash in sub-zero temperatures: what mix you need
Two fluids do most of the work of protecting your car from freezing: the antifreeze in the cooling system and the screenwash in the washer bottle. Get either wrong in a cold snap and you could be looking at a cracked engine block or a windscreen you cannot clear.
This guide explains what each fluid does, how to check them and the right mix for UK winters.
Last updated 9 November 2026. Always follow your car’s handbook and the instructions on the product.
Key takeaways
- Antifreeze (coolant) stops the water in your engine’s cooling system freezing in winter and boiling in summer.
- Most cars use a 50:50 mix of antifreeze and water, which typically protects to well below any UK winter temperature.
- Use the type of coolant your manufacturer specifies. Mixing types can cause problems.
- Plain water in the washer bottle freezes at 0C. Use a screenwash concentrate at the strength shown for the temperatures you expect.
- Never open the coolant cap on a hot engine.
In this guide
- What antifreeze does
- What antifreeze mix do you need?
- Which coolant does your car need?
- How to check your coolant
- Screenwash: why water is not enough
- How to top up screenwash
- Frequently asked questions
- Sources and further reading
What antifreeze does
Your engine’s cooling system is filled with coolant: a mix of antifreeze (usually based on ethylene glycol) and water. Antifreeze lowers the freezing point and raises the boiling point of the mixture. It also contains corrosion inhibitors that protect the radiator, water pump and engine.
If the coolant is too weak and freezes, it expands. That can split hoses, crack the radiator and, in the worst cases, crack the engine block. Ice can also stop coolant circulating, so the engine overheats even on a cold day.
Read more in our guide to the importance of engine coolant.
What antifreeze mix do you need?
For most cars, the standard is a 50:50 mix of antifreeze and water. Check the bottle for the exact protection level, but a 50:50 mix typically protects to around minus 35C, which is far colder than UK winters get.
Using more antifreeze than water does not always give more protection, so stick to the ratio in your handbook. Many garages and shops sell ready-mixed coolant, which takes the guesswork out.
Which coolant does your car need?
Coolants come in different chemical types (often described as OAT, HOAT or IAT, and sometimes by colour). Colour is not a reliable guide, so check your handbook or ask a parts shop for the coolant that matches your car’s specification.
Mixing incompatible coolants can reduce corrosion protection and, in some cases, cause a sludge that blocks the system. If you are not sure what is in there, use a universal coolant that says it is compatible, or ask a garage to check and change it.
How to check your coolant
- Wait until the engine is cold. The cooling system is pressurised when hot and can spray scalding liquid.
- Find the coolant expansion tank under the bonnet. It is usually a translucent plastic bottle with MIN and MAX marks.
- Check the level is between MIN and MAX.
- Top up if needed with the correct pre-mixed coolant.
- Check the strength. An antifreeze tester (hydrometer or refractometer) costs a few pounds and shows the protection level. Garages can also test it during a service.
If you need to top up the coolant often, there may be a leak, so get it checked.
Screenwash: why water is not enough
Winter roads are covered in salt and grime that smears across your windscreen. A working washer system is essential, and the Highway Code requires you to keep your windscreen clean.
Plain water freezes at 0C, and can freeze in the bottle, pipes and jets overnight. A good screenwash concentrate lowers the freezing point and cuts through salt and grime. Most bottles show the mix needed for different temperatures, for example a stronger ratio for minus 10C than for minus 5C.
In winter, mix to the strength recommended for the coldest temperatures you expect, or use a ready-mixed winter screenwash.
How to top up screenwash
- Find the washer bottle under the bonnet. The cap usually has a windscreen and water symbol.
- Mix the concentrate and water to the winter strength on the bottle, or use ready-mixed winter screenwash.
- Pour it in until it is near the top.
- Run the washers for a few seconds to draw the new mix through the pipes.
Never put coolant in the washer bottle, or washing-up liquid, which can leave streaks and damage paint. See our step-by-step guide on how to top up your screenwash.
Frequently asked questions
How often should antifreeze be changed?
It depends on the car and the coolant type. Many modern coolants last several years. Check your service schedule, and ask the garage to test the strength at your winter service.
Can I just top up with water?
In an emergency you can add a little water to get you going, but it dilutes the antifreeze. Get the mix checked and corrected before a cold snap.
My washers have frozen. What do I do?
Run the engine with the heater on the windscreen and the jets often thaw. Our guide to fixing frozen doors, locks and washer jets has more.