Coolant Mixture Calculator
Work out how much antifreeze and how much water your system needs. If the mix is already wrong, find out how much to drain and replace to fix it.
Your cooling system
- Figures are for ethylene glycol. Propylene glycol protects slightly less at the same percentage.
- Always use distilled water. Tap water deposits scale and accelerates corrosion in aluminium.
- Never mix coolant chemistries. Match what the manufacturer specifies, by specification not by colour.
Why 50/50 and not more
Pure antifreeze is worse at its job than a mixture. Water carries heat better than glycol does, so a system running straight concentrate actually runs hotter. Glycol brings freeze protection and a higher boiling point, water brings heat transfer, and 50/50 is the compromise that has held up for decades.
Going to 60% buys you another 28 degrees of freeze protection and costs you some cooling capacity. Past 70% the freeze point starts climbing back up, which surprises people.
The floor is 33%
Antifreeze is not just glycol. It carries the corrosion inhibitor package that protects the water pump seal, the heater core and every aluminium surface in the engine. Dilute below about a third and there is not enough of that left, regardless of what the climate demands.
Fixing a mix that is already wrong
You cannot add your way from 30% to 50% by pouring in concentrate, because the system is already full. Something has to come out first.
Drain a measured amount, replace it with the same volume of concentrate, and the maths works out. Enter your current percentage above and it tells you how much. Test with a refractometer rather than one of the floating ball hydrometers, which are close to useless.
Frequently asked questions
Can I just use straight antifreeze?
No. Straight glycol transfers heat worse than a mixture, so the engine runs hotter, and the freeze point is actually higher than a 60% mix. It also costs more. There is no situation where 100% concentrate is the right answer in a road vehicle.
Does coolant colour tell me what it is?
No, and treating it as though it does causes real damage. Colour is dye, and different manufacturers use the same colour for completely different chemistries. Match the specification your manufacturer lists, such as a Dex-Cool or G-series number, not the colour in the bottle.
Why distilled water?
Tap water carries dissolved minerals that drop out as scale on the hottest surfaces in the engine, which is exactly where you need heat to move. It also brings chlorides that attack aluminium. Distilled water costs about a dollar a gallon and removes the problem.
How does a pressure cap raise the boiling point?
Raising the pressure raises the temperature at which the coolant boils, by roughly 3 degrees Fahrenheit per psi. A 15 psi cap on a 50/50 mix pushes the boiling point from around 226F to about 270F. A cap that has lost its seal takes that margin away, which is why a weak cap causes overheating that no amount of new coolant fixes.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.