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About this tool
Work out coolant and water litres for your radiator capacity, with the freeze point and cap-pressure boil point of the mix.
The Coolant Mix Ratio Calculator converts a cooling-system capacity and a target glycol percentage into exact litres of coolant and distilled water, then reads the resulting freeze point and boil point off the published glycol/water property curves. It also adds the boiling headroom your radiator cap contributes, calculated from the water saturation temperature at the raised pressure. Use it when refilling after a flush, when moving a car to a colder region, or when checking whether an existing mix is strong enough.
Open Coolant Mix Ratio Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Freeze and boil points are interpolated from the published glycol/water tables, not guessed.
Shows the extra boiling headroom the radiator cap gives, calculated from saturation temperature.
Handles both undiluted concentrate and ready-to-use 50:50 premix, and flags impossible targets.
A 50:50 mix by volume is the normal specification: it freezes at about -37 °C and boils at about 108 °C before the cap is considered. Manufacturers generally allow anywhere from 33% to 60% glycol, but not below 33%, because the corrosion inhibitor package is then too dilute to protect aluminium and gaskets.
No — neat ethylene glycol is worse in both directions. Freeze protection peaks near 68% glycol and then reverses, so pure concentrate freezes at only about -13 °C, and undiluted glycol carries heat away much more slowly than a mix, so the engine actually runs hotter.
Use distilled or deionised water. The calcium and magnesium in tap water form scale on the hottest surfaces inside the head and radiator, and the dissolved chlorides accelerate corrosion, which is exactly what the inhibitor package is there to prevent.
A 1.1 bar (about 16 psi) cap raises the boiling point by roughly 22 °C, which takes a 50:50 mix from about 108 °C to around 130 °C. This is why a failed or wrongly rated cap shows up as boiling over even when the coolant itself is fine.
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