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Coolant Mix Calculator

Mix antifreeze and water to your target ratio

Result

4.50
3.00

How it is calculated

The tool splits the total volume by the concentration you want: antifreeze = capacity × target% ÷ 100 and water = capacity − antifreeze. The capacity can be in litres, quarts or gallons; the answers come out in the same unit, rounded to two decimals.

It assumes the antifreeze is undiluted full-strength concentrate and that you are filling an empty system with the whole capacity. The target must be between 0 and 100 and the capacity must be above zero. It does not account for coolant that stays in the engine after a partial drain, for pre-mixed 50/50 coolant (which is already half water) or for how much freeze protection a given percentage gives. Follow your vehicle maker's recommended concentration and use the water type they specify.

Worked example

A cooling system holding 7.5 litres, with a target of 60% antifreeze.

Antifreeze: 7.5 × 60 ÷ 100 = 4.50 litres. Water: 7.5 − 4.50 = 3.00 litres. Pouring in 4.5 litres of concentrate and 3 litres of water fills the system with a 60/40 mix. For 50% on the same capacity, the split would be 3.75 and 3.75.

Try this example →

Common mistakes

  • Pouring pre-mixed 50/50 coolant into the antifreeze figure. Pre-mix is already half water, so using the concentrate quantity would give a weaker mix than planned.
  • Entering the capacity of the radiator only. Use the total system volume including the engine block, heater and reservoir.
  • Forgetting the coolant left after a partial drain. If 3 litres of old coolant remain, the new quantities cover only the part you refill.
  • Mixing units, such as a capacity in gallons and a measuring jug in litres.
  • Assuming a higher percentage always protects more. Very strong mixes lose heat transfer and some freeze protection, so stay within the maker's recommendation.

About this calculator

This calculator works out how much antifreeze concentrate and water to mix for a target coolant ratio, by splitting your system's capacity at the antifreeze percentage you choose. For reference, with ethylene or propylene glycol antifreeze, a standard 50/50 mix protects to roughly −37°C (−34°F), while richer mixes protect against colder temperatures up to a point (pure, undiluted glycol actually protects less than a 60–70% mix, since water is needed for the chemistry to work and for heat transfer). Antifreeze is sold either as full-strength concentrate or already pre-mixed at 50/50; the amount shown here is full-strength concentrate (a pre-mixed product needs no extra water), so getting the dilution right matters for both the target ratio and the total volume the cooling system holds.

Mechanics and DIY car owners use this before a coolant flush or top-up to hit the manufacturer-recommended concentration for their climate, since too weak a mix risks freezing damage in winter, and — less intuitively — too strong a mix reduces heat transfer efficiency and can actually raise the freezing point again.

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Comments (1)

  • Editorial team

    The two results come back in whatever volume unit you typed for capacity, so litres in gives litres out. An 8 litre system at 50 percent, for instance, is 4.00 of concentrate and 4.00 of water. The percentage is the share of concentrate by volume; it does not tell you the freeze protection, which comes from the chart on the label of your coolant. What mix does your climate or your car's manual call for?

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