Calculators · Ownership

Coolant mixture and engine oil top-up calculator

Engine coolant is a mix of antifreeze concentrate and water, and the ratio decides how well it protects against freezing, boiling and corrosion — usually 50/50. These calculators give the quantities for a fresh fill, how much old coolant to drain and replace with concentrate to strengthen a weak mix, and how much oil to add from a dipstick reading. Always use the coolant type and oil grade in your vehicle handbook.

Coolant quantities for a fresh fill

Splits the cooling system capacity into antifreeze concentrate and water for the chosen strength. Use distilled or deionised water; tap water can leave scale. Skip this if you buy ready-mixed coolant.

Results
Concentrate—Undiluted antifreeze to add.
Water—Distilled or deionised water to add.

Formula
concentrate = capacity × % / 100
water = capacity − concentrate

Strengthening a weak coolant mix

If a tester shows the coolant is too weak, drain some of it and top up with pure concentrate. This calculates exactly how much to drain to reach the target strength in one step.

Results
Drain and replace with concentrate—Remove this much coolant, then refill with the same amount of pure concentrate.

Formula
drain = capacity × (target − current) / (100 − current)

Engine oil to add from the dipstick

The distance between the minimum and maximum marks on a dipstick equals a fixed volume, often about one litre. From where the oil sits between the marks, this gives how much to add. Add a little less, wait a few minutes and recheck, rather than overfill.

Results
Oil to add—Amount needed to reach the MAX mark.

Formula
add = span × (100 − level) / 100

Worked example: an 8-litre cooling system

  1. A 50% fill needs 4 L of concentrate and 4 L of distilled water.
  2. If the tester reads only 30%, drain 8 × 20 / 70 = 2.3 L and refill with pure concentrate to reach 50%.
  3. With oil 40% of the way from MIN to MAX on a 1-litre dipstick span, add about 0.6 L.

Typical freeze protection of ethylene-glycol coolant (check the product label)

ConcentrationProtection down to about
33%−18 °C
40%−25 °C
50%−37 °C
60%−52 °C

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About these results

Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.

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