Starlight Tools

Engine Coolant Mixture Calculator

Calculate antifreeze concentrate and water for a fresh fill, find how much mixed coolant to drain and replace, or check the concentration of two known quantities. All values stay in this browser tab.

Use the vehicle specification—not coolant colorOnly use coolant approved for the exact vehicle and follow its label, concentration range, water specification, filling, bleeding, and testing procedure. Never remove a coolant cap while the engine or cooling system is hot.

Enter the coolant mixture

Use one unit throughout; the ratio is unchanged.

Use the manual's total fill capacity.

Commonly 50%; permitted range varies.

Privacy: values are calculated locally. They are not uploaded, stored, or included in analytics events.

Fresh-fill amounts

Coolant mixture formulas

For a fresh fill, multiply system capacity by the target concentrate fraction:

concentrate = capacity × target % ÷ 100
water = capacity − concentrate

To strengthen an existing, fully mixed system by draining coolant and replacing the same volume with concentrate:

replace with concentrate = capacity × (target fraction − current fraction) ÷ (1 − current fraction)

To dilute it by replacing mixed coolant with water:

replace with water = capacity × (current fraction − target fraction) ÷ current fraction

The correction assumes the old coolant is homogeneous before draining, the drained amount has the current concentration, concentrate is treated as the product's full concentrate, and the system returns to the entered full capacity.

How the freeze-protection estimate works

Freeze point is not calculated as a straight average of water and pure glycol. This tool linearly interpolates between the published Zerex DEX-COOL Concentrate points below solely as an illustrative ethylene-glycol product reference:

40% → −24 °C   |   50% → −36 °C   |   60% → −48 °C   |   70% → −67 °C

Important: It does not extrapolate below 40% or above 70%. Coolant chemistry, concentrate formulation, product conventions, measurement method, and vehicle limits vary. Treat your exact coolant label, owner/service manual, and a compatible calibrated tester as authoritative.

How to plan a coolant fill or correction

  1. Identify the specified coolant. Match the vehicle's exact specification; color is not a reliable compatibility standard.
  2. Work only on a cold system. Follow the manual before opening any cap or draining coolant.
  3. Use reliable inputs. Get total capacity from service information and current concentration from the correct tester—not reservoir color or a guess.
  4. Calculate the planning volume. For an adjustment, the result is the amount of well-mixed old coolant to drain and replace with concentrate or water.
  5. Fill, bleed, and verify. Follow the manufacturer procedure, check for leaks and correct cold level, then retest concentration after circulation and full cool-down.

Engine coolant mixture FAQs

How much concentrate is needed for a 50/50 mix?

For a fully drained 12 L system, the simple planning amounts are 6 L concentrate and 6 L of the water specified by the coolant manufacturer. Residual flush water or trapped old coolant changes the real result, so test after filling.

Can I add concentrate without draining first?

Only when the system has the required free capacity and the vehicle procedure allows it. This calculator's adjustment mode holds total capacity constant, so it calculates a quantity of mixed coolant to drain before adding the same quantity of concentrate or water.

Why does correcting 30% to 50% require replacing more than 20% of capacity?

Because the drained liquid already contains 30% concentrate. Replacing 20% of capacity removes some antifreeze along with the water. The calculator accounts for that removed concentrate.

Is the displayed freeze point exact?

No. It interpolates a published ethylene-glycol DEX-COOL product table only from 40% through 70%. It is a reference estimate, not a measurement or a specification for another coolant.

Is more antifreeze always better?

No. Higher concentration is not universally safer and can reduce heat transfer or exceed the vehicle's permitted range. Many systems call for around 50%, but some specify a narrower or different range.

Can I mix different coolant colors or types?

Do not infer compatibility from color. Check the exact coolant specification and product documentation. Some vehicle manufacturers explicitly warn that mixing coolant types or colors can harm the engine or cooling-system components.

Are my values tracked?

No. The calculation runs locally in your browser and does not upload, store, or attach inputs to analytics events.

Limits and coolant-safety disclaimer

  • The calculator assumes additive volumes, a known total capacity, perfect mixing, and no trapped flush water, old coolant, air, leaks, overflow, or inaccessible fluid.
  • It does not determine coolant specification, compatibility, additive condition, service life, corrosion protection, boiling protection, cap pressure, water quality, or whether a cooling system needs repair.
  • The freeze estimate is not valid for propylene-glycol coolant, waterless coolant, another product's data, mixtures outside 40–70%, or coolant contaminated with other fluids.

Safety disclaimer: Hot pressurized coolant can cause severe burns. Work only as the vehicle procedure permits on a fully cooled system. Ethylene-glycol antifreeze is toxic if swallowed; keep it away from children and animals, clean spills promptly, use suitable protective equipment and containers, and recycle or dispose of used coolant under local rules. Seek urgent medical or poison-control help for suspected ingestion.

Methodology and sources

Last reviewed: August 2, 2026. Concentration guidance, the reference freeze table, compatibility limits, and health precautions were checked against manufacturer and U.S. government sources:

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