Quick answer
Use this coolant mix calculator to work out how much concentrate and distilled water to use for a fresh fill, or how much old coolant to drain and replace to bring a weak mix up to strength. It also estimates the freeze point of the result.
Key takeaways
- Fresh fill: concentrate = capacity × target %.
- Strengthening: drain V × (target − current) ÷ (100 − current), refill with concentrate.
- Measure your current mix with a refractometer.
- Your coolant label and manual override estimates.
Calculator
Coolant mix calculator
Assumes a full-strength (100%) concentrate. Freeze points are interpolated from published glycol/water tables [EG] [PG]. Your coolant label overrides these estimates.
The formulas
- Fresh fill: concentrate = V × t ÷ 100, water = V − concentrate.
- Raise strength (current p%, target t%): drain and replace x = V × (t − p) ÷ (100 − p) with concentrate. With 50/50 premix instead: x = V × (t − p) ÷ (50 − p), which only works for targets below 50%.
- Dilute: drain and replace V × (p − t) ÷ p with distilled water.
These are simple mass-balance formulas. Real systems never drain completely, so test again after a run with the heater on.
Measure the mix first
Tool for this job: a coolant refractometer
A refractometer reads freeze protection from a single drop. Pick one with a scale for your glycol type (ethylene or propylene) and automatic temperature compensation.
- Robinair 75240 coolant & battery refractometer Check price on Amazon
- 4-in-1 antifreeze refractometer Check price on Amazon
Examples of the tool type, not a ranking or a test by us. Check the scale, adapters and fit before you buy.
As an Amazon Associate I earn from qualifying purchases. Commissions never change which products we list or what the data says.
A refractometer reads freeze protection from one drop; see coolant refractometers.
Check your mix ratio's freeze protection
Open calculator →Concentrates with a checked listing
| Product | Form | Codes listed | Buy |
|---|---|---|---|
| Prestone Platinum All Vehicles Antifreeze + Coolant Prestone · not published | Concentrate and Premix 50/50 | WSS-M97B51-A1, WSS-M97B55-A, WSS-M97B44-D | Check prices on Amazon |
| Prestone All Vehicles Antifreeze + Coolant (Universal) Prestone · not published | Concentrate and Premix 50/50 | No code listed | Check price on Amazon |
| PEAK All Vehicles Concentrate Antifreeze + Coolant PEAK · not published | Concentrate | No code listed | Check price on Amazon |
| PEAK DEX-COOL Concentrate Antifreeze + Coolant PEAK · OAT | Concentrate | GMW 3420, GM 6277M | Check price on Amazon |
| PEAK Global Lifetime Concentrate Antifreeze + Coolant PEAK · OAT | Concentrate | No code listed | Check price on Amazon |
| PEAK TITANIUM Concentrate Antifreeze + Coolant PEAK · not published | Concentrate | No code listed | Check price on Amazon |
Only buy a concentrate that lists your manual's code. Premix vs concentrate is explained in premix vs concentrate.
As an Amazon Associate I earn from qualifying purchases. Commissions never change which products we list or what the data says.
Typical targets
Most manuals call for about 50%. ASTM D3306 notes concentrates are generally used at 40–60%; Hyundai's manual says 35–60% and Mercedes-Benz sheet 310.1 says 55% at most. Above about 60–70% ethylene glycol, freeze protection gets worse, not better. Check freeze and boil points with the freeze & boil calculator.
Frequently asked questions
How much coolant does my car hold?
The capacity is in your owner's manual, usually under "capacities" or "specifications". A small car is often in single-digit litres; check yours.
Should I use tap water?
Use distilled or deionised water unless your manual says otherwise. Hyundai's manual, for example, allows only deionised, distilled or soft water.
New to cooling systems? Start with how a car cooling system works. Looking for a product? Search the coolant directory or match your manual's spec with Find my car coolant. See also coolant by car make and coolants by brand.
Tools for this job
4-in-1 antifreeze refractometer (EG and PG scales, ATC)
Reads freeze protection from one drop, on separate ethylene and propylene glycol scales.
Examples of the tool type, not a test or ranking by us. Check fit and instructions before you buy. See the full cooling system tools guide.
As an Amazon Associate I earn from qualifying purchases. Commissions never change which products we list or what the data says.