How much antifreeze concentrate or 50/50 premix and water to use, or how much to drain and refill, to reach a glycol percentage or a freeze protection in degrees F.
Coolant Mix Calculator - Antifreeze to Water Ratio
How much antifreeze concentrate or 50/50 premix and water to use, or how much to drain and refill, to reach a glycol percentage or a freeze protection in degrees F.
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Coolant mix ratio for a fresh fill or a correction
Tell the coolant mix calculator how much the cooling system holds, whether it is empty or full, what the tester reads now and what you want: a glycol percentage or the coldest temperature to survive. It returns how much concentrate or 50/50 premix and how much distilled water to use, or how much to drain and refill, in gallons, quarts or liters. Freezing and boiling points come from Dow's published table for ethylene glycol by volume, in °F and °C.
Inputs, top to bottom
- Capacity and unit - the total cooling system capacity from the owner's manual, including the heater core and the reservoir, in gallons, quarts or liters.
- System state - empty (drained and flushed, or a new engine) or full of coolant you want to strengthen or weaken.
- Current glycol - for a full system only: the reading of a refractometer or a coolant tester, or 0 if it holds plain water.
- What you add - full-strength concentrate or 50/50 premix. Premix can never take the mix above 50%.
- Target - a glycol percentage from 10 to 57.8%, or a protection temperature in °F, from which the calculator finds the percentage.
Before and after one drain and refill
The quick start system, measured before and after the change.
Before: 30% glycol
Freezes at 3.5 °F (-15.9 °C). Boils at 220 °F at sea level without a pressure cap.
After: 50% glycol
Freezes at -35.1 °F (-37.2 °C), boils at 226 °F. The cost: 1.00 gal of concentrate and one gallon of old coolant to dispose of.
The same system full of plain water (a flush) needs 1.75 gal drained and replaced. Starting from empty it would be 1.75 gal of concentrate and 1.75 gal of distilled water.
Ethylene glycol freezing and boiling points by volume
Rows straight from the Dow DOWTHERM SR-1 product sheet (typical values, boiling at 101 kPa). The calculator interpolates between them. Dow recommends picking a row at least 5 °F (3 °C) colder than the lowest temperature you expect.
| Glycol, vol % | Freezes, °F | Freezes, °C | Boils, °F |
|---|---|---|---|
| 18.1 | 17.9 | -7.8 | 216 |
| 27.7 | 6.7 | -14.1 | 220 |
| 32.6 | -0.2 | -17.9 | 221 |
| 37.5 | -8.1 | -22.3 | 222 |
| 42.5 | -17.5 | -27.5 | 224 |
| 47.6 | -28.9 | -33.8 | 225 |
| 52.7 | -42.0 | -41.1 | 227 |
| 57.8 | -54.9 | -48.3 | 230 |
Between 47.6 and 57.8% each extra point of glycol buys about 2.5 °F of protection, while the boiling point rises only 5 °F over the same span.
How much to drain to reach 50%
For a full 3.5 gal system, refilled with concentrate. Drain = capacity x (target - current) / (100 - current).
| Tester reads now | Freezes now | Drain and add concentrate |
|---|---|---|
| 0% (water) | 32.0 °F | 1.75 gal |
| 20% | 15.8 °F | 1.31 gal |
| 25% | 10.1 °F | 1.17 gal |
| 30% | 3.5 °F | 1.00 gal |
| 35% | -4.1 °F | 0.81 gal |
| 40% | -12.8 °F | 0.58 gal |
| 45% | -23.1 °F | 0.32 gal |
Concentrate or premix: what each saves
Take the same 3.5 gal system at 25% and a target of 40%. With concentrate you drain and replace 0.70 gal. With premix it takes 2.10 gal, three times as much liquid through the system, because each gallon of premix brings only half a gallon of glycol.
Premix wins when you start from empty and want exactly 50%: no measuring, no separate water. Concentrate wins every time you correct a full system, and it is the only way above 50%. If the tester reads too strong, the fix is water, not more coolant: from 65% to 50% the same system needs 0.81 gal drained and replaced with distilled water.
Coolant mix questions
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