Antifreeze mix ratio calculator
The antifreeze mix ratio sets the freeze point: a 50/50 mix of concentrate and water protects to about −29 °F according to the manufacturer table, a 1:2 mix only to about 1 °F. Find the freeze point of your mix, the share needed for a target temperature and how much to drain when topping up.
Example: Calculate: Freeze point of a mix · Concentrate 5 · Water 5 · Volume unit: Quarts (US) · Temperature unit: °F → Freeze protection: 50 % concentrate: protects down to about -29.3 °F. Source: Dow: DOWTHERM SR-1 (96 wt% ethylene glycol + inhibitors), Product Information – table „Typical Freezing and Boiling Points of DOWTHERM SR-1 Fluid“ (vol% SR-1 → freezing point, 0 to 60.5 vol%; 65.8 to 77.1 vol% only „below -50°C“; 82.6 vol% → −46.8 °C, 99.5 vol% → −19.4 °C) and note „at least 3°C (5°F) lower than the expected lowest ambient temperature“; copy hosted by Universidad de Sevilla (read 10.10.2026). Updated: .
How it is calculated
The freeze point depends on the share of concentrate by volume. Share = concentrate ÷ (concentrate + water) × 100. For that share the calculator reads the freeze point from Dow’s table for a concentrate of 96 % ethylene glycol plus corrosion inhibitors (DOWTHERM SR-1) and interpolates linearly between two rows. The results are estimates: Dow calls them “typical numbers only”, not specifications, and other antifreeze brands may differ.
Antifreeze chart for a 10-quart system
| Protects down to | Share of concentrate | Concentrate in 10 qt |
|---|---|---|
| 0 °F | 33.9 % | 3.39 qt |
| −20 °F | 45.7 % | 4.57 qt |
| −30 °F | 50.3 % | 5.03 qt |
| −34 °F | 51.9 % | 5.19 qt |
| −40 °F | 54.4 % | 5.44 qt |
Common mixes: 50/50 gives about −34.1 °C (−29.3 °F), one part concentrate to two parts water (33.3 %) about −17.3 °C (0.8 °F). Dow advises picking a temperature at least 3 °C (5 °F) below the lowest temperature you expect.
More is not better
Exact values in the table go up to 60.5 % concentrate with −48.3 °C (−54.9 °F); for 65.8 to 77.1 % Dow only states “below −50 °C (−60 °F)”. After that the freeze point rises again: according to Dow, 82.6 % freezes at −46.8 °C and almost pure concentrate (99.5 %) already at −19.4 °C (−3 °F). Never fill concentrate undiluted.
Making weak coolant stronger
If protection is too weak, drain part of the coolant and add the same amount of concentrate. Amount to drain = capacity × (new share − old share) ÷ (100 − old share). Example: 10 qt testing at 0 °F (33.9 %), wanted −30 °F (50.3 %): drain 2.47 qt and replace it with concentrate. Then run the engine and test again.
Safety
- MedlinePlus describes ethylene glycol as colorless, odorless and sweet-tasting, and poisonous if swallowed; most ethylene glycol poisonings come from antifreeze. Catch drained coolant, keep containers closed and do not leave it standing open.
- If someone swallows it, get medical help or call Poison Help (1-800-222-1222) right away; do not make the person throw up unless poison control or a health care provider tells you to.
- Your owner’s manual says which coolant type your car needs; do not mix types without approval.
For other mixing jobs use the dilution calculator; to switch between °F and °C use Fahrenheit to Celsius.
Frequently asked questions
What antifreeze mix ratio protects to −20 °F?
About 45.7 % concentrate according to the Dow table – 4.57 quarts of concentrate in a 10-quart system. With the recommended 5 °F margin (−25 °F) it is about 48 %.
How cold can a 50/50 mix go?
Equal parts concentrate and water (50 %) freeze at about −34 °C, or −29 °F, in the manufacturer table.
Can I use more concentrate for more protection?
According to Dow the freeze point stays below −50 °C (−60 °F) up to about 77 % concentrate – the table gives no exact values there – and then rises again: 82.6 % freezes at −46.8 °C, undiluted concentrate at −19.4 °C (−3 °F). This calculator works up to 60.5 %.
How much coolant should I drain to raise protection?
Capacity × (new share − old share) ÷ (100 − old share). From 0 °F to −30 °F in a 10-quart system you drain 2.47 quarts and add the same amount of concentrate.
Does this chart work for every antifreeze?
It applies to an ethylene glycol concentrate with 96 % glycol. Premixed 50/50 coolant, propylene glycol and concentrates with a different glycol content have other values – follow the bottle label.
Sources and legal basis
- Dow: DOWTHERM SR-1 (96 wt% ethylene glycol + inhibitors), Product Information – table „Typical Freezing and Boiling Points of DOWTHERM SR-1 Fluid“ (vol% SR-1 → freezing point, 0 to 60.5 vol%; 65.8 to 77.1 vol% only „below -50°C“; 82.6 vol% → −46.8 °C, 99.5 vol% → −19.4 °C) and note „at least 3°C (5°F) lower than the expected lowest ambient temperature“; copy hosted by Universidad de Sevilla (read 10.10.2026)
- MedlinePlus (U.S. National Library of Medicine): Ethylene glycol poisoning – „colorless, odorless, sweet-tasting chemical. It is poisonous if swallowed“, „Most ethylene glycol poisonings occur due to the ingestion of antifreeze“ (read 10.10.2026)
- MedlinePlus: Antifreeze poisoning – toxic agents ethylene glycol, methanol, propylene glycol; seek medical help immediately, do not induce vomiting (read 10.10.2026)
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