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Coolant Mix Ratio Calculator — Freeze & Boil Protection

Find the right coolant-to-water mix for your cooling system with freeze protection, boil protection, and exact quart amounts

50/50 Mix

6.0 qt coolant

Water

6.0 qt

Freeze Point

-34°F

Boil Point

310°F

Leave at 0 for a fresh fill. Enter existing amounts if topping off.

Mix Recipe

6.0
Quarts Coolant
6.0
Quarts Water

Protection Range

-34°F
Freeze Protection
310°F
Boilover (w/ cap)

Without radiator cap: 265°F boilover

Mix Ratio Comparison

30/70−4°F freeze
40/60−12°F freeze
50/50−34°F freeze
60/40−62°F freeze
70/30−84°F freeze

All Ratios

RatioCoolantWaterFreezeBoil
30/703.6 qt8.4 qt-4°F235°F
40/604.8 qt7.2 qt-12°F245°F
50/506.0 qt6.0 qt-34°F265°F
60/407.2 qt4.8 qt-62°F276°F
70/308.4 qt3.6 qt-84°F287°F

Frequently Asked Questions

Q

What is the best coolant-to-water ratio?

50/50 coolant-to-water is the standard recommendation for most vehicles and climates. It provides freeze protection to -34°F and boil protection to 265°F (310°F with a 16 PSI radiator cap). Only use 60/40 or 70/30 in extreme cold climates.

  • 50/50: standard for most vehicles, freeze to -34°F, boil to 265°F
  • 60/40: cold climates, freeze to -62°F, slightly reduced heat transfer
  • 70/30: extreme cold only, freeze to -84°F, worst heat transfer
  • Never exceed 70% coolant — protection actually decreases above 70%
  • Never run pure water — no freeze protection and causes corrosion
RatioFreeze PointBoil PointBest For
30/70-4°F235°FMild climates only
50/50-34°F265°FMost vehicles (standard)
60/40-62°F276°FCold climates
70/30-84°F287°FExtreme cold only
Q

How many quarts of coolant does my car need?

Most 4-cylinder cars hold 6–8 quarts, V6 engines hold 10–14 quarts, and V8 trucks hold 14–18 quarts. For a 50/50 mix on a 12-quart system, you need 6 quarts of coolant and 6 quarts of distilled water.

  • 4-cylinder compact: 6–8 quarts total capacity
  • V6 sedan/SUV: 10–14 quarts total capacity
  • V8 truck/SUV: 14–18 quarts total capacity
  • Heavy-duty diesel: 20–28 quarts total capacity
  • Check owner’s manual for exact capacity — includes heater core and hoses
Vehicle TypeCapacity50/50 Coolant50/50 Water
4-cyl compact7 qt3.5 qt3.5 qt
V6 sedan12 qt6.0 qt6.0 qt
V8 truck16 qt8.0 qt8.0 qt
Diesel truck24 qt12.0 qt12.0 qt
Q

Can I use tap water instead of distilled water?

Always use distilled water. Tap water contains minerals (calcium, magnesium) that cause scale buildup inside the radiator and heater core, reducing cooling efficiency. Hard water can also accelerate corrosion of aluminum components.

  • Distilled water: pure H2O, no minerals, no scale buildup
  • Tap water: contains dissolved minerals that form scale deposits
  • Scale reduces heat transfer and can clog heater core passages
  • Hard water regions (150+ ppm) are worst for cooling systems
  • Pre-mixed coolant already uses deionized water — add nothing
Water TypeTDS (ppm)Scale RiskRecommendation
Distilled0–5NoneAlways use this
Soft tap50–150LowEmergency only
Hard tap150–300ModerateAvoid
Very hard300+HighNever use
Q

What is the difference between ethylene glycol and propylene glycol coolant?

Ethylene glycol is the standard automotive coolant with better heat transfer and lower cost. Propylene glycol is less toxic (pet-safe) but transfers heat about 5% less efficiently. Both provide similar freeze/boil protection at the same concentrations.

  • Ethylene glycol: better heat transfer, lower cost, toxic if ingested
  • Propylene glycol: pet-safe, food-grade available, 5% less efficient
  • Both have similar freeze and boil protection at same concentration
  • Never mix the two types in the same system
  • Most vehicles use ethylene glycol — check owner’s manual
PropertyEthylene GlycolPropylene Glycol
ToxicityHighly toxicLow toxicity (pet-safe)
Heat TransferBetter (baseline)~5% less efficient
Cost (per gallon)$12–$18$18–$28
Freeze (50/50)-34°F-28°F

Example Calculations

112-Quart System, 50/50 Fresh Fill

Inputs

System Capacity12 quarts
Target Ratio50/50
Existing FluidEmpty (fresh fill)

Result

Coolant Needed6.0 quarts
Water Needed6.0 quarts
Freeze Protection-34°F
Boil Protection310°F (with cap)

12 quarts × 50% = 6 quarts of each. The 50/50 mix protects from -34°F to 265°F open, or 310°F with a standard 16 PSI radiator cap.

216-Quart Truck, 60/40 for Cold Climate

Inputs

System Capacity16 quarts
Target Ratio60/40
Existing FluidEmpty (fresh fill)

Result

Coolant Needed9.6 quarts
Water Needed6.4 quarts
Freeze Protection-62°F
Boil Protection321°F (with cap)

16 × 60% = 9.6 quarts coolant, 16 × 40% = 6.4 quarts water. The 60/40 mix protects to -62°F, suitable for northern US and Canadian winters.

Formulas Used

Coolant Mix Volumes

Coolant (qt) = Capacity × Ratio%; Water (qt) = Capacity × (1 − Ratio%)

Calculates the volume of concentrated coolant and distilled water needed for a target mix ratio based on total system capacity.

Where:

Capacity= Total cooling system capacity in quarts
Ratio%= Coolant concentration as decimal (50% = 0.50)

Boil Protection with Cap

Boil Point (cap) = Base Boil Point + ~45°F

A standard 16 PSI radiator cap raises the boiling point by about 45°F above the atmospheric boiling point of the coolant mix.

Where:

Base Boil= Boiling point at atmospheric pressure for the mix ratio
45°F= Approximate boil point increase from 16 PSI cap pressure

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Last Updated: Mar 9, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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