Freezing Point Calculator

Estimate the freezing point of salt water (brine) or an antifreeze and water mix based on its concentration.

How to use the Freezing Point Calculator

Choose salt water or antifreeze.
Enter the concentration in percent.
Read the freezing point in °C and °F.

How it works

Dissolved substances lower the freezing point of water — the reason roads are salted and engines use antifreeze. For salt water, the freezing point drops until about 23.3% salt, which freezes at −21.1 °C (−6 °F); that is why rock salt stops working in very cold weather. For ethylene glycol coolant, a 50/50 mix protects to about −37 °C (−34 °F). Values are interpolated from published tables.

Freezing points

Mixture	°C	°F
Pure water	0	32
Seawater (3.5% salt)	-1.9	28.6
10% salt	-6.6	20.1
20% salt	-16.5	2.3
30% antifreeze	-16	3.2
50% antifreeze	-37	-34.6
60% antifreeze	-52	-61.6

Frequently asked questions

At what temperature does salt water freeze?

Seawater freezes at about −1.9 °C (28.6 °F); saturated brine at about −21 °C (−6 °F).

What is the freezing point of 50/50 antifreeze?

About −37 °C (−34 °F).

Why not use 100% antifreeze?

Pure ethylene glycol freezes at about −13 °C and transfers heat poorly — a water mix works better.

Does salt melt ice at any temperature?

Rock salt becomes ineffective below about 15 °F (−9 °C) on roads.