The temperature on the bag is usually the eutectic point, a lab number. The number that matters on a lot is the practical working temperature, and the gap between them is huge.1
Eutectic vs. practical
The eutectic temperature is the lowest temperature at which a deicer solution can stay liquid, reached only at one exact concentration. Salt brine at 23.3% sodium chloride won't freeze until about −6°F.1 On real pavement, though, the brine keeps getting diluted by the snow and ice it melts, so it starts refreezing at much warmer temperatures. That's why practical guidance puts rock salt's useful floor around 15°F, with 20°F and up preferred.1
| Deicer | Practical working temp | Eutectic (lab only) | At concentration |
|---|---|---|---|
| Sodium chloride (rock salt, salt brine) | 15°F (20°F+ preferred) | −6°F | 23.3% |
| Magnesium chloride | 0°F (10°F+ preferred) | −28°F | 21.6% |
| Calcium chloride | 0°F (10°F+ preferred) | −60°F | 29.8% |
| Calcium magnesium acetate (CMA) | 20°F | −17°F | 32.5% |
| Potassium acetate | 0°F | −76°F | 49% |
Practical temperatures from the Minnesota DOT Anti-Icing Guide. Your mileage varies with sun, traffic, snowfall rate and how much water the deicer has to work against.
What that means on the job
- Above about 20°F pavement: plain or treated rock salt does nearly everything, and it's the cheapest melting you can buy.
- 15°F to 20°F: plain salt slows down. Treated salt (pre-wetted with mag or calcium chloride) or pre-wetting at the spinner keeps it working.2
- Below 15°F: plain dry salt isn't recommended. Use treated blends, calcium or magnesium chloride for the critical spots, and sand for traction on the hazardous areas.3
- Below 0°F: chemistry gets expensive and slow. Plow clean, treat the walks, entrances and ramps that matter, and use abrasives for grip.3
Know your chlorides
| Deicer | Low-temp performance | Speed | Concrete | Plants | Metal | Cost |
|---|---|---|---|---|---|---|
| Sodium chloride (rock salt) NaCl | Works best above about 15°F pavement temperature; slows sharply below that | Moderate. Needs moisture to make brine before it melts | Low risk on cured, sound concrete. Repeated freeze-thaw can scale new (under 1 year) or poor-quality concrete | Harmful in quantity: chloride and sodium build up in soil along walks and lots | Corrosive to steel, rebar and vehicles | $ Lowest |
| Magnesium chloride MgCl₂ | Works into the single digits and below zero, commonly cited to around −5°F to −10°F | Fast. Gives off a little heat as it dissolves | Usually rated gentler on concrete than calcium chloride, but high concentrations can still react with cement paste | Generally less harmful to plants than rock salt at normal rates | Corrosive (a chloride), often sold with corrosion inhibitors | $$ Mid |
| Calcium chloride CaCl₂ | The cold specialist, commonly cited to around −20°F to −25°F | Fastest. Releases heat as it dissolves and pulls moisture from the air | Can contribute to scaling if overused; follow label rates | Can burn plants at high rates | Corrosive to metal; leaves a slick residue if overapplied | $$$ Highest |
| Potassium chloride KCl | Weak in real cold. Most useful above about 20°F–25°F | Slow | Low risk on sound concrete | Gentler on plants at normal rates (potassium is a plant nutrient), but still a chloride | Corrosive (a chloride), mildly | $$ Mid–high for what it does |
Next in the guideApplication rates that actually work →