Wind Chill & Heat Index Calculator

The formulas

Both numbers use the official National Weather Service equations. Heat index combines temperature and humidity to estimate how hot it feels in the shade — sweat evaporates less in humid air, so your body can't cool itself. Direct sun can add up to 15°F more. Wind chill estimates how cold exposed skin feels as wind strips away body heat, and how fast frostbite can set in.

On the trail

• In heat, start at first light, rest in shade and size your water with the water calculator. See also how to stay cool at camp.
• In cold wind, a windproof shell does more than another fleece. Size your sleep system with the sleeping bag guide and read cold weather camping.

Frequently asked questions

What heat index is too hot to hike?

The National Weather Service treats a heat index of 103°F and above as dangerous, with heat exhaustion and heat stroke likely during prolonged exertion. Many hikers avoid strenuous hiking when it's above 90°F.

At what wind chill does frostbite occur?

At a wind chill around -18°F, frostbite can develop on exposed skin in about 30 minutes. At about -32°F it can take 10 minutes, and at -48°F five minutes or less.

Does wind chill affect water bottles or tents?

No. Wind chill describes heat loss from skin. Objects only cool to the actual air temperature, though wind makes them get there faster.