The thermometer says 90°F but it feels like 105. Why? Humidity. Just enter the temperature and humidity, and our free heat index calculator tells you exactly how hot it feels on your body. Uses the official National Weather Service formula so you can stay safe in the heat.
Modify the values and click the calculate button to use
| RH | 80° | 82° | 84° | 86° | 88° | 90° | 92° | 94° | 96° | 98° | 100° | 102° | 104° | 106° | 108° | 110° |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 40% | 80 | 81 | 83 | 85 | 88 | 91 | 94 | 97 | 101 | 105 | 109 | 114 | 119 | 124 | 130 | 136 |
| 45% | 80 | 82 | 84 | 87 | 89 | 92 | 96 | 100 | 104 | 109 | 114 | 119 | 124 | 130 | 137 | 143 |
| 50% | 81 | 83 | 85 | 88 | 91 | 95 | 99 | 103 | 108 | 113 | 118 | 124 | 131 | 137 | 144 | 152 |
| 55% | 81 | 84 | 86 | 89 | 93 | 97 | 101 | 106 | 112 | 117 | 124 | 130 | 137 | 145 | 153 | 161 |
| 60% | 82 | 84 | 88 | 91 | 95 | 100 | 105 | 110 | 116 | 123 | 129 | 137 | 145 | 153 | 162 | 171 |
| 65% | 82 | 85 | 89 | 93 | 98 | 103 | 108 | 114 | 121 | 128 | 136 | 144 | 153 | 162 | 172 | 182 |
| 70% | 83 | 86 | 90 | 95 | 100 | 106 | 112 | 119 | 126 | 134 | 143 | 152 | 161 | 172 | 182 | 194 |
| 75% | 84 | 88 | 92 | 97 | 103 | 109 | 116 | 124 | 132 | 141 | 150 | 160 | 171 | 182 | 193 | 206 |
| 80% | 84 | 89 | 94 | 100 | 106 | 113 | 121 | 129 | 138 | 148 | 158 | 169 | 181 | 193 | 205 | 219 |
| 85% | 85 | 90 | 96 | 102 | 110 | 117 | 126 | 135 | 145 | 155 | 167 | 179 | 191 | 204 | 218 | 233 |
| 90% | 86 | 92 | 98 | 105 | 113 | 122 | 131 | 141 | 152 | 164 | 176 | 189 | 202 | 216 | 231 | 247 |
| 95% | 88 | 94 | 101 | 109 | 117 | 127 | 137 | 148 | 160 | 172 | 185 | 199 | 214 | 229 | 245 | 262 |
| 100% | 89 | 96 | 104 | 112 | 121 | 132 | 143 | 155 | 168 | 181 | 195 | 210 | 226 | 243 | 260 | 278 |
Heat index is sometimes called the "feels like" temperature. It's what your body experiences when you combine air temperature with relative humidity.
Here's the thing: your body cools itself by sweating. When sweat evaporates, it takes heat away from your skin. But if the air is already full of moisture (high humidity), your sweat can't evaporate as easily. So you feel hotter than the actual temperature.
Think of it like this: You're in a 90°F room with a fan. You feel fine because the fan helps sweat evaporate. Now imagine that same 90°F room but it's super humid, like a steam room. You'd feel like you're melting. That's the heat index.
The National Weather Service created the heat index to help people understand the real danger of hot weather. It's not just about being uncomfortable. It's about staying alive.
Our calculator uses the official formula from the National Weather Service. You just need two numbers:
Air temperature
In Fahrenheit or Celsius
Relative humidity
As a percentage (%)
Plug those in, and boom. You get the heat index. It's that simple.
💡 Pro Tip: The formula only works for temperatures above 80°F (about 27°C) and humidity above 40%. Below that, the heat index is pretty close to the actual temperature. So don't bother calculating on a cool spring day.
90°F with 60% humidity. You might think, "It's hot, but I can handle it." But the heat index is 97°F. That's a big difference. Your body feels like it's almost 100°F. That's when you start to feel really uncomfortable.
85°F with 80% humidity. That sticky, gross beach day. The heat index jumps to 95°F. Even though the thermometer says 85, your body is working hard to cool down. You'll sweat buckets and feel exhausted.
100°F but only 20% humidity. The heat index is actually 96°F — lower than the actual temperature! Because the air is dry, your sweat evaporates easily. So you feel cooler than the thermometer says. That's why desert heat feels different than jungle heat.
Heat index isn't just a fun fact. It's a safety tool. When the heat index gets high, your body can overheat. That leads to heat exhaustion or even heat stroke. Both are serious.
These numbers are for the average healthy adult. Kids, elderly people, and pets are more sensitive. More on that in a bit.
Football practice in August? That's a recipe for heat illness. Coaches use heat index to decide if practice should be shorter, have more water breaks, or be canceled. Many schools have rules about this.
Working on a roof in July is brutal. Companies use heat index to schedule breaks and keep workers safe. Some jobs even stop when the heat index hits a certain number.
Is it safe to take the kids to the park? Parents check heat index to know when to stay indoors. Same goes for people caring for elderly relatives.
Humidity can ruin a good run. Checking the heat index helps you decide if you should run early morning, late evening, or skip it altogether.
Dogs can't sweat like humans. They cool down by panting. High heat index is extra dangerous for them. Pet owners use it to decide if it's safe to walk their dog.
Mistake 1: Thinking It's Just the Temperature
Some people think heat index is just a fancy name for temperature. Nope. It's a completely different number that accounts for humidity.
Mistake 2: Ignoring It on "Cooler" Days
Just because it's 85°F doesn't mean it's safe. With high humidity, that 85°F can feel like 95°F. Don't let the lower number fool you.
Mistake 3: Not Adjusting for Activity
Heat index tells you how it feels when you're just standing around. If you're exercising or working, your body generates more heat. So you need to be even more careful.
Mistake 4: Forgetting About Sunlight
The heat index is calculated for shade. If you're in direct sunlight, it can feel 10-15°F hotter. So add a few degrees if you're out in the sun.
You've probably heard of wind chill in the winter. It's the opposite of heat index. Wind chill tells you how cold it feels when you combine temperature with wind speed.
Heat index tells you how hot it feels when you combine temperature with humidity. So they're like cousins. Both help you understand what the weather actually feels like to your body.
Drink water before you feel thirsty. Thirst means you're already dehydrated.
Take breaks in the shade or air conditioning.
Wear light-colored, loose clothing. Dark colors absorb heat.
Avoid the hottest part of the day. That's usually 10 AM to 4 PM.
Know the signs of heat exhaustion: Heavy sweating, weakness, dizziness, nausea, headache.
Know the signs of heat stroke: Hot, red, dry skin; confusion; loss of consciousness. This is a medical emergency. Call 911.
Children's bodies heat up faster than adults. They also don't sweat as efficiently. So a heat index that's uncomfortable for you could be dangerous for them. Keep kids indoors when the heat index is above 90°F.
Older adults have a harder time regulating body temperature. They might not feel thirsty even when they're dehydrated. Check on elderly neighbors and relatives during heat waves.
Dogs and cats can overheat quickly. Never leave them in a car, even with the windows cracked. On high heat index days, keep walks short and avoid hot pavement. If it's too hot for your hand, it's too hot for their paws.
Heart disease, high blood pressure, and obesity can make heat more dangerous. Some medications also affect how your body handles heat. If you have a health condition, talk to your doctor about heat safety.
The heat index wasn't always a thing. In the 1970s, a scientist named Robert G. Steadman started working on a formula to measure how hot it really feels. He published his research in 1979. The National Weather Service adopted it in the 1980s.
Before that, people just guessed. "It feels hot" wasn't very helpful. Now we have a number that tells us exactly how dangerous the heat is. Pretty cool, right?
Fun fact: The heat index is sometimes called the "apparent temperature" or "feels like" temperature. Different weather services use slightly different formulas, but they all do the same thing: tell you how hot it feels.
Before going for a run or bike ride
Before starting outdoor work or yard work
Before taking kids to the playground
Before walking your dog
Before planning a picnic, barbecue, or outdoor event
During a heat wave to know when it's safe to go outside
When you're traveling to a different climate
The heat index is one of those things that seems complicated but is actually pretty simple. It's just a way to measure how hot it really feels. And it's super useful for staying safe in hot weather.
So next time you're about to head outside on a hot day, check the heat index first. It might change your plans. And that's a good thing.
Use our Heat Index Calculator anytime you need to know the real deal. It's free, it's fast, and it could keep you safe.
Heat index is how hot it actually feels to your body when you combine air temperature with humidity. Your body cools itself by sweating. When humidity is high, sweat doesn't evaporate as well, so you feel hotter than the thermometer says. It's like the "feels like" temperature you see on weather apps.
You can use our Heat Index Calculator. Just enter the air temperature and relative humidity. The calculator does the math for you using the official formula from the National Weather Service. It works best for temperatures above 80°F and humidity above 40%.
Actual temperature is what the thermometer reads. Heat index is what your body feels. For example, if it's 90°F with 60% humidity, the heat index is 97°F. So your body feels like it's 97°F, even though the thermometer says 90°F.
For most healthy adults, heat index above 90°F means be careful. Above 103°F is dangerous. Above 125°F is extremely dangerous. For kids, elderly people, and pets, any heat index above 90°F can be risky. Always listen to your body and take breaks.
Yes, high heat index can lead to heat stroke, which is a medical emergency. When your body can't cool down, your internal temperature rises. This can damage your organs and even be fatal. That's why it's so important to take heat warnings seriously and stay hydrated.
Heat index measures how hot it feels in summer. Wind chill measures how cold it feels in winter. Heat index uses temperature and humidity. Wind chill uses temperature and wind speed. They're both "feels like" numbers, just for different seasons.
It's all about humidity. In a dry place like Arizona, 85°F might feel comfortable because sweat evaporates easily. In a humid place like Florida, 85°F can feel like 95°F because sweat doesn't evaporate. Same temperature, totally different experience.
If the heat index is above 90°F, be careful — run early morning or late evening when it's cooler. Above 103°F, it's best to skip outdoor exercise entirely. Your body generates extra heat during exercise, making the effective heat index even higher.
Heat index is designed for outdoor shade conditions. Indoors with air conditioning, humidity is usually lower so the heat index doesn't apply the same way. But in non-air-conditioned spaces like garages or sheds, the heat index can still be dangerously high.
Different services may use slightly different formulas or humidity sources. The NWS formula is the standard in the US. Some apps use a simplified version. Our calculator uses the official NWS Rothfusz equation for the most accurate results.
High heat index at night makes it harder to fall asleep because your body needs to cool down to initiate sleep. The ideal sleeping temperature is 65-70°F. If the heat index is above 80°F at night, sleep quality drops significantly.
The highest heat index ever recorded was approximately 178°F in Dhahran, Saudi Arabia in July 2003, with an air temperature of 108°F and a dew point of 95°F. These conditions are unsurvivable for more than a few hours without artificial cooling.