Heat Index Calculator
Use this heat index calculator to find the real feels-like temperature from air temperature and humidity, using the official National Weather Service formula and danger scale. Switch tabs to work out wind chill with frostbite times, or dew point and comfort level.
- Official NWS formulas
- °F and °C
- Danger levels and safety tips
- Free, private, no sign-up
Use the shade temperature from a weather report. The heat index assumes shade and a light breeze.
Feels like
100°F
Extreme caution
Use the sustained wind speed at about 5 feet (1.5 m), as given in most forecasts.
Feels like
-19°F
High risk
The dew point is the temperature at which air becomes saturated and dew forms. It is the best single measure of how humid it feels.
Dew point
65°F
Muggy
A heat index calculator built for real decisions
Most heat index calculators give you one number. This one tells you what the number means for your body and covers cold and humidity too.
Three calculators in one
Heat index, wind chill and dew point share one clean interface, so you can check summer heat or winter cold in seconds.
Official formulas
Uses the NWS Rothfusz regression with its humidity adjustments, and the 2001 NWS and Environment Canada wind chill formula.
Danger scale and safety tips
A color-coded gauge shows exactly where you sit on the risk scale, with clear advice for each level.
Frostbite times
Wind chill results include how quickly exposed skin can freeze, based on the official wind chill risk categories.
Humidity or dew point
Enter relative humidity or dew point, whichever your weather app shows. The calculator converts between them.
Private and instant
No sign-up and no tracking. Results update as you drag the sliders, in °F or °C.
How to use the heat index calculator
Grab the current temperature and humidity from any weather app.
- Choose your unitsPick °F with mph, or °C with km/h. Values convert automatically.
- Enter the air temperatureType it in or drag the slider. Use the shade temperature, not a sunny thermometer.
- Add humidity or dew pointUse relative humidity, or switch to dew point if that is what your forecast shows.
- Read the feels-like temperatureCheck the danger level on the gauge and follow the safety advice.
- Check wind chill or dew pointSwitch tabs for cold weather or to see how humid it really feels.
How the heat index is calculated
When the air is humid, sweat evaporates more slowly, so your body loses heat less efficiently. The heat index puts a number on that effect. The National Weather Service first uses a simple formula, and if the result is 80°F or higher it switches to the full Rothfusz regression.
Small corrections apply when humidity is below 13% at 80 to 112°F, or above 85% at 80 to 87°F. This calculator applies both automatically.
Worked example: 90°F at 60% humidity
- Air temperature
- 90°F (32°C)
- Relative humidity
- 60%
- Simple formula check
- above 80°F
- Rothfusz regression
- 99.7°F
- Heat index
- 100°F (38°C), extreme caution
Heat index chart
Find the air temperature across the top and the relative humidity down the side. Colors match the NWS risk levels.
| RH | 80°F27°C | 84°F29°C | 88°F31°C | 92°F33°C | 96°F36°C | 100°F38°C | 104°F40°C | 108°F42°C | 112°F44°C |
|---|---|---|---|---|---|---|---|---|---|
| 40% | 80 | 83 | 88 | 94 | 101 | 109 | 119 | 130 | 137+ |
| 50% | 81 | 85 | 91 | 99 | 108 | 118 | 131 | 137+ | 137+ |
| 60% | 82 | 88 | 95 | 105 | 116 | 129 | 137+ | 137+ | 137+ |
| 70% | 83 | 90 | 100 | 112 | 126 | 137+ | 137+ | 137+ | 137+ |
| 80% | 84 | 94 | 106 | 121 | 137+ | 137+ | 137+ | 137+ | 137+ |
| 90% | 86 | 98 | 113 | 131 | 137+ | 137+ | 137+ | 137+ | 137+ |
| 100% | 89 | 104 | 121 | 137+ | 137+ | 137+ | 137+ | 137+ | 137+ |
Heat index danger levels
| Heat index | Level | Possible effects with prolonged exposure or activity |
|---|---|---|
| 80 to 90°F (27 to 32°C) | Caution | Fatigue is possible. |
| 91 to 103°F (33 to 39°C) | Extreme caution | Heat cramps and heat exhaustion are possible. |
| 104 to 124°F (40 to 51°C) | Danger | Heat cramps and heat exhaustion are likely; heat stroke is possible. |
| 125°F+ (52°C+) | Extreme danger | Heat stroke is highly likely. |
Values assume shade. Full sunshine can make it feel up to 15°F (8°C) hotter. To stay safe, use the water intake calculator to plan hydration, and size your air conditioner with the BTU calculator.
How wind chill is calculated
Wind strips heat away from exposed skin, so it feels colder than the air temperature. The current formula, adopted by the US and Canada in 2001, models heat loss from the face at walking speed.
For example, 0°F with a 15 mph wind gives a wind chill of −19°F (−28°C), where exposed skin can freeze in 10 to 30 minutes.
Wind chill chart
| Wind | 40°F4°C | 30°F−1°C | 20°F−7°C | 10°F−12°C | 0°F−18°C | −10°F−23°C | −20°F−29°C | −30°F−34°C | −40°F−40°C |
|---|---|---|---|---|---|---|---|---|---|
| 5 mph8 km/h | 36 | 25 | 13 | 1 | −11 | −22 | −34 | −46 | −57 |
| 10 mph16 km/h | 34 | 21 | 9 | −4 | −16 | −28 | −41 | −53 | −66 |
| 15 mph24 km/h | 32 | 19 | 6 | −7 | −19 | −32 | −45 | −58 | −71 |
| 20 mph32 km/h | 30 | 17 | 4 | −9 | −22 | −35 | −48 | −61 | −74 |
| 25 mph40 km/h | 29 | 16 | 3 | −11 | −24 | −37 | −51 | −64 | −78 |
| 30 mph48 km/h | 28 | 15 | 1 | −12 | −26 | −39 | −53 | −67 | −80 |
| 40 mph64 km/h | 27 | 13 | −1 | −15 | −29 | −43 | −57 | −71 | −84 |
| 50 mph80 km/h | 26 | 12 | −3 | −17 | −31 | −45 | −60 | −74 | −88 |
| 60 mph97 km/h | 25 | 10 | −4 | −19 | −33 | −48 | −62 | −76 | −91 |
Frostbite risk by wind chill
| Wind chill | Risk | Exposed skin can freeze in |
|---|---|---|
| 0 to −9°C (32 to 16°F) | Low | Unlikely |
| −10 to −27°C (14 to −17°F) | Moderate | Hypothermia risk with long exposure |
| −28 to −39°C (−18 to −38°F) | High | 10 to 30 minutes |
| −40 to −47°C (−40 to −53°F) | Very high | 5 to 10 minutes |
| −48 to −54°C (−54 to −65°F) | Severe | 2 to 5 minutes |
| −55°C and colder (−67°F) | Extreme | Under 2 minutes |
Dew point and comfort
Relative humidity changes as the temperature rises and falls during the day, so it can be misleading. The dew point stays steadier and tells you how much moisture is actually in the air. The calculator uses the Magnus formula, accurate to within a fraction of a degree in normal weather.
| Dew point | How it feels |
|---|---|
| Below 55°F (13°C) | Dry to comfortable |
| 55 to 59°F (13 to 15°C) | Comfortable, slightly humid |
| 60 to 64°F (16 to 18°C) | Sticky |
| 65 to 69°F (18 to 21°C) | Muggy and uncomfortable |
| 70 to 74°F (21 to 24°C) | Oppressive |
| 75°F (24°C) and above | Miserable, heat illness more likely |
For example, 86°F (30°C) at 50% humidity has a dew point of about 65°F (18°C), which most people find muggy. Need a quick unit swap? Try the Celsius to Fahrenheit converter.
Why check the feels-like temperature
The thermometer only tells half the story. Here is how this calculator helps.
Plan safe workouts. Move runs and training to cooler hours when the heat index climbs.
Protect outdoor workers. Schedule breaks, water and shade based on real heat stress.
Look after vulnerable people. Children, older adults and pets feel heat and cold faster.
Dress for winter. Know when exposed skin is at risk and how long you can safely stay out.
Understand humidity. Dew point shows why the same temperature can feel fine one day and awful the next.
Learn the science. Formulas, charts and worked examples make it ideal for students and teachers.
Heat index calculator FAQs
Quick answers about feels-like temperature, wind chill and dew point.
The heat index, or feels-like temperature, combines air temperature and relative humidity to show how hot it feels to the human body. Humid air slows the evaporation of sweat, so your body cools less efficiently and it feels hotter than the thermometer reads.
The National Weather Service rates a heat index of 80 to 90°F as caution, 91 to 103°F as extreme caution, 104 to 124°F as danger and 125°F or higher as extreme danger. Heat stroke becomes possible in the danger range, especially during physical activity.
Yes. Heat index values assume shady, light-wind conditions. Direct sunshine can make it feel up to 15°F (8°C) hotter, so take extra care in full sun.
Wind chill uses the 2001 formula adopted by the US and Canada: 35.74 + 0.6215T - 35.75V^0.16 + 0.4275T x V^0.16, with T in °F and V in mph. It applies at 50°F or colder with wind of at least 3 mph. For example, 0°F with a 15 mph wind feels like -19°F.
On the official wind chill scale, exposed skin can freeze in 10 to 30 minutes at a wind chill of -28°C (about -18°F), in 5 to 10 minutes at -40°C, in 2 to 5 minutes at -48°C, and in under 2 minutes at -55°C or colder.
Most people feel comfortable when the dew point is below about 55°F (13°C). From 60 to 64°F it starts to feel sticky, 65 to 69°F feels muggy and 70°F or above feels oppressive.
Relative humidity compares the moisture in the air with the maximum it can hold at that temperature, so it rises and falls as the temperature changes. Dew point is the temperature at which the air would become saturated, which makes it a better measure of how humid it actually feels.
The full NWS formula is designed for 80°F (27°C) and above. Below that, the calculator uses the simpler NWS formula, and the result stays close to the actual air temperature.
Heat stroke is a medical emergency. Warning signs include a body temperature of 103°F (39.4°C) or higher, hot red skin, confusion and a fast pulse. Call emergency services, move the person somewhere cool and cool them with water while you wait.
Know how hot it really feels before you head out
Check the heat index, wind chill or dew point in seconds and get clear advice for every level of risk.
Estimates for general guidance only. Always follow official heat and cold warnings from your local weather service. Heat stroke and severe frostbite are medical emergencies: call your local emergency number.