Is the Heat Index Calculator free to use?

Yes, our Heat Index Calculator is completely free to use with no registration required.

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Heat Index Calculator

Calculate the heat index (feels like temperature) based on air temperature and relative humidity or dew point. Understand heat danger levels, health risks, and safety tips using the National Weather Service Rothfusz regression formula.

Air Temperature (°F)

Relative Humidity (%)

Quick Calculate

Heat Index

100°F

Feels Like

Danger Level

Extreme Caution

Health Risk

Heat cramps & exhaustion possible

Heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.

Heat Index Formula (NWS Rothfusz Regression)

HI = -42.379 + 2.0490T + 10.1433R - 0.2248TR - 0.00684T² - 0.0548R² + 0.00123T²R + 0.000853TR² - 0.00000199T²R²

Where T = air temperature (°F) and R = relative humidity (%)

This formula is used by the National Weather Service (NWS) and is valid for temperatures at or above 80°F (27°C) with relative humidity of 40% or more.

Potential Effects of Heat Index

Celsius	Fahrenheit	Effects
27-32°C	80-90°F	Caution — Fatigue with prolonged exposure. Heat cramps possible.
32-41°C	90-105°F	Extreme Caution — Heat cramps and heat exhaustion possible. Heat stroke possible with continued activity.
41-54°C	105-130°F	Danger — Heat cramps and heat exhaustion likely. Heat stroke probable with continued activity.
Over 54°C	Over 130°F	Extreme Danger — Heat stroke imminent.

Heat Safety Tips

Stay Hydrated

Drink water regularly, even if you do not feel thirsty. Thirst is a late sign of dehydration.

Limit Outdoor Activity

Schedule strenuous activities for early morning or evening when heat index values are lower.

Wear Appropriate Clothing

Choose lightweight, light-colored, loose-fitting clothing. Use sunscreen with SPF 30 or higher.

Take Breaks

Rest frequently in shaded or air-conditioned areas. Monitor yourself and others for signs of heat-related illness.

Never Leave Children or Pets in Vehicles

Car interior temperatures can rise 20°F (11°C) in just 10 minutes, even with windows cracked.

Heat Index Chart (°F)

Temp (°F)	40%	45%	50%	55%	60%	65%	70%	75%	80%	85%	90%	95%
80°F	80	80	81	81	82	82	83	84	84	85	86	88
85°F	84	85	86	88	89	91	93	95	97	99	102	105
90°F	91	92	95	97	100	103	106	109	113	117	122	127
95°F	99	102	105	109	113	118	123	128	134+	140+	147+	154+
100°F	109	114	118	124	129	136+	143+	150+	158+	167+	176+	185+
105°F	121	127	134+	141+	149+	157+	166+	176+	187+	198+	209+	221+
110°F	136+	143+	152+	161+	171+	182+	194+	206+	219+	233+	247+	262+
115°F	152+	162+	173+	184+	197+	210+	224+	239+	255+	272+	289+	308+
120°F	170+	182+	196+	210+	225+	241+	258+	276+	295+	315+	336+	358+

Color coding: black = safe, yellow = caution, orange = extreme caution/danger, red = extreme danger. Exposure to full sunshine can increase heat index values by up to 14°F (8°C).

How to Use

1
Select your unit system — Imperial (°F) or Metric (°C)
2
Choose your input mode: enter either relative humidity (%) or dew point temperature
3
Enter the current air temperature — heat index is only valid for temperatures at or above 80°F (27°C)
4
If using relative humidity mode, enter the humidity percentage (0-100%)
5
If using dew point mode, enter the dew point temperature — the calculator converts it to relative humidity automatically
6
View your heat index temperature, danger level, and health risk assessment
7
Use the quick calculate buttons for common hot weather scenarios
8
Understanding the key inputs — heat index measures how hot it feels when humidity is factored in. High humidity prevents sweat from evaporating efficiently, reducing your body's ability to cool itself
9
Interpreting the results — the danger level color coding helps you quickly assess outdoor safety. Exposure to full sunshine can increase heat index values by up to 14°F (8°C)
10
Check the heat index chart below for a comprehensive overview across a range of temperatures and humidity levels to plan outdoor activities safely

Examples

Good Examples

Hot summer day with moderate humidity

90°F air temperature with 60% relative humidity → heat index of 100°F — Extreme Caution: heat cramps and heat exhaustion possible

Humid tropical conditions

95°F with 80% humidity → heat index of 131°F — Extreme Danger: heat stroke is imminent. Seek air conditioning immediately

Mild warm day, safe for outdoor exercise

82°F with 45% humidity → heat index of 83°F — Caution: generally safe with adequate hydration

Using dew point to calculate feels-like temperature

88°F with dew point of 72°F → relative humidity ≈ 60% → heat index of 97°F — good for assessing conditions from weather reports that show dew point

Comparing conditions for outdoor workers

Monday: 88°F with 50% humidity (HI 93°F). Tuesday: 88°F with 80% humidity (HI 105°F). Tuesday is significantly more dangerous despite the same air temperature

Planning outdoor activity in desert heat

105°F with 20% humidity → heat index ≈ 106°F. Despite low humidity, extreme heat still poses a danger — drink plenty of water and limit sun exposure

Bad Examples

Using heat index for cool temperatures

Heat index formula is not valid below 80°F (27°C) — at cooler temperatures, humidity does not significantly affect how hot it feels

Ignoring heat index and only looking at air temperature

An 88°F day with 80% humidity feels like 105°F — dressing and hydrating for 88°F alone puts you at risk of heat exhaustion

Confusing heat index with actual temperature

A heat index of 110°F does not mean the air is 110°F — it measures perceived temperature based on your body's ability to cool itself through evaporation

Forgetting that shade and wind reduce effective heat index

The standard heat index assumes shady, light-wind conditions. Direct sunlight can add up to 14°F to the value, while a breeze helps with cooling

Common Mistakes

Using heat index below 80°F (27°C) — the formula only applies at hot temperatures where humidity impedes the body's cooling through sweat evaporation
Confusing heat index temperature with actual air temperature — heat index is a "feels like" measure that accounts for humidity's effect on body cooling
Ignoring heat index when planning outdoor activities — air temperature alone does not reflect the true danger of hot weather, especially in humid climates
Not increasing water intake in high heat index conditions — dehydration risk increases dramatically when humidity is high because sweat does not evaporate as efficiently
Assuming heat index only matters in summer — heat index can be dangerously high in tropical climates year-round, and in industrial settings with high humidity
Forgetting that direct sunshine increases heat index by up to 14°F — the standard heat index is calculated for shade conditions; full sun exposure makes it significantly worse
Not accounting for heat index when exercising — physical activity generates additional body heat, making the effective heat index even higher during exercise
Using dew point and relative humidity interchangeably without conversion — dew point is a more stable measure of moisture in the air, while relative humidity varies with temperature throughout the day

Frequently Asked Questions

What is the heat index formula?

The National Weather Service heat index formula (Rothfusz regression) is: $\text{HI} = -42.379 + 2.0490T + 10.1433R - 0.2248TR - 0.00684T^2 - 0.0548R^2 + 0.00123T^2R + 0.000853TR^2 - 0.00000199T^2R^2$ , where $T$ is temperature in °F and $R$ is relative humidity in %. This formula was adopted by the NWS and is valid for temperatures above 80°F with humidity of 40% or more.

At what heat index temperature does heat stroke occur?

Heat stroke risk increases significantly when the heat index exceeds 125°F (52°C), which the NWS classifies as "Extreme Danger." However, heat-related illness can begin at lower values — heat cramps and exhaustion are possible above 90°F, and likely above 103°F. Children, elderly, and those with chronic conditions are at higher risk even at lower heat index values.

Does direct sunlight increase the heat index?

Yes. The NWS heat index is calculated for shady, light-wind conditions. Exposure to direct sunshine can increase heat index values by up to 14°F (8°C). If you are planning outdoor activities in full sun, add approximately 14°F to the calculated heat index for a more accurate picture of the perceived temperature.

What is the difference between heat index and wind chill?

Heat index measures how hot it feels in summer when humidity slows the body's ability to cool itself through sweating. Wind chill measures how cold it feels in winter when wind accelerates heat loss from exposed skin. Both provide a "feels like" temperature that is more useful than the actual air temperature for assessing comfort and safety.

How does humidity affect the heat index?

Higher humidity increases the heat index because moisture in the air reduces the evaporation rate of sweat — your body's primary cooling mechanism. At 90°F with 40% humidity, the heat index is about 91°F, but at 90°F with 90% humidity, the heat index jumps to approximately 122°F.

How do I convert dew point to relative humidity for heat index?

Dew point and relative humidity are related but measure moisture differently. The Magnus formula converts between them: given temperature $T$ and dew point $T_d$ (both in °C), $\text{RH} = 100 \times \exp\left(\frac{a \cdot T_d}{b + T_d} - \frac{a \cdot T}{b + T}\right)$ where $a = 17.27$ and $b = 237.7$ . Our calculator does this conversion automatically when you select dew point mode.

How does the heat index affect outdoor exercise?

High heat index values significantly increase the risk of heat-related illness during exercise. When the heat index exceeds 90°F, reduce exercise intensity, take frequent breaks, and drink water every 15-20 minutes. Above 105°F, outdoor exercise should be avoided entirely. Watch for symptoms like dizziness, nausea, or excessive sweating.

What is the difference between heat index and wet bulb temperature?

Heat index uses air temperature and relative humidity to estimate perceived temperature. Wet bulb temperature measures the lowest temperature that can be achieved through evaporative cooling and accounts for humidity and temperature together. Wet bulb temperature is often used for industrial and military applications, while heat index is the standard for public weather forecasts.

At what heat index should outdoor activities be canceled?

The NWS recommends modifying outdoor activities when the heat index reaches 90°F and canceling strenuous outdoor activities when it exceeds 105°F. For children and elderly, activities should be modified at lower thresholds. Many schools and sports organizations use 105°F as the cutoff for canceling outdoor practices and events.

How to stay safe on high heat index days?

Stay hydrated by drinking water regularly (even before you feel thirsty), schedule outdoor activities for early morning or evening, wear lightweight and light-colored clothing, take frequent breaks in shade or air conditioning, and watch for signs of heat exhaustion. The CDC recommends never leaving children or pets in vehicles, as interior temperatures can rise 20°F in just 10 minutes.