Heat Index Calculator — Free Online Apparent Temperature Tool

How to Use the Heat Index Calculator

1. Enter the air temperature: Type the current outdoor air temperature in degrees Fahrenheit into the Temperature field. Use a reading from a shaded thermometer or your local weather service for the most accurate starting point. The heat index formula is designed for temperatures at or above 80°F where humidity becomes a significant factor in perceived warmth. The calculator still works at lower temperatures but the heat index value will closely match the actual temperature.
2. Enter the relative humidity: Input the current relative humidity as a percentage. Relative humidity tells you how saturated the air is with water vapor. You can find this from weather apps, weather station reports, or a personal hygrometer. In the summer months across the southern and southeastern United States, relative humidity commonly ranges from 50% to 90%, which is where the heat index becomes most significant.
3. Review the heat index value: The calculator instantly displays the heat index in both Fahrenheit and Celsius. This number represents the effective temperature your body perceives. If the thermometer reads 95°F but the heat index says 110°F, your body is under the same heat stress as it would be in 110°F dry air.
4. Check the danger level and health effects: The results show a color-coded danger level from Normal through Extreme Danger, along with a specific warning and a description of potential health effects. Use this information to decide whether outdoor activities are safe, how frequently to hydrate, and whether to reschedule strenuous work to cooler hours.

All results update in real time. Experiment with different humidity levels to see how dramatically moisture in the air affects the perceived temperature, especially at temperatures above 90°F where the impact accelerates rapidly.

Heat Index Formula

Variables Explained

• T (Temperature): The ambient air temperature in degrees Fahrenheit, measured in the shade with proper ventilation. This is the starting point for the calculation. The full regression equation is applied when the simple heat index calculation yields a result of 80°F or higher. Below that threshold, a simpler formula is used since humidity has minimal impact at lower temperatures.
• RH (Relative Humidity): The percentage of moisture in the air relative to the maximum the air can hold at that temperature. At 100% relative humidity, the air is fully saturated and no additional evaporation can occur, which eliminates your body's primary cooling mechanism. Even at 60% to 70% humidity, sweat evaporation is significantly impaired at high temperatures.
• HI (Heat Index): The resulting apparent temperature in degrees Fahrenheit that accounts for the combined effects of heat and humidity on the human body. The regression equation includes adjustments for extreme conditions: one subtracts a correction factor when humidity is below 13% at moderate temperatures, and another adds a correction when humidity exceeds 85% at temperatures between 80°F and 87°F.

Step-by-Step Example

Suppose the temperature is 96°F with a relative humidity of 65%:

1. First, check the simple formula: 0.5 x (96 + 61.0 + (96 - 68) x 1.2 + 65 x 0.094) = 88.5°F
2. Since 88.5 exceeds 80, apply the full regression equation
3. Compute: -42.379 + (2.049 x 96) + (10.143 x 65) - (0.2248 x 96 x 65) - ...
4. After all terms: the heat index is approximately 121°F
5. Danger Level: Danger

At 96°F with 65% humidity, the heat index is roughly 121°F, placing conditions firmly in the Danger zone. Heat stroke is possible with prolonged exposure, and all outdoor activities should include frequent shade breaks, aggressive hydration, and buddy systems to watch for heat illness symptoms.

Practical Examples

Example 1: Maria's Outdoor Wedding

Maria is planning an outdoor wedding in Houston in July. The forecast shows 94°F with 70% humidity. She uses the heat index calculator to assess guest comfort:

• Temperature: 94°F
• Humidity: 70%
• Heat index result: approximately 119°F
• Danger level: Danger

With a heat index near 119°F, Maria decides to move the ceremony to 6:30 pm when temperatures will be lower, set up large tent canopies for shade, provide bottled water at every table, and rent industrial misting fans. She also shortens the outdoor portion to 30 minutes and moves the reception indoors to an air-conditioned ballroom.

Example 2: Coach Rivera's Football Practice

Coach Rivera runs high school football practice in Georgia during August. The afternoon reading shows 92°F with 75% humidity. He checks the heat index before sending players onto the field:

• Temperature: 92°F
• Humidity: 75%
• Heat index result: approximately 117°F
• Danger level: Danger

Following the state athletic association guidelines, Coach Rivera limits practice to helmets-only for the first 3 days and mandates 10-minute water breaks every 20 minutes. He moves practice to 7 am, cuts the session from 2 hours to 90 minutes, and assigns an athletic trainer to monitor players for heat illness symptoms. Any player showing signs of dizziness or nausea is immediately removed and cooled down.

Example 3: Linda's Garden Work Schedule

Linda is a retired gardener in Florida who spends several hours a day maintaining her extensive flower and vegetable gardens. In June, the typical afternoon conditions are 88°F with 80% humidity:

• Temperature: 88°F
• Humidity: 80%
• Heat index result: approximately 103°F
• Danger level: Extreme Caution

Linda restructures her gardening schedule to avoid the hottest hours. She works from 6:30 am to 9:30 am, takes a long break indoors through the heat of the day, and returns for a brief evening session from 5:30 pm to 7 pm. She keeps a water bottle nearby at all times and wears a wide-brimmed hat, sunscreen, and lightweight long sleeves to protect against both sun and heat.

Example 4: Construction Manager Jake's Crew Safety

Jake manages a construction crew building a commercial complex in Phoenix during July. While Phoenix has dry heat, a monsoon day brings 105°F with 35% humidity:

• Temperature: 105°F
• Humidity: 35%
• Heat index result: approximately 108°F
• Danger level: Danger

Jake implements OSHA-recommended heat illness prevention measures. He sets up shaded rest areas on site, requires workers to drink one cup of water every 15 to 20 minutes, schedules the most physically demanding tasks for early morning, and runs a buddy system where workers monitor each other for heat stress signs. He uses the heat index calculator daily to determine whether to modify the work schedule or shut down operations entirely.

Heat Index Reference Table

Values are approximate heat index temperatures. Dashes indicate conditions where the heat index exceeds 150°F (extremely dangerous). Add 10-15°F for direct sun exposure.

Tips and Complete Guide

Recognizing and Preventing Heat-Related Illness

Heat-related illnesses progress through stages of increasing severity. Heat cramps are the mildest form, causing painful muscle spasms usually in the legs or abdomen during heavy exertion. Heat exhaustion follows with symptoms including heavy sweating, weakness, cold and clammy skin, nausea, dizziness, and a rapid weak pulse. If untreated, heat exhaustion can progress to heat stroke, which is a life-threatening medical emergency characterized by body temperature above 103°F, hot and dry skin (sweating stops), rapid strong pulse, confusion, and potential loss of consciousness. Prevention involves staying hydrated by drinking water before you feel thirsty, wearing appropriate clothing, taking regular breaks in shade or air conditioning, and avoiding peak heat hours between 10 am and 4 pm.

Hydration Guidelines by Heat Index Level

Your hydration needs increase dramatically with the heat index. At heat index values below 90°F, drink at least 8 ounces of water every 20 minutes during physical activity. Between 90°F and 103°F, increase intake to 8 ounces every 15 minutes and add electrolyte drinks if sweating heavily for more than an hour. Above 103°F, drink 8 to 12 ounces every 10 to 15 minutes. Avoid waiting until you feel thirsty because thirst is a delayed signal that your body is already beginning to dehydrate. Also avoid alcohol and excessive caffeine during high heat index periods because both promote dehydration. If your urine is dark yellow, you are not drinking enough water.

Heat Index Versus Dry Heat

Many people assume that dry heat is always more comfortable than humid heat, and this is generally correct. In dry climates like Arizona or Nevada, temperatures of 110°F are common in summer, but low humidity (10% to 20%) allows sweat to evaporate quickly, providing effective cooling. The heat index at 110°F with 10% humidity is about 105°F. However, the same 110°F in a humid environment with 50% humidity produces a heat index above 150°F, which is immediately life-threatening. That said, dry heat has its own dangers because the rapid evaporation can mask dehydration since you may not notice how much you are sweating. Always hydrate aggressively in both humid and dry heat conditions. For cold weather planning, use our wind chill calculator to assess conditions on the opposite end of the spectrum.

Common Mistakes to Avoid

• Relying on raw temperature alone: A 92°F day at 30% humidity (heat index 93°F) is vastly different from 92°F at 75% humidity (heat index 117°F). Always check the heat index to understand the true danger level.
• Forgetting to add sun exposure adjustment: The heat index formula assumes shade. If you are in direct sunlight, add 10 to 15°F to the calculated value. Many outdoor workers and athletes underestimate their heat exposure because they use shade-based heat index numbers.
• Drinking only water during extended heat exposure: When sweating heavily for more than 60 minutes, you lose electrolytes (sodium, potassium) that water alone cannot replace. Use a balanced electrolyte drink to prevent hyponatremia, which occurs when sodium levels drop too low from excessive water intake without electrolyte replacement.
• Ignoring early heat illness symptoms: Heat cramps and early heat exhaustion symptoms like dizziness, headache, and excessive sweating are warning signs. Pushing through these symptoms can rapidly progress to heat stroke, which has a mortality rate of 10% to 50% depending on treatment speed.
• Assuming acclimation removes all risk: While heat acclimation over 7 to 14 days does improve your body's cooling efficiency, it does not eliminate heat illness risk. Even acclimated individuals can suffer heat stroke at extreme heat index values above 115°F.

Frequently Asked Questions