Heat Index Interactive Calculator

High humidity doesn't just make hot days uncomfortable. It makes it harder for your body to lose heat, so a 92°F afternoon can become much more dangerous than you'd expect. This Heat Index Calculator gives you the "feels like" temperature, combining air temperature and humidity. It's not just for weather forecasts—heat index matters when planning factory shifts, athletic practice, HVAC setpoints, and outdoor jobs. Below, you'll find the Rothfusz regression formula, an example calculation, some nuts-and-bolts physiology, and answers to practical questions like how altitude or indoor settings affect the numbers.

What is Heat Index?

Heat index is the "feels like" temperature for your body, not just the air temperature. Once you add humidity, sweat can't evaporate as well, so your body can't cool itself. That's why a humid 88°F day can feel as bad as a dry 100°F.

Simple Explanation

Picture working in a raincoat on a hot day—your sweat just pools, and you overheat quickly. Humidity works the same way. Heat index tells you how much extra stress that humidity adds. The higher the humidity, the further off the "feels like" temperature gets from just reading the thermometer.

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Visual Diagram: Heat Index Mechanism

Heat Index Interactive Calculator

How to Use This Calculator

1. Select your calculation mode — heat index from T and RH, solve for required humidity, solve for required temperature, compare 2 conditions, or run a full heat risk assessment.
2. Enter the air temperature in °F and relative humidity as a percentage (0–100).
3. If using the Risk Assessment mode, also select your activity level and enter the exposure duration in hours.
4. Click Calculate to see your result.

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Heat Index Equations

The National Weather Service's Rothfusz regression covers when temperature is over 80°F and humidity is high enough to matter. Below 80°F, you can use a simpler formula and save yourself the hassle of all the exponents.

Use the formula below to calculate heat index.

Low Humidity Adjustment: When RH < 13% and 80°F ≤ T ≤ 112°F:

Use the formula below to calculate the low humidity adjustment.

High Humidity Adjustment: When RH > 85% and 80°F ≤ T ≤ 87°F:

Use the formula below to calculate the high humidity adjustment.

Simplified Steadman Equation: For temperatures below 80°F where full regression introduces unnecessary complexity:

Use the formula below to calculate heat index under cooler conditions.

Simple Example

Air temperature: 95°F. Relative humidity: 60%.

Plug into the Rothfusz equation: HI ≈ 107°F.

This means "feels like" jumps 12°F over what the thermometer says—enough to move you into the "Danger" zone where heat exhaustion can show up fast.

Theory & Practical Applications

Physiological Basis of Heat Index

Heat index is about how hard your body has to work to keep cool when it's hot and humid—not just the actual air temperature. Sweating off a liter of water takes around 580 kcal, so evaporation is your main cooling resource anytime the surrounding air is nearing, or above, skin temperature. Once the air is above 90°F and humidity goes up, there's less "room" in the air for your sweat to evaporate. At 80% RH and 90°F, the air's capacity to absorb sweat is only a quarter of what it is at 20% RH. When sweat doesn't evaporate, blood flow to the skin ramps up to dump heat by radiation—but this also slightly increases your heat production at the same time. That's why heavy labor in the humidity is much harder. The Rothfusz formula works because it lumps all these effects into a set of terms that ramp up quickly as both temperature and humidity climb, especially above 90°F.

Critical Engineering Limitations

There's a catch with the standard heat index formula. It's based on a "reference person"—about 147 pounds, average height, walking at 3.1 mph, in full sun, light wind (5.8 mph). Most real-world work doesn't match that. Workers in vapor-barrier suits (or PPE) can have internal heat loads much higher than calculated, since sweat can't escape. Same story with athletes on an open field if you add solar load—sunlight can make it feel 10–15°F hotter than the heat index suggests. Wind makes a difference, too. The standard formula assumes a light breeze. Still air can make it feel 3–7°F hotter, and strong wind does the opposite. For HVAC or workspace planning, make sure you look at airflow near workers, not just the air temp and humidity. Even a gentle breeze can noticeably reduce the apparent heat, but heavy clothing takes that benefit away quickly.

Industrial and Athletic Applications

OSHA work/rest cycles are based on heat index: below 91°F, you can work without scheduled breaks. Go above that and you need more rest—at 91–103°F, you’re looking at 25% rest per hour, and above 115°F, only brief shifts are safe for heavy work. These numbers are for acclimatized workers—if it's your first hot week of the year, you should bump those rest periods up even more. In sports, trainers track both heat index and wet bulb globe temperature (WBGT), since WBGT takes solar load and wind into account. A WBGT over 82°F means practice should be cut short, and over 92°F, it's often cancelled. The actual heat index in full sun will be even worse, so check both numbers if you're responsible for safety planning.

HVAC Design and Validation

For industrial HVAC, you can't just look up a heat index value and call it a day. You need to account for all heat sources—including moisture from people and processes—because these drive up both temperature and humidity. Keep a warehouse at 78°F, 45% RH (HI = 78°F), and most people will feel fine. Let humidity drift up to 65% and suddenly everyone feels 4°F warmer, which can impact accuracy and productivity. For data centers, it's a balancing act—you aim for safe temperatures for equipment (usually 77°F, 40% RH), but don't forget the support crew occasionally working inside. Too low humidity raises static risks, but too high makes it uncomfortable for maintenance. Heat index is one check, but you have to juggle comfort, safety, and equipment requirements.

Worked Example: Construction Site Heat Safety Planning

Scenario: It's 96°F with 68% relative humidity in Houston. You're assigning roofing crews for a four-hour shift.

Step 1: Calculate Base Heat Index

Use the Rothfusz formula for 96°F, 68% RH:

HI = -42.379 + 2.04901523(96) + 10.14333127(68) - 0.22475541(96)(68)
- 0.00683783(96)² - 0.05481717(68)² + 0.00122874(96)²(68)
+ 0.00085282(96)(68)² - 0.00000199(96)²(68)²

HI = -42.379 + 196.705 + 689.747 - 1468.338
- 62.987 - 253.285 + 772.474
+ 398.123 - 79.946

HI = 120.1°F

This is "Extreme Danger" territory for heat stress.

Step 2: Account for Solar Loading

Roofers in direct sun? Add about 13°F as a ballpark:

Effective HI = 120.1 + 13 = 133.1°F

Step 3: Apply OSHA Work-Rest Guidelines

With HI at 133°F for heavy labor, only 25% of your shift can be actual work without a higher risk. On a 4-hour job:

1 hour work, 3 hours of rest in shade or cooled space—often in 15/45 or 20/60 minute increments per cycle.

Step 4: Calculate Cooling Station Requirements

Cooling zones need to get the HI down at least 25–30°F below the work zone. At 75°F and 50% RH (HI ≈ 75°F), you reach that target. If the rest area is hotter or more humid, recovery won't be effective.

Step 5: Hydration Requirements

NIOSH: 1 cup water every 15–20 minutes. For a 12-person crew over four hours, you’ll need at least 10 gallons—plan for more to cover spillage and extra thirst.

Safety Conclusion: At HI = 120°F (133°F in full sun), keep roofing to early morning or run 15:45 work/rest cycles. Don't expect normal productivity in this heat—workers can only manage a quarter of the usual output. That’s why contractors in hot climates shift outdoor jobs to cooler months whenever possible.

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