RapidRatio

Heat Index Calculator

Calculate Heat Index

Generally defines felt heat starting at 80°F (26.7°C).
Percent between 0 and 100.

Result

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Enter temperature and relative humidity, then click Calculate.

A 90°F day with 40% humidity feels very different from 90°F at 80% humidity—yet the thermometer reads the same. If you have ever cut grass or poured concrete in August, you know sweat stops evaporating when the air is already heavy with moisture, and that is when heat exhaustion shows up fast. The heat index (apparent temperature) puts a number on that sticky feeling. Enter air temperature and relative humidity, choose °F or °C, and get the NWS-style heat index plus a safety advisory band.

How to Use This Heat Index Calculator

  • Select temperature units. Pick Fahrenheit (°F) or Celsius (°C). Relative humidity is always entered as a percent (0–100).
  • Enter air temperature. Use shade air temperature when possible—the NWS heat index assumes shaded, lightly breezy conditions, not direct sun on blacktop.
  • Enter relative humidity (%). Values must fall between 0 and 100. Weather apps and hygrometers report this directly.
  • Click Calculate. The apparent temperature appears in the result panel with an NWS advisory label (Caution through Extreme Danger). Expand step-by-step details to see which formula branch ran.

Wet-bulb temperature tells a similar story from an evaporative-cooling angle—compare both on the Wet Bulb Calculator. For winter exposure, switch to the Wind Chill Calculator.

Heat Index Formulas and Practical Applications

Your body cools by evaporating sweat. When humidity is high, that evaporative path clogs up and core temperature rises even though the dry thermometer has not moved. The heat index combines temperature T (°F) and relative humidity R (%) into one apparent value using the Rothfusz regression employed by the National Weather Service:

HI = −42.379 + 2.04901523×T + 10.14333127×R − 0.22475541×T×R − 0.00683783×T² − 0.05481717×R² + 0.00122874×T²×R + 0.00085282×T×R² − 0.00000199×T²×R²

At lower combined heat levels the tool uses a simpler average formula instead, then switches to the full regression when results would exceed the simple model's range. Two NWS adjustments may apply after the base calculation: a subtraction when humidity is very low (R < 13% and 80°F ≤ T ≤ 112°F) and an addition when humidity is very high (R > 85% and 80°F ≤ T ≤ 87°F). The step-by-step panel shows exactly which path ran for your inputs.

Worked example: T = 90°F and R = 60% typically produce a heat index near 100°F—Extreme Caution territory for outdoor labor. That is the difference between "hot" and "schedule the pour for dawn."

NWS advisory classifications

  • Caution (80–90°F / 27–32°C): Fatigue possible with prolonged exposure or activity.
  • Extreme Caution (90–103°F / 32–39°C): Heat cramps and exhaustion possible.
  • Danger (103–124°F / 39–51°C): Heat exhaustion likely; heat stroke possible.
  • Extreme Danger (125°F / 52°C and above): Heat stroke highly likely with continued exposure.

Frequently Asked Questions

Why does humidity make the air feel warmer?

Sweat evaporation removes heat from your skin. When the air is already near saturation, sweat stays on your skin longer and your body retains more heat—so the same thermometer reading feels hotter.

Does wind reduce the heat index?

Breeze helps sweat evaporate, which can feel cooler. The standard NWS heat index assumes a light wind near 5.7 mph (9.2 km/h) in the shade. Calm, sunny conditions often feel worse than the calculated index.

At what temperature does the heat index matter?

NWS guidance focuses on apparent temperatures from about 80°F (27°C) upward. Below that range, humidity still affects comfort but the regression is less meaningful; the tool may report normal comfort conditions.

Is heat index the same as wet-bulb temperature?

No. Heat index is a human-comfort scale tied to temperature and humidity. Wet-bulb is a thermodynamic property used in engineering and heat-stress science. They correlate in hot weather but answer different questions—compare both on their respective calculators when planning outdoor work.

Why does the step-by-step show two different formulas?

The NWS uses a simplified average formula when combined temperature and humidity produce a low apparent value, and the full Rothfusz regression otherwise. Low-humidity and high-humidity adjustment terms may further tweak the result—the step panel lists every branch applied.

Disclaimer. RapidRatio is informational only. Apparent temperature indices are estimates based on standard shade environments. Direct exposure to full sunlight can increase heat index values by up to 15°F (8°C). Consult local weather advisories during heat waves.