BYTETOOLS

Heat Index Calculator

Work out how hot it really feels from temperature and humidity using the NWS Rothfusz regression, with the official warning band and the dew point.

70
105.9
Heat index (°F)
41.1
Heat index (°C)
+15.9 °F
vs air temperature
26.0
Dew point (°C)

Danger

Heat cramps and heat exhaustion are likely, and heat stroke is possible with continued activity.

How this number was reached

  • Air temperature = 90.0 °F (32.2 °C)
  • Relative humidity = 70%
  • Method = NWS Rothfusz multiple regression
  • Heat index = 105.92 °F = 41.07 °C
  • Dew point (Magnus–Tetens) = 26.04 °C = 78.87 °F

The Rothfusz regression reproduces the published NWS heat index table to within about ±1 °F. At 90 °F and 70% humidity it returns 105.9 °F, which the NWS chart shows as 106.

Warning bands

Heat indexBand
Below 80 °F (27 °C)No elevated risk
80–90 °F (27–32 °C)Caution
90–103 °F (32–39 °C)Extreme Caution
103–124 °F (39–51 °C)Danger
125 °F (52 °C) and aboveExtreme Danger

The bands are the US National Weather Service categories. They assume shade and a light wind — direct sun can add as much as 15 °F to the apparent temperature. These are estimates for general information and are not medical advice.

What is the Heat Index Calculator?

The ByteTools Heat Index Calculator applies the US National Weather Service procedure exactly as published.

  • Full NWS procedure: Steadman average below 80 °F, Rothfusz regression above
  • Both official adjustments applied and named when they fire
  • Results in °F and °C, with the gap from the air temperature shown
  • Caution, Extreme Caution, Danger and Extreme Danger bands with NWS advice
  • Dew point from the Magnus–Tetens formula for extra context
  • Humidity slider alongside the number field, and a full band reference table

How to use the Heat Index Calculator

  1. 1

    Enter the air temperature and choose Fahrenheit or Celsius — switching units converts the number for you.

  2. 2

    Type the relative humidity as a percentage, or drag the humidity slider.

  3. 3

    Read the heat index in both units, the difference from the air temperature and the dew point.

  4. 4

    Check the coloured warning band for what the NWS advises at that level.

  5. 5

    Open the working panel to see which formula and which correction were applied, then copy the results.

About the Heat Index Calculator

The ByteTools Heat Index Calculator applies the US National Weather Service procedure exactly as published. It first tries the simple Steadman average and, when that comes out at 80 °F or above, falls through to the Rothfusz multiple regression, then applies the two standard corrections: a subtraction when humidity is under 13% between 80 and 112 °F, and an addition when humidity is over 85% between 80 and 87 °F.

You get the heat index in both Fahrenheit and Celsius, the difference from the air temperature, the NWS warning band from Caution through to Extreme Danger, and the dew point derived from the Magnus–Tetens formula for context. At 90 °F and 70% humidity it returns 105.9 °F, which the published NWS chart shows as 106.

These are estimates for general information and are not medical advice. The bands assume shade and a light wind — direct sun can add as much as 15 °F to how hot it feels. Everything runs in your browser and nothing is uploaded.

Frequently asked questions

What is the heat index?

It is the apparent temperature — how hot the air feels to a human body once humidity is taken into account. Sweat is your cooling system, and humid air slows evaporation, so 90 °F at 70% humidity feels like 106 °F. It is sometimes called the humiture or the apparent temperature.

How is the heat index calculated?

The National Weather Service uses the Rothfusz regression, a nine-term polynomial in temperature and relative humidity fitted to Steadman's 1979 tables. Below an 80 °F result a simpler average is used instead, and two corrections handle very dry and very humid corners of the chart.

At what heat index is it dangerous?

The NWS calls 80–90 °F Caution, 90–103 °F Extreme Caution, 103–124 °F Danger and 125 °F and above Extreme Danger. In the Danger band heat exhaustion is likely with prolonged exposure and heat stroke becomes possible, so activity should be cut back sharply.

How accurate is the heat index?

The regression reproduces the published NWS table to within about ±1 °F, but the table itself models a particular person — 5'7", 147 lb, in shade, with a light breeze. Direct sun can add up to 15 °F, and age, fitness, medication and acclimatisation all shift the real risk considerably.

What is the difference between heat index and dew point?

Dew point is the temperature at which the air would become saturated, and it measures the absolute amount of moisture present. Heat index combines that moisture with the air temperature to say how hot it feels. A dew point above 20 °C usually feels muggy regardless of the thermometer.

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