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Heat Index Calculator

NWS Rothfusz regression. Free online Heat Index Calculator. Calculate heat index online — fast, accurate, mobile-friendly, no signup needed.

Heat index (°F)
105.922021

Derivation

  1. ├── 01GivenT = 90, RH = 70
  2. ├── 02Formula2.04901523 × t-42.379+10.14333127 × a-.22475541 × t × a-.00683783 × t²-.05481717 × a²+.00122874 × t² × a+85282e-8 × t × a²-199e-8 × t² × a²
  3. └── 03Compute Heat index (°F)105.922021
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§01What is

Understanding the Heat Index Calculator

The Heat Index Calculator computes Heat index (°F) from 2 inputs: temp (°f), humidity (%). NWS Rothfusz regression.

Quick calculators for the math that shouldn’t need a notepad — instant, accurate, private to your browser. The Heat Index Calculator sits in that toolkit — it NWS Rothfusz regression. Enter your numbers above and the result updates instantly; every step of the math is shown in the Derivation panel so you can see exactly how the answer was reached.

§02The Formula

How it’s calculated

2.04901523 × t-42.379+10.14333127 × a-.22475541 × t × a-.00683783 × t²-.05481717 × a²+.00122874 × t² × a+85282e-8 × t × a²-199e-8 × t² × a²

Where

T
Temp (°F)
RH
Humidity (%)
§03Practical Example

Step-by-step walkthrough

Scenario

Apply the formula to a realistic set of inputs: Temp (°F) = 90, Humidity (%) = 70.

  1. 01Start by noting the input — Temp (°F): 90.
  2. 02Start by noting the input — Humidity (%): 70.
  3. 03Substitute these values into the formula: 2.04901523 × t-42.379+10.14333127 × a-.22475541 × t × a-.00683783 × t²-.05481717 × a²+.00122874 × t² × a+85282e-8 × t × a²-199e-8 × t² × a²
  4. 04Compute Heat index (°F): the calculator returns 105.922.
  5. 05Cross-check the answer by opening the Derivation panel above — every line of math is shown so you can follow the computation end-to-end.
§04Variants

Common Heat Index Problems

The formula gets rearranged depending on which variable you need. Here are the patterns you’ll run into in the real world — find the one that matches your problem and follow the worked steps.

01 · PATTERN

Temp (°F) halved

T = 45 (from 90)

Keep every other input at its default and halve the temp (°f). See how heat index (°f) responds.

  1. 01New Temp (°F): 45
  2. 02Baseline Heat index (°F): 105.922
  3. 03New Heat index (°F): 111.905
  4. 04Heat index (°F) increases by 5.6% → use this sensitivity to plan for real-world variation.
02 · PATTERN

Temp (°F) doubled

T = 180 (from 90)

Keep every other input at its default and double the temp (°f). See how heat index (°f) responds.

  1. 01New Temp (°F): 180
  2. 02Baseline Heat index (°F): 105.922
  3. 03New Heat index (°F): 937.446
  4. 04Heat index (°F) increases by 785% → use this sensitivity to plan for real-world variation.
03 · PATTERN

Humidity (%) halved

RH = 35 (from 70)

Keep every other input at its default and halve the humidity (%). See how heat index (°f) responds.

  1. 01New Humidity (%): 35
  2. 02Baseline Heat index (°F): 105.922
  3. 03New Heat index (°F): 89.1574
  4. 04Heat index (°F) decreases by 15.8% → use this sensitivity to plan for real-world variation.
04 · PATTERN

Humidity (%) doubled

RH = 140 (from 70)

Keep every other input at its default and double the humidity (%). See how heat index (°f) responds.

  1. 01New Humidity (%): 140
  2. 02Baseline Heat index (°F): 105.922
  3. 03New Heat index (°F): 182.211
  4. 04Heat index (°F) increases by 72% → use this sensitivity to plan for real-world variation.
§05FAQ

Frequently asked questions

Yes. The calculator implements the standard formula as documented and returns exact floating-point results. No approximations are used unless noted in the formula.
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