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Summer Heat Index Calculator

Enter the air temperature (°F) and relative humidity (%).

Result

108.9 °F
42.7 °C
Danger

How it is calculated

Enter the air temperature T in degrees Fahrenheit and the relative humidity R in percent. Following the National Weather Service procedure, the tool first computes the simple Steadman estimate 0.5 × (T + 61 + (T − 68) × 1.2 + R × 0.094). If the average of that and T is below 80 °F, this simple value is the answer. Otherwise it applies the Rothfusz regression, a polynomial in T and R, plus two NWS adjustments: one for humidity under 13% at 80–112 °F and one for humidity above 85% at 80–87 °F. The result is shown in °F and °C with a risk label: below 80 no risk, then Caution (below 91), Extreme Caution (below 104), Danger (below 126) and Extreme Danger. Humidity outside 0–100% or T outside −60 to 150 °F gives no result.

Worked example

Take T = 95 °F and R = 55%. The simple estimate is 0.5 × (95 + 61 + 27 × 1.2 + 55 × 0.094) = 96.8. Its average with T is (96.8 + 95) ÷ 2 = 95.9, which is at least 80, so the full regression is used. Neither humidity adjustment applies at 55%. The polynomial gives 108.9 °F. In Celsius that is (108.9 − 32) × 5 ÷ 9 = 42.7 °C. A value between 104 and 126 falls in the Danger category, which is what the tool reports.

Try this example →

Common mistakes

  • Entering the temperature in Celsius. Typing 35 for a hot 35 °C day is read as 35 °F, giving a heat index near 31 °F and No risk. Convert first (35 °C = 95 °F).
  • Using dew point or absolute humidity instead of relative humidity. The field is relative humidity in percent.
  • Entering humidity as a fraction (0.55) rather than a percentage (55).
  • Using shade temperature figures for a person standing in full sun. The index is defined for shade, so direct sunshine can make it feel hotter than shown.
  • Trusting extreme inputs. At very high temperature and humidity together, such as 110 °F and 60%, the formula returns numbers like 171 °F that are extrapolations rather than reliable readings.

About this calculator

The heat index, sometimes called the “feels-like” temperature, estimates how hot the air actually feels to the human body when high humidity is factored in alongside air temperature, because humidity slows the evaporation of sweat that the body relies on to cool itself. The US National Weather Service computes it using the Rothfusz regression, a multi-term polynomial fit built to approximate Steadman’s 1979 apparent-temperature model. This calculator follows the full NWS procedure: it first uses Steadman's simpler formula, and only when that result reaches about 80 °F does it switch to the Rothfusz regression, adding the NWS corrections for very low and very high humidity.

The NWS groups the result into risk categories — Caution, Extreme Caution, Danger and Extreme Danger — each tied to the likelihood of heat-related illness such as heat cramps, heat exhaustion or heat stroke with prolonged exposure or physical activity, and forecasters use these thresholds to issue heat advisories and excessive-heat warnings.

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Comments (1)

  • Editorial team

    Compare the same air temperature at two humidity levels to see why humid days feel so different. At 90 °F with 40 percent humidity the index is 90.7 °F, in the Caution band, but at 70 percent it is 105.9 °F, which falls in Danger. The thermometer reads the same in both cases. Use a forecast humidity figure for the hour you will be outside, not a daily average. What time of day do you usually plan outdoor work or exercise?

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