Heat Index Calculator (Feels Like Temperature)
Enter the air temperature and relative humidity to see how hot it feels in the shade, and the heat risk category. The NWS equation is only valid for warm, humid conditions, and the result tells you when you are outside that range.
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Formula
NWS procedure, with temperature T in degrees F and relative humidity R in percent:
- Simple estimate:
HI = 0.5 × (T + 61 + (T − 68) × 1.2 + R × 0.094). If the average of this andTis below 80, that is the answer. - Otherwise the Rothfusz regression:
HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R² - If
R < 13and80 ≤ T ≤ 112, subtract((13 − R) ÷ 4) × √((17 − |T − 95|) ÷ 17). - If
R > 85and80 ≤ T ≤ 87, add((R − 85) ÷ 10) × ((87 − T) ÷ 5). - Celsius inputs are converted to F and the result back to C.
Worked example
90 F and 70% humidity: the simple estimate averages to 91, which is above 80, so the regression applies. Neither adjustment is needed, and the heat index is 105.9 F, in the danger band (103 to 125 F). In Celsius, 32 C at 70% gives 40.4 C.
The NWS equation, with its adjustments, can differ by a couple of degrees from the printed chart: at 80 F and 100% humidity it gives 89.3 F, while the chart reads about 87 F.
Limits
Valid roughly for 80 to 110 F with at least 40% humidity, in shade and light wind. Outside it, a warning is shown.
Frequently asked questions
What is the heat index?
The heat index is the temperature your body feels once humidity is combined with the air temperature, because sweat evaporates more slowly in humid air. At 90 F and 70% humidity it is about 105.9 F. It assumes shade and light wind.
Which formula does this use?
The National Weather Service Rothfusz regression, with its two adjustments. If relative humidity is below 13% and the temperature is 80 to 112 F, a term is subtracted. If humidity is above 85% and the temperature is 80 to 87 F, a term is added. Below about 80 F the simple averaging formula is used instead, so 70 F at 50% gives 69.1 F.
When is the result not reliable?
The regression was fitted for roughly 80 to 110 F and humidity of 40% or more. Outside it, as at 95 F and 10% humidity (89.4 F), the figure is only an estimate and the calculator shows a warning. Direct sun can add up to about 15 F to how hot it feels.
What do the risk categories mean?
They follow the NWS chart: caution from 80 F, extreme caution from 90 F, danger from 103 F and extreme danger from 125 F. Young children, older adults and people working outdoors are affected sooner than these thresholds suggest.