Heat Index Calculator

What the heat index calculator does

The heat index is how hot it actually feels when humidity is taken into account, not just what the thermometer reads. This calculator estimates it from the air temperature and the relative humidity, giving the result in both Fahrenheit and Celsius.

Below is what the heat index is, why humid heat is so much harder on the body, the equation behind it, and what the numbers mean for safety.

How to use it

1. Enter the air temperature in degrees Celsius.
2. Enter the relative humidity as a percentage.
3. Press Calculate for the heat index, shown in both Fahrenheit and Celsius, or Reset to clear it.

What the heat index is

The heat index, sometimes called the apparent temperature or the feels-like temperature, combines air temperature and humidity into a single number that reflects how hot the human body actually perceives the conditions. On a humid day, the air feels far hotter than the thermometer suggests, and the heat index puts a figure on that extra discomfort by reporting the temperature that dry air would need to feel equally hot.

So a reading of 32 degrees Celsius with high humidity might carry a heat index of 41 degrees, meaning it feels as punishing as 41-degree dry heat. The thermometer alone misses this, because it measures only the air's temperature and not how the body copes with it. The heat index fills that gap, which is why weather services issue heat warnings based on it rather than on raw temperature.

Why humidity makes heat dangerous

The reason humidity matters so much comes down to how the body sheds heat. When you are hot, you sweat, and the sweat evaporates from your skin, carrying heat away and cooling you. This evaporative cooling is the body's main defence against overheating, and it works well in dry air, which readily soaks up the moisture.

Humid air, though, is already close to full of water vapour, so it accepts sweat only slowly. Evaporation stalls, the cooling fails, and the body's heat builds up with nowhere to go. This is why the same temperature feels far worse and is far more dangerous when the air is humid: the hotter it is and the more humid it is, the less the body can cool itself. The heat index captures this combined effect, which is why it tracks the real risk of heat illness better than temperature alone.

The equation it uses

The calculator uses the Rothfusz regression, the formula adopted by the US National Weather Service for the heat index. It is a polynomial fitted to detailed studies of human heat balance, combining temperature and humidity along with their squares and their product:

HI = c₁ + c₂T + c₃R + c₄TR + c₅T² + c₆R² + c₇T²R + c₈TR² + c₉T²R²

Here T is the temperature in Fahrenheit, R is the relative humidity in percent, and c₁ through c₉ are fixed coefficients. The calculator converts your Celsius input to Fahrenheit, applies the regression, and adds small correction factors for very low or very high humidity, then reports the result back in both scales. The many terms let the formula bend to match how heat and humidity interact, which is not a simple proportion.

What the numbers mean for safety

The heat index maps onto recognised levels of risk, which is its main practical purpose. As a rough guide using the National Weather Service bands, a heat index up to around 32 degrees Celsius calls for caution, with fatigue possible on prolonged exertion. From roughly 32 to 39 degrees is extreme caution, where heat cramps and heat exhaustion become likely with activity. From about 39 to 51 degrees signals danger, with heat exhaustion probable and heatstroke possible, and above about 51 degrees is extreme danger, where heatstroke is highly likely.

These bands explain why a humid 35-degree day can be genuinely hazardous while a dry one of the same temperature is merely uncomfortable. They are why outdoor workers, athletes, and event organisers watch the heat index closely, scheduling rest, shade, and water around it. Treating the heat index as a safety gauge rather than just a comfort figure is the sensible way to use it, and this calculator gives you the number to judge by.

Working range and precision

The heat index is designed for hot conditions, and the Rothfusz regression is meant for temperatures of roughly 27 degrees Celsius (80 degrees Fahrenheit) and above, where heat stress is a real concern. Below that, the idea of a feels-like heat does not really apply, and the figure is best ignored. The calculator takes the temperature in Celsius and the humidity as a percentage, and reports the heat index in both Fahrenheit and Celsius to two decimal places. It is an estimate of perceived heat, not a precise physical temperature.

A worked example

Suppose the air temperature is 32 degrees Celsius (about 90 degrees Fahrenheit) and the relative humidity is 70 percent.

The Rothfusz regression gives a heat index of about 105 degrees Fahrenheit, or roughly 40 degrees Celsius. So although the thermometer reads 32 degrees, the humid air makes it feel like 40-degree heat. That places the conditions in the danger band, where heat exhaustion becomes likely with exertion, a much more serious situation than the plain temperature would suggest.

Questions people ask

What is the heat index?

The temperature it feels like when humidity is combined with the air temperature. Humid air makes heat feel hotter, and the heat index reports the equivalent dry-air temperature for that sensation.

Why does humid heat feel worse than dry heat?

Because the body cools itself by evaporating sweat, and humid air absorbs that sweat only slowly. Evaporation stalls, cooling fails, and the same temperature becomes far harder and more dangerous to endure.

How is the heat index calculated?

Using the National Weather Service's Rothfusz regression, a polynomial in temperature and humidity fitted to studies of human heat balance, with small corrections at very low or very high humidity.

What heat index is dangerous?

As a rough guide, around 39 to 51 degrees Celsius signals danger, with heat exhaustion likely and heatstroke possible, and above about 51 degrees is extreme danger. Lower values warrant caution with exertion.

References

A quick note on where the science comes from. The heat index and its Rothfusz regression are defined by the US National Weather Service, and the concept of apparent temperature is described in the Wikipedia article on heat index. The HyperPhysics overview from Georgia State University covers the underlying role of evaporative cooling.

1. National Weather Service (NOAA), The Heat Index Equation. https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml
2. Wikipedia, Heat index. https://en.wikipedia.org/wiki/Heat_index
3. HyperPhysics, Evaporative Cooling. http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/evap.html