Heat Index Calculator

Heat Index Calculator

Introduce ambient temperature and relative humidity values in the heat index calculator and find out the risk of heat stress.

What is the Heat Index?

What is the Heat Index?

The Heat Index incorporates the information on ambient temperature and relative humidity to assess the risks during heat waves and warn about the conditions dangerous for human health.

This is particularly important during moist heat waves, when temperatures alone are not sufficient to assess the heat stress experienced by the human body (Cvijanovic et al. 2023).

There are a number of heat indices in operation, the one presented here is the Extended Heat Index , a corrected and extended version of National Weather Service (NWS) Heat Index derived by Lu and Romps (2022).

Based on the NWS chart, we present 5 heat stress levels (‘Safe’, ‘Caution’, ‘Extreme Caution’, ‘Danger’ and ‘Extreme danger’) corresponding to different combinations of temperatures and relative humidity.

Why humidity and not only temperature?

When exposed to heat stress, the human body cools through sweating - evaporation of this sweat cools the body. However, as humidity of the surrounding air increases the rate of evaporation decreases, making our natural cooling mechanism less efficient. In other words, we can not sweat enough to prevent overheating. At certain combinations of humidity and temperature, evaporation of sweat can no longer happen and sweating is no longer an effective way of cooling us down. Such conditions are extremely dangerous. Heat indices are designed to warn us about the onset of dangerous conditions so we can act to protect ourselves. According to the Extended Heat Index Chart, at 80% humidity, dangerous conditions commence already at a temperature of 31 ° C. At 20% humidity, however, dangerous conditions do not start until the temperature passes 40 °C. This is why we need to consider humidity too and not only ambient temperatures.


Recommendations to protect yourself from heat waves


. if you don't think your home is safe during extreme heat episodes, think about nearby locations where you can stay cool and plan how you will get there before the start of dangerous conditions
. try to make your home more heat-proof –dark curtains, outside shutters are more effective than the inside ones, heat insulation...
. make contingency plans in case of power or water outages, ensure you have enough water
. talk to your doctor about how to prepare if you have a medical condition or are taking medication


Heat index values from the calculator above can give you an idea of the level of danger expected at your current conditions. Below are some recommendations on the actions you can follow in case of high and extreme heat stress. Remember that different people have different sensitivities to heat and that older people, children and people with chronic health conditions or medications can experience difficulties at lower heat stress levels compared to the fit and healthy adults

. Make sure you
hydrate enough.
. replenish salts and minerals if sweating a lot (fruit and fruit juices and sport drinks can help), avoid coffee and very sweet drinks that promote dehydration
. limit physical activity as that will increase your body temperature.
. stay in the coolest area of your home, stay away from the sun, use air-conditioning if available.
. remove excess clothing, wear light breathable materials.
. monitor your body temperature and cool yourself down with cold water as needed.
. be careful with the use of fans if they are only moving very hot air around the room they can make the situation worse. Placing ice between you and the fan can help.
. check on vulnerable people
around you.
. make sure your pets are staying cool too, their levels of sensitivity to heat can be lower than yours.

Where do these results come from?

The results provided on this page are based on the Extended Heat Index , a corrected and extended version of National Weather Service (NWS) Heat Index derived by Lu and Romps (2022). A simplified version of the index is presented in the table below. The colour coding is based on the NSW Heat Index danger classification (Table 2) except for the black region that indicates nominally fatal heat index value of 93ºC (366 K), suggested by Lu and Romps (2022). With sustained exposure, this region is considered fatal for a large fraction, if not a majority, of the population.

The US National Weather Services describes the danger levels associated with different Heat Index ranges as follows:

* The Heat Index threshold of 93ºC was not suggested by the NWS, but by Lu and Romps (2022) and has not been considered in the original NSW classification.


The heat index values were devised for shady, light wind conditions; exposure to full sunshine can increase heat index values by up to 15°F (8°C). Exposure to strong winds, particularly with very hot, dry air, can make the conditions experienced even more hazardous, while the cool breeze can help decrease the heat stress. Note that the vulnerability to heat varies from person to person depending on many factors including age, chronic conditions and overall fitness level.

For a more accurate calculation of the risk of heat stress, especially outdoors, other meteorological variables, such as wind speed or solar radiation, should be taken into account. In any case, the results provided on this page are merely orientations based on scientific literature and do not constitute medical advice or diagnosis. ISGlobal declines all responsibility for the use that users may make of the information obtained through this website, as well as for any aspect related to the health guidelines shown on it. In case of doubt or in the event of any possible health problem, it is recommended to consult your health centre of reference.


The heat index calculator is an ISGlobal project developed by Dr. Ivana Cvijanovic, Laura Chica and Pau Rubio. The heat index used is the Extended National Weather Service (NWS) Heat Index (Lu and Romps 2022). Graphic design: Maria Beltran. Web development: Óscar Rodríguez / Sonicon