Air conditioning is one of the most important issues of Climate Justice. Air conditioning is the main electricity consumer in buildings equipped with air conditioning, while the waste for nothing is tens of percent or more. Overcooling below the Comfort Heat Index is not for comfort, actually it causes overcooling discomfort and a health hazard. The lower is the air conditioning temperature, the longer is its operation time. Unlike many other GHG mitigation measures, setting the air conditioning to the Comfort Heat Index does not involve expenses or investment, on the contrary, it significantly reduces energy cost (mainly due to shorter operating time) and reduces investment in oversized air conditioning units. The purpose of the Heat Index online calculator according to this work is to assist in setting the air conditioning unit to the correct temperature based on Comfort Heat Index. The Heat Index online calculator according to this work can be found at: https://nowagreen.ct.ws/HI