What this calculator does

This heat index calculator estimates how hot the air feels to people when warm air and high relative humidity act together. It is useful for interpreting summer conditions that feel much more stressful than the dry-bulb temperature alone suggests.

The tool is designed for quick weather interpretation, communication, and planning. It helps explain why a day at 90 F can feel manageable in dry air but much more dangerous when moisture prevents sweat from evaporating efficiently. Heat index is a comfort and risk indicator, not a replacement for measured air temperature.

Inputs explained

• Air temperature: Enter the ambient dry-bulb air temperature in either F or C.
• Relative humidity: Enter the percentage of moisture in the air relative to saturation at that temperature.
• Unit selector: Choose the temperature unit that matches the value you entered so the calculator can convert internally without changing the displayed result.

How it works / method

The calculator follows the same logic used in common NOAA and National Weather Service guidance. It starts with a simpler warm-weather estimate and then applies the Rothfusz regression for hotter and more humid conditions. The result is mapped to a practical risk band so the number is easier to interpret during planning, reporting, or safety conversations.

Formula used

In this page's engine, T is handled in F for the main regression and RH is relative humidity in percent. The calculator also applies NOAA-style low-humidity and very-high-humidity adjustments in the standard warm-weather range.

Step-by-step example

Suppose the air temperature is 95 F and the relative humidity is 55 percent. The result should be interpreted as a practical warning signal rather than a literal thermometer reading.

1. Enter 95 for air temperature and choose F.
2. Enter 55 for relative humidity.
3. The calculator estimates a heat index a little above 107 F.
4. That falls into a dangerous range where heat illness becomes more likely during outdoor activity.
5. If those same conditions occur in direct sunshine, the real outdoor burden on the body can be even higher.

Use cases

• Checking whether planned outdoor work or sports sessions should be shortened, rescheduled, or supplied with extra cooling breaks.
• Explaining why a modest rise in humidity can materially change summer discomfort even when the thermometer barely changes.
• Comparing forecast days when the air temperature is similar but the moisture burden is very different.
• Adding context to local weather reports, preparedness pages, and plain-language safety messaging.

Assumptions and limitations

• Heat index is built for warm weather and human heat stress, not for cold weather, engineering heat transfer, or indoor HVAC sizing.
• The regression is most relevant in the normal warm-humid operating range used by NOAA guidance and is not a universal law of human physiology.
• The value is less informative when unusual clothing, radiant heat, intense physical work, or limited airflow dominate the actual exposure.
• A comfort index can support decisions, but site safety policies may require WBGT or direct field measurements instead.

If the question is about direct sun, gear, or work-rest scheduling, compare this page with the WBGT calculator before making a risk decision.

Sources & references

• NOAA National Weather Service - Heat index guidance
• NOAA National Weather Service - Wind chill chart and formula
• NOAA National Weather Service - Wet Bulb Globe Temperature overview
• Bureau of Meteorology - Apparent temperature background
• Bureau of Meteorology - Apparent temperature glossary
• Environment and Climate Change Canada - Humidex guidance
• Schema.org - FAQPage and WebApplication vocabulary