Heat Index Humidity Calculator

Scenario comparison

Save several temperatures, humidity levels, or sun settings and compare the resulting apparent heat.

Result visualization

Heat-index reference table

The grid estimates heat index in degrees Fahrenheit. Your nearest temperature and humidity cell is outlined after calculation.

Formula used

For sufficiently warm conditions, the calculator uses the Rothfusz regression with temperature in degrees Fahrenheit and relative humidity as a percentage. For milder conditions, it uses a simplified apparent-temperature estimate.

Low-humidity adjustment

When humidity is below 13 percent and temperature is between 80°F and 112°F, a small downward adjustment can be applied.

High-humidity adjustment

When humidity exceeds 85 percent and temperature is between 80°F and 87°F, a small upward adjustment can be applied.

Sunlight handling

The official shade-based heat index does not directly include solar radiation. This page therefore displays sunlight as a separate, clearly labeled estimate rather than silently changing the regression.

How to use this calculator

1. Enter measured conditions

Enter air temperature and select Fahrenheit, Celsius, or Kelvin. Add relative humidity from zero to one hundred percent. Use a shaded weather reading whenever you want a standard heat-index result.

2. Select exposure details

Choose full shade, partial sunlight, or direct sunlight. Wind, activity, exposure duration, ventilation, clothing, acclimatization, age, and pregnancy settings do not redefine the official heat index. They add practical context and more cautious guidance.

3. Review the result panel

The result appears above the form. It includes measured temperature, calculated heat index, a separate sun-adjusted estimate, the humidity effect, risk classification, formula details, and recommended precautions.

4. Compare or export

Add scenarios to the comparison table, save calculations to browser history, copy a formatted summary, export a CSV record, create a shareable URL, or use the print dialog to save a PDF.

Understanding heat index and humidity

Heat index describes how hot warm air may feel when relative humidity slows the evaporation of perspiration. The body normally releases heat partly through sweat evaporation. When the surrounding air already contains substantial moisture, evaporation can become less effective. Skin may remain wet while internal heat continues rising, especially during exercise, heavy work, or prolonged exposure.

The calculated number is not a direct measurement of core body temperature. It is an environmental indicator that helps compare conditions. Two people can respond differently because of hydration, medications, illness, age, clothing, work intensity, acclimatization, and access to shade or cooling. The same heat index can therefore create very different levels of strain.

Heat index compared with humidex

Both heat index and humidex describe apparent warmth, but they use different equations and reporting conventions. Heat index is commonly expressed as an equivalent temperature, while humidex is an index derived from air temperature and moisture conditions. Values from the two systems should not be treated as interchangeable.

Heat index compared with wet-bulb temperature

Wet-bulb temperature represents cooling potential from evaporation under specific measurement conditions. It is not the same as heat index. Occupational and athletic heat decisions may require wet-bulb globe temperature, which also considers radiant heat and wind. This calculator does not replace a calibrated workplace heat-stress assessment.

Why direct sunlight matters

Standard heat-index values assume shade and relatively light wind. Sunlight adds radiant heat that can increase the body's heat load. The sun option on this page provides an approximate display adjustment for awareness. It should not be interpreted as an official meteorological value.

Formula limitations

Heat-index equations are most meaningful in warm, humid conditions. Very cool, very dry, highly windy, or intensely sunny environments may require other measures. Indoor factories, kitchens, foundries, protective suits, and enclosed vehicles can also create radiant or metabolic heat loads not represented by ordinary weather inputs.

Practical heat-safety habits

Plan heavy activity during cooler hours, drink fluids regularly, use shaded rest areas, increase exposure gradually over several days, and use a buddy system. Clothing should allow heat to escape when the task permits. Employers and organizers should use formal heat plans, emergency procedures, and locally applicable guidance.

Early heat illness may involve headache, dizziness, unusual fatigue, nausea, cramps, weakness, or heavy sweating. Confusion, collapse, seizures, or loss of consciousness require urgent action. Move the person to a cooler place, begin rapid cooling when safe, and contact emergency services.