This heat index calculator estimates how hot the air feels from Fahrenheit temperature and relative humidity. Enter those two values to apply the standard multi-term heat index formula. The default example uses 92°F and 60 percent humidity and returns about 104.7°F.
Coaches, event planners, and outdoor workers use it when humidity makes a warm day more stressful. For cold weather planning, pair awareness with local forecasts rather than treating heat index as a year-round metric.
How the formula works
The Rothfusz regression blends temperature and humidity with several product and square terms. Higher humidity at the same temperature raises heat index. The output is an apparent temperature in Fahrenheit, not a humidity reading.
Worked example
Temperature 92°F, relative humidity 60 percent. Heat index ≈ 104.7°F.
| Input | Value |
|---|---|
| Temperature | 92°F |
| Relative humidity | 60% |
| Heat index | ~104.7°F |
How to use the fields
- Temperature must be in degrees Fahrenheit.
- Relative humidity is a percent from 0 to 100 for the same place and time.
Why humidity matters
Sweat cools you when it evaporates. Moist air slows that process. Two days at the same thermometer reading can feel very different if humidity jumps. Heat index puts that difference into one number you can compare across days.
Common mistakes
- Entering Celsius without converting
- Mixing morning humidity with afternoon temperature
- Treating heat index as a sun exposure score (shade and radiant load are separate)
- Ignoring rest, water, and clothing once the number looks high
Planning checklist
- Pull temperature and humidity from the same forecast hour.
- Recheck near start time if weather shifts.
- Raise rest breaks when heat index climbs through the day.
- Watch people who are new to heat or who wear heavy gear.
Indoor versus outdoor
Indoor air can still feel oppressive with high humidity and poor airflow. The same formula applies if your sensors are accurate. Outdoor sun and dark pavement can make conditions feel worse than the shade based air inputs alone.
Reading the number with context
Use heat index as a planning flag, not a personal medical limit. Age, medications, and acclimatization change risk. When agencies issue heat alerts, follow those rules even if your own spot reading looks milder.
Scheduling outdoor work
Move heavy tasks to cooler morning hours when heat index is lower. Recheck mid afternoon because humidity and temperature can climb together. If the number jumps several degrees between forecasts, shorten work blocks and add shade breaks even if the crew feels fine at the start.
Sports practices should treat rising heat index as a signal to cut full pads time, add water breaks, and watch new athletes closely. A number near the default example already sits in a range where caution beats toughness contests.
Hydration and clothing
Light, breathable clothing and more frequent water intake matter more as heat index rises. Dark, heavy layers trap heat. The calculator cannot see your clothing, so translate a high reading into practical gear choices yourself.
Electrolyte needs vary by person and sweat rate. Use heat index as the trigger to follow a hydration plan you already trust, not as a prescription for a specific drink brand or dose.
Comparing two forecast hours
Run the tool twice with morning and afternoon pairs from the same forecast. The difference shows how much “extra heat” humidity adds across the day. Share both readings with volunteers or staff so arrival times and break plans are based on the hotter window, not the milder one.
If indoor venues lack air conditioning, measure indoor temperature and humidity with a simple sensor and compute heat index the same way. Crowded rooms can feel worse than the outdoor shade reading suggests.
What this number is not
Heat index is not a UV index, not a pollution index, and not a medical diagnosis. Pair it with common sense about sun exposure, air quality alerts, and personal health history. When agencies issue heat warnings, those public rules override a single spot calculation from this page.
Limitations
The polynomial is calibrated for hot, humid conditions. Very cool temperatures are outside its usual use. Wind, solar radiation, and clothing insulation are not part of this two input model.
For related unit and percent checks while you plan materials, try the unit converter or the percentage calculator.