Formula used
The calculator first evaluates the simpler Steadman expression. When its averaged result reaches 80°F, it applies the National Weather Service Rothfusz regression and its low-humidity or high-humidity adjustment.
Variables: T is air temperature in °F. RH is relative humidity in percent. HI is the heat index in °F.
Humidex: Humidex = TC + 0.5555(e − 10), using vapor pressure from dew point.
Wind apparent temperature: AT = TC + 0.33e − 0.70ws − 4.00.
How to use this calculator
- Choose humidity mode or dew-point mode.
- Enter air temperature and select Fahrenheit, Celsius, or automatic detection.
- Enter humidity or dew point, then add wind speed if available.
- Select the environment, sunlight, activity, clothing, age, acclimatization, and exposure duration.
- Optionally enable custom warning thresholds.
- Select Calculate heat index. Review the standard result before optional planning guidance.
- Copy the summary, export CSV, or print the page to a PDF file.
Example data
| Air temperature | Humidity | Environment | Sun | Activity | Expected standard heat index |
|---|---|---|---|---|---|
| 95°F | 55% | Outdoor | Shade | Moderate | About 110°F |
| 32°C | 65% | Outdoor | Full sun | Heavy | Calculate shade value, then show sun estimate |
| 88°F | Dew point 73°F | Indoor | None | Light | Humidity is derived first |
Heat index chart
This compact chart uses the same NWS calculation. Values are shade estimates in degrees Fahrenheit.
| RH \ Temp | 80°F | 85°F | 90°F | 95°F | 100°F | 105°F | 110°F |
|---|---|---|---|---|---|---|---|
| 40% | 80 | 84 | 91 | 99 | 109 | 121 | 136 |
| 50% | 81 | 86 | 95 | 105 | 118 | 134 | 152 |
| 60% | 82 | 89 | 100 | 113 | 129 | 149 | 171 |
| 70% | 83 | 93 | 106 | 123 | 143 | 166 | 194 |
| 80% | 84 | 97 | 113 | 134 | 158 | 187 | 219 |
| 90% | 86 | 102 | 122 | 147 | 176 | 209 | 247 |
| 100% | 89 | 108 | 132 | 161 | 195 | 234 | 278 |
Direct sunlight can increase heat index values by up to 15°F. Wind, clothing, activity, health, and exposure time also matter.
Risk levels and practical meaning
| Category | Heat index | General effect |
|---|---|---|
| Caution | 80–89°F | Fatigue is possible with prolonged exposure or activity. |
| Extreme caution | 90–102°F | Heat cramps or heat exhaustion are possible. |
| Danger | 103–124°F | Heat cramps or heat exhaustion are likely. Heat stroke is possible. |
| Extreme danger | 125°F or higher | Heat stroke is highly likely with continued exposure. |
Heat index, humidex, apparent temperature, and WBGT
Heat index combines shade temperature and relative humidity. It is familiar in United States forecasts and uses a regression designed for hot conditions.
Humidex is a Canadian index based on Celsius temperature and vapor pressure. Its number is interpreted like a temperature, but it does not carry a degree symbol.
Apparent temperature can include humidity and wind. This calculator uses the Australian Bureau of Meteorology non-radiation approximation for an adult outdoors in shade.
Wet-bulb globe temperature considers more environmental heat-stress factors. Outdoor work, sports, direct sun, protective clothing, and strenuous activity may require a measured or formally calculated WBGT instead.
Calculation history
Successful calculations are stored only in this browser. Clear the list whenever it is no longer needed.
| Date | Temperature | Humidity | Heat index | Sun estimate | Risk |
|---|
Frequently asked questions
What is the heat index?
It estimates how hot the air feels when relative humidity is combined with air temperature.
Is the heat index measured in sunlight?
Standard values assume shade and light wind. Full sunshine can raise the experienced value by as much as 15°F.
When is the Rothfusz equation appropriate?
The regression is mainly used when the preliminary heat index reaches about 80°F. Lower values use a simpler equation.
Can dew point replace relative humidity?
Yes. The calculator derives relative humidity from air temperature and dew point before computing heat index.
Why can the heat index be lower than air temperature?
Dry conditions can support faster evaporation. The simple apparent-temperature estimate may therefore be lower.
Does wind change the NWS heat index?
The standard NWS heat index assumes light wind. Wind is shown separately through the apparent-temperature calculation.
Do clothing and activity alter the official result?
No. They create additional planning flags because they affect heat strain but are outside the NWS equation.
Is humidex the same as heat index?
No. Both describe perceived heat, but they use different equations, scales, and meteorological conventions.
Can this calculator set workplace break schedules?
No. Employers should use applicable regulations, workplace programs, acclimatization plans, and qualified safety guidance.
What symptoms require urgent action?
Confusion, collapse, seizures, or altered consciousness can indicate heat stroke and require emergency care.
Why can extreme inputs be unreliable?
Regression equations are fitted to limited observations. Extrapolation beyond their data range can create meaningless values.
Important limitations
This calculator offers general educational estimates. It does not diagnose illness, determine fitness for work, replace local forecasts, or establish regulatory compliance.
Heat risk depends on health, medications, hydration, sleep, acclimatization, age, clothing, activity, radiant heat, airflow, and exposure duration. A single index cannot represent every person or setting.
For occupational decisions or strenuous outdoor activity, use an approved heat-safety program and the index required by your local authority. Emergency symptoms require immediate professional help.
Sources and methodology
The equations and general categories are based on public meteorological guidance. Review current local warnings before making heat-safety decisions.