Heat Index Calculator
Calculate the heat index, feels-like temperature, and heat risk level from air temperature and humidity.
Calculator is for informational purposes only. Terms and Conditions
Choose the setup
Select the humidity method and preferred unit system.
Enter the known values
Use shaded, light-wind weather values for the official heat index result.
Visual Check
The gauge scale matches the heat risk thresholds used by the calculator.
Solution
Live result, risk level, quick checks, warnings, and solution steps.
Quick checks
- Risk level—
Show solution steps See the equation, substitutions, assumptions, and result path
- Enter values to see the full solution steps and checks.
Source, Standards, and Assumptions
Calculation basis, constants, assumptions, and limitations.
The calculator uses the NOAA/NWS heat index method with the Rothfusz regression when applicable.
- Assumptions will appear after a valid calculation.
On this page
Calculator Guide
How to Use the Heat Index Calculator
The Heat Index Calculator above estimates how hot the air feels by combining air temperature with humidity. Enter temperature and relative humidity, or switch to dew point mode if that is the weather value you have, then use the result to understand the apparent temperature and heat risk level.
The heat index is most useful for hot, humid outdoor conditions. It helps explain why 95°F can feel closer to 109°F when humidity is high, even though the measured air temperature has not changed.
Quick Answer
Heat index is the apparent temperature, or how hot the weather feels to the body after humidity is included. The calculator uses air temperature and relative humidity; in dew point mode, it first estimates relative humidity and then calculates heat index.
Try a quick comparison
After calculating one result, change only the humidity value above and watch how quickly the heat index changes. This is the fastest way to understand why humid heat is often more dangerous than dry heat at the same air temperature.
When not to rely on heat index alone
Official heat index values assume shade and light wind. Do not use heat index alone for direct sun, heavy work, protective clothing, hot roofs, pavement, vehicles, or formal worksite heat stress decisions. OSHA notes that WBGT is more accurate than heat index for worksite heat hazard assessment.
Inputs and Outputs Used by the Calculator
The calculator needs a temperature input and a humidity input. Relative humidity is the direct formula input, while dew point mode is useful when your weather source gives dew point instead of humidity.
| Type | Value | What It Means | Common Unit |
|---|---|---|---|
| Input | Air temperature | The shaded air temperature used in the heat index equation. | °F or °C |
| Input | Relative humidity | The amount of moisture in the air compared with saturation at that temperature. | Percent |
| Input option | Dew point | A humidity-related temperature that can be converted into relative humidity. | °F or °C |
| Output | Heat index | The apparent or feels-like temperature for the body. | °F or °C |
| Output | Risk level | A screening category such as Caution, Extreme Caution, Danger, or Extreme Danger. | Category |
Heat Index Formula
The standard heat index calculation uses air temperature in °F and relative humidity in percent. For hot and humid conditions, the calculator uses the NOAA/NWS Rothfusz regression.
Main Heat Index Formula
In this formula, \(HI\) is the heat index in °F, \(T\) is air temperature in °F, and \(R\) is relative humidity in percent.
Formula source note
The NOAA/NWS Weather Prediction Center heat index equation explains the Rothfusz regression, the simplified preliminary calculation, and the humidity adjustment rules. The regression is empirical and should not be treated as valid for every extreme temperature and humidity combination.
What the Variables Mean
The heat index formula is long, but the actual inputs are simple: temperature and humidity. The calculator handles the repeated \(T\), \(R\), squared, and interaction terms automatically.
\(HI\)
Heat index, also called apparent temperature or feels-like temperature. The standard formula returns \(HI\) in °F.
\(T\)
Air temperature in °F. If you enter °C, the calculator converts it to °F before using the formula.
\(R\)
Relative humidity in percent. For example, 55% is entered as \(R=55\), not \(0.55\).
Dew point
Dew point is not in the Rothfusz formula directly. In dew point mode, the calculator estimates relative humidity first.
How to Use the Calculator
Use the calculator with shaded weather values when you want the official heat index estimate. Then compare the result with the risk category, full-sun note, and activity warnings.
Choose the humidity method
Select relative humidity if you know it. Select dew point if your weather report gives dew point instead.
Enter air temperature
Use the actual air temperature, not pavement temperature, vehicle dashboard temperature, or a surface temperature reading.
Check units
Use °F or °C consistently. The equation is Fahrenheit-based internally, but the calculator can show the answer in either unit.
Review the result
Read the heat index, risk range, difference from actual temperature, and estimated full-sun apparent heat if selected.
Activity level note
Activity level does not change the official heat index value. It changes the warning context because physical work, sports, and heavy exertion increase heat stress beyond the weather value alone.
How to Interpret Heat Index Results
A higher heat index means your body has a harder time cooling itself. The result becomes especially important when the calculated value moves from Caution into Danger or Extreme Danger ranges.
| Heat Index | Risk Level | What It Suggests |
|---|---|---|
| Below 80°F | Low | Heat index is usually close to the actual air temperature. |
| 80°F to 90°F | Caution | Fatigue is possible with prolonged exposure or physical activity. |
| 90°F to 105°F | Extreme Caution | Heat cramps and heat exhaustion become possible with prolonged exposure or activity. |
| 105°F to 130°F | Danger | Heat cramps or heat exhaustion are likely, and heat stroke is possible with continued exposure. |
| 130°F or higher | Extreme Danger | Heat stroke or sunstroke is highly likely with continued exposure. |
What to do with the result
Use the heat index to plan outdoor work, sports, yard work, events, and heat breaks.
What changes it most?
Humidity often drives the difference between air temperature and feels-like temperature.
Sanity check
If humidity is high, the heat index should usually be higher than the air temperature in hot weather.
Full-sun estimate is not the official heat index
If the calculator shows an estimated full-sun apparent heat value, treat it as an additional cautionary estimate. The official heat index result is still the shaded, light-wind value.
Input Checklist Before You Trust the Answer
Heat index errors usually come from using the wrong weather value, mixing units, or applying the result to conditions that are hotter than a shaded weather station reading.
Use air temperature
Do not use roof, asphalt, dashboard, or equipment surface temperature as the air temperature input.
Enter humidity as percent
Use \(R=55\) for 55% relative humidity. Do not enter \(0.55\).
Check dew point mode
Dew point should normally be less than or equal to the air temperature.
Remember the shade assumption
Direct sun can make the apparent heat feel hotter than the official heat index.
Worked Example
This example shows how the calculator estimates heat index from a common hot-weather condition: 95°F air temperature and 55% relative humidity.
Formula
Substitution
Final answer
Heat index \(\approx109^\circ F\). This is reasonable because 95°F with high humidity should feel much hotter than the measured air temperature, and 109°F falls in the Danger range.
Why Humidity Raises the Heat Index
Humidity raises the heat index because it slows sweat evaporation. When sweat evaporates efficiently, the body cools better; when humid air limits evaporation, the same air temperature feels hotter.
The numbered visual avoids embedded text labels so it stays readable on mobile and does not create overlapping or black-background text.
1. Lower humidity
Sweat evaporates more easily, so the body cools more effectively and the heat index stays closer to the actual air temperature.
2. Higher humidity
Sweat evaporates more slowly, so the body loses less heat and the same air temperature feels hotter.
Reference Checks for Common Heat Index Values
Reference values help you tell whether the calculator result is plausible. A hot temperature with low humidity may feel only slightly hotter, while the same temperature with high humidity can jump into a dangerous range.
90°F and 70% humidity
Heat index is about \(106^\circ F\), which is in the Danger range.
95°F and 50% humidity
Heat index is about \(105^\circ F\), near the start of the Danger range.
100°F and 60% humidity
Heat index is about \(129^\circ F\), near the Extreme Danger threshold.
Below 80°F
Heat index is usually treated as close to the actual air temperature.
Risk range source note
The National Weather Service publishes heat index risk ranges and heat disorder guidance on pages such as its heat index reference chart. Treat the ranges as screening guidance, not exact medical cutoffs.
Design Notes and Practical Ranges
Heat index is not an engineering design load, but it is a useful environmental screening value. Use it to compare hot-weather scenarios, plan activity timing, and decide when a more detailed heat stress method is needed.
Public weather screening
Heat index works well for general feels-like temperature and public heat awareness.
Outdoor work
Use heat index as an early warning, then consider WBGT for work/rest planning, PPE, sun, wind, and radiant heat.
Direct sun
Full sun, hot asphalt, rooftops, and machinery can make the real heat stress higher than the shaded heat index.
What to do when the heat index is high
Reduce strenuous activity, take breaks in shade or air conditioning, drink water, check on vulnerable people, and avoid scheduling heavy outdoor work during the hottest part of the day.
Heat illness warning
If someone shows serious heat illness signs such as confusion, fainting, slurred speech, loss of consciousness, or hot red skin during extreme heat, treat it as an emergency and follow local emergency guidance.
Units and Conversions
The Rothfusz heat index regression is based on °F and percent relative humidity. If you enter °C, the calculator converts temperature to °F, calculates heat index, and then converts the answer back to °C if requested.
Temperature Conversion
Hidden unit trap
Relative humidity is entered as a percent. Enter 65 for 65%, not 0.65. Dew point should use the same temperature unit style you select in the calculator.
Why the equation uses Fahrenheit
If your weather app reports feels-like temperature in °C, the calculator may still perform the heat index equation internally in °F before converting the final result back to °C.
Heat Index vs Temperature, Dew Point, and WBGT
Heat index is one way to describe hot-weather risk, but it is not the only heat measure. Temperature, dew point, and WBGT each answer a different question.
| Measure | Uses | Best For |
|---|---|---|
| Air temperature | Measured temperature of the air. | Basic weather reporting. |
| Heat index | Combines air temperature and humidity. | Feels-like temperature and heat risk screening. |
| Dew point | Shows how much moisture is in the air. | Understanding muggy or oppressive conditions. |
| WBGT | Accounts for heat stress factors such as temperature, humidity, sun, wind, and radiant heat. | Worksite, athletic, and heat stress management decisions. |
NIOSH explains that WBGT is recommended for occupational exposure limits and alert limits, while heat index is still useful when WBGT equipment is not available.
Common Mistakes
The heat index equation is sensitive to humidity and units. Small input mistakes can produce a result that looks precise but does not describe the real outdoor condition.
Do
- Use shaded air temperature from a reliable weather observation.
- Enter humidity as a percent from 0 to 100.
- Use the full-sun estimate only as an added warning, not as the official heat index.
- Use WBGT guidance for occupational or athletic heat stress decisions when needed.
Don’t
- Do not enter 0.55 when you mean 55% relative humidity.
- Do not use pavement or roof temperature as the air temperature.
- Do not assume heat index includes direct sunlight.
- Do not treat risk categories as exact medical thresholds.
Troubleshooting Unrealistic Results
If the heat index result looks wrong, check the humidity, units, and formula range first. The calculator may still compute a number even when the inputs do not describe a realistic outdoor weather condition.
Result is too high
Check whether humidity was entered as 85 instead of 8.5, whether dew point is above air temperature, or whether a surface temperature was entered.
Result is too low
Confirm the humidity input is not accidentally entered as a decimal and that the output unit is not set to °C when you expected °F.
Heat index equals temperature
This can be normal below about 80°F or when humidity is low enough that the apparent temperature does not rise much.
Risk seems understated
Remember that direct sun, heavy work, PPE, hot surfaces, and poor airflow can make conditions more hazardous than the heat index alone suggests.
Assumptions and Limitations
The heat index is an estimate for human apparent temperature under shaded, light-wind conditions. It is not a full heat stress model and does not replace professional judgment, medical guidance, or worksite heat safety procedures.
Shade assumption
Official values are based on shade and light wind. Full sun can make conditions feel much hotter.
Formula range
The Rothfusz regression is empirical and is not intended for every extreme temperature and humidity combination.
Missing risk factors
Heat index does not include clothing, PPE, acclimatization, workload, hydration, radiant heat, or local health conditions.
Safety decisions
For work, sports, or emergency decisions, use applicable guidance and consider WBGT or site-specific heat stress evaluation.
Key Terms
These terms help connect the calculator inputs, formula, and risk interpretation.
Heat index
The apparent temperature that combines air temperature and humidity.
Relative humidity
The percentage of moisture in the air compared with saturation at the same temperature.
Dew point
The temperature at which air becomes saturated with moisture.
WBGT
Wet bulb globe temperature, a more complete heat stress metric for work and athletics.
Apparent temperature
The temperature the body feels after humidity and other comfort factors are considered.
Heat Index Calculator FAQ
What is the heat index?
The heat index is the apparent or feels-like temperature that combines air temperature and relative humidity. It estimates how hot conditions feel to the human body when humidity reduces sweat evaporation.
How do you calculate heat index?
Heat index is calculated from air temperature in degrees Fahrenheit and relative humidity in percent. For hot and humid conditions, the calculator uses the NOAA/NWS Rothfusz regression and applies the standard low-humidity or high-humidity adjustment when applicable.
Does heat index include sunlight?
No. Official heat index values assume shade and light wind. Direct sunshine, hot pavement, rooftops, and radiant heat can make conditions feel hotter than the calculated heat index.
What heat index is dangerous?
A heat index around 105°F or higher is commonly treated as a dangerous screening range because heat cramps or heat exhaustion are likely and heat stroke is possible with prolonged exposure or physical activity.
What is the difference between heat index and WBGT?
Heat index uses air temperature and humidity. Wet bulb globe temperature, or WBGT, accounts for additional heat stress factors such as sun, wind, and radiant heat, so it is more appropriate for worksite and athletic heat stress decisions.