Heat Index Calculator
Calculate the heat index from temperature and humidity and see the danger category.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
Heat index (°F)
Dots show recalculated samples. Other inputs stay fixed.
About this calculator
Heat index estimates how hot it feels in shade with light wind from air temperature and relative humidity. Sun exposure can raise heat stress; this result does not capture direct sun, activity, hydration, or personal health. The NWS method uses a simpler estimate below its 80°F range and humidity adjustments in specified conditions; predictions are approximate. At hotter temperatures, combinations outside the published NWS chart are rejected instead of extrapolated.
Worked example
calculated with the default values below
Air temperature 95 °F · Relative humidity 55 % · Temperature unit Fahrenheit Heat index 108.9 °F · Heat index 42.7 °C · Above the actual temperature 13.9 °F · Heat-index increase above air temperature 7.7 °C · Category Danger · Precaution Heat exhaustion likely, heat stroke possible
Method and assumptions
HI = mean(T, HI_simple) if preliminary HI<80°F; otherwise Rothfusz(T,RH)+humidity adjustments NWS method: preliminary HI = mean(T, simple estimate); if preliminary HI ≥80°F, use Rothfusz regression with low-RH (<13%, 80–112°F) and high-RH (>85%, 80–87°F) adjustments. Approximation assumes shade/light wind; reject T >112°F or RH outside 0–100%. Heat index estimates how hot it feels in shade with light wind from air temperature and relative humidity. Sun exposure can raise heat stress; this result does not capture direct sun, activity, hydration, or personal health. The NWS method uses a simpler estimate below its 80°F range and humidity adjustments in specified conditions; predictions are approximate. At hotter temperatures, combinations outside the published NWS chart are rejected instead of extrapolated. The references explain definitions and methods; any extra preset must be checked against the assumptions on this page.
Data sources
The references explain definitions and methods; any extra preset must be checked against the assumptions on this page.
Disclaimer
These calculators give general estimates. Sizing, pricing, availability and standards vary by brand, retailer and country, so treat the output as a starting point and confirm it against the seller's or authority's own figures.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
HI = mean(T, HI_simple) if preliminary HI<80°F; otherwise Rothfusz(T,RH)+humidity adjustments
- T
- Air temperature, °F
- R
- Relative humidity, %
- HI
- Heat index, °F
NWS method: preliminary HI = mean(T, simple estimate); if preliminary HI ≥80°F, use Rothfusz regression with low-RH (<13%, 80–112°F) and high-RH (>85%, 80–87°F) adjustments. Approximation assumes shade/light wind; reject T >112°F or RH outside 0–100%.
Frequently asked questions
How does the heat-index estimate work below 80°F?
It first calculates the NWS simple estimate and averages it with air temperature. If that preliminary value reaches 80°F, it applies the Rothfusz regression and the specified low- or high-humidity adjustment. This is an approximation for shade and light wind.
Does the result describe my full heat exposure?
No. Direct sun, exertion, hydration, clothing, acclimatization, and health affect heat strain but are not inputs. Risk can remain below an alert band; follow local weather and heat-safety guidance.
Which inputs and limits apply to Heat Index Calculator?
Use the units shown beside each field. These are form limits, not recommended settings. Air temperature (Fahrenheit): 40–110; Air temperature (Celsius): 4.4444444444–43.3333333333; Relative humidity: 0–100 % HI = mean(T, HI_simple) if preliminary HI<80°F; otherwise Rothfusz(T,RH)+humidity adjustments NWS method: preliminary HI = mean(T, simple estimate); if preliminary HI ≥80°F, use Rothfusz regression with low-RH (<13%, 80–112°F) and high-RH (>85%, 80–87°F) adjustments. Approximation assumes shade/light wind; reject T >112°F or RH outside 0–100%. Heat index estimates how hot it feels in shade with light wind from air temperature and relative humidity. Sun exposure can raise heat stress; this result does not capture direct sun, activity, hydration, or personal health. The NWS method uses a simpler estimate below its 80°F range and humidity adjustments in specified conditions; predictions are approximate. At hotter temperatures, combinations outside the published NWS chart are rejected instead of extrapolated. The references explain definitions and methods; any extra preset must be checked against the assumptions on this page.
What is a complete worked example for Air temperature = 95 °F; Relative humidity = 55 %?
Inputs: Air temperature = 95 °F; Relative humidity = 55 %; Temperature unit = Fahrenheit. Calculation: HI = mean(T, HI_simple) if preliminary HI<80°F; otherwise Rothfusz(T,RH)+humidity adjustments. Results: Heat index = 108.9 °F; Heat index = 42.7 °C; Above the actual temperature = 13.9 °F; Heat-index increase above air temperature = 7.7 °C; Category = Danger; Precaution = Heat exhaustion likely, heat stroke possible.