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Weather calculators

Heat Index Calculator

This calculator estimates heat index (apparent temperature) from air temperature and relative humidity or dew point using the National Weather Service (NWS) Rothfusz regression. Enter °F or °C; when temperature is below 80°F or relative humidity below 40%, heat index equals air temperature. Optional direct-sun adds 15°F (8.3°C). It shows NWS-style risk bands and does not model wind, saunas, or enclosed-vehicle heat.

By Jeff Beem

Updated

01

Settings

02

Air temperature

03

Moisture input

%

Adds 15°F (8.3°C) to the heat index when checked.

Results

Caution

Fatigue possible with prolonged exposure.

Feels like

86.3 °F

30.2 °C

Note: The Steadman heat index algorithm is designed for ambient air temperatures between 80°F and 120°F. Results for extreme temperatures are theoretical approximations.

How to use this calculator

Under 01 Settings, pick °F or °C and result decimals. Enter air temperature in 02 (defaults 85°F / 29.4°C). In 03 Moisture input, use relative humidity or switch to dew point. Check In direct sunlight to add 15°F (8.3°C). The results panel shows feels-like heat index and an NWS-style risk banner. Heat index applies at 80°F and 40% RH or above; below that, feels-like matches air temperature.

Reading feels-like heat

Section 01 Settings sets units and decimals. Enter air temperature in 02, moisture in 03, and optional direct sun. The dark Results panel shows Feels like heat index plus an NWS-style risk banner.

Example: 85°F (29.4°C), 50% relative humidity

Defaults: air 85°F (29.4°C) and 50% relative humidity with In direct sunlight off. Rothfusz heat index ≈ 86.5°F (30.3°C) → Caution banner (80–90°F band). Thermometer and feels-like differ by only a degree or two at moderate humidity; the gap widens as you raise the section 03 humidity slider.

Humidity slider vs feels-like headline

Try 95°F in section 02 with the section 03 slider at 30%: relative humidity below 40% means the widget returns 95°F feels-like (no Rothfusz boost). Move the slider to 80% and the same air temperature jumps to about 133.8°F—Extreme Danger on the results banner.

Dew point toggle in section 03

Switch moisture input from Rel. humidity to Dew point when you have a dew-point observation. Default dew point 18.9°C (66°F) pairs with default air temperature to yield about 50% relative humidity internally. Either path feeds the same Feels like output.

Direct sun and formula limits

The Rothfusz regression targets roughly 80–120°F air in shaded, light-wind conditions. Checking In direct sunlight adds 15°F before the risk banner updates. Above 150°F air the widget flags non-meteorological extremes—use occupational heat standards for furnaces and enclosed vehicles, not this outdoor model alone.

Heat index calculator: NWS Rothfusz and risk bands

Apparent temperature from air temperature and humidity or dew point using the National Weather Service (NWS) Rothfusz regression. Shaded baseline; optional +15°F in direct sun.

What this calculator does

The widget computes heat index (apparent temperature) from air temperature and relative humidity, or from temperature and dew point with humidity derived internally. It uses the NWS Rothfusz regression when temperature ≥ 80°F and relative humidity ≥ 40%; otherwise returns air temperature. The optional direct-sun checkbox adds 15°F (8.3°C). The results panel displays NWS-style risk bands from Caution through Extreme Danger. It does not model wind speed, radiant load in complex environments, or occupational compliance rules.
  • Applies when:
    T≥80∘FT \geq 80^\circ\text{F} and RH≥40%\text{RH} \geq 40\%
  • Direct sun:
    Optional +15∘F15^\circ\text{F} (8.3°C) to the computed heat index

How the math works

With defaults—air 85°F and 50% relative humidity—the Rothfusz regression evaluates to about 86.5°F feels-like (≈ 30.3°C). That lands in the Caution band (80–90°F heat index) on the results banner.
The regression combines temperature and humidity in one apparent-temperature value. When either input falls below the activation window (air under 80°F or RH under 40%), the widget skips the polynomial and sets feels-like equal to air temperature—so a dry 95°F afternoon at 30% RH still reads 95°F, while the same thermometer at 80% RH jumps to about 133.8°F.
Dew-point mode converts moisture to relative humidity first via Magnus-Tetens, then applies the same formula. Checking In direct sunlight adds a flat 15°F (8.3°C) after the regression, which can bump a borderline Caution value into Extreme Caution on sunny pavement.

Limits of the model

Heat index assumes shaded, light-wind outdoor air. It does not replace wet-bulb globe temperature (WBGT) for sports or OSHA heat stress programs. Strong wind, clothing, acclimatization, and radiant heat from surfaces can shift real discomfort. Inputs above 150°F trigger meteorological warnings; values above 250°F are rejected as out of range. Use occupational standards for industrial and enclosed-vehicle scenarios.

Heat Index Calculator FAQ

What is heat index?

Heat index is apparent temperature: how hot it feels when humidity slows sweat evaporation. The widget uses the National Weather Service (NWS) Rothfusz regression when air temperature is at least 80°F (27°C) and relative humidity at least 40%; otherwise the Feels like line in the results panel matches air temperature. Check In direct sunlight to add about 15°F (8.3°C) per NWS guidance.

Can I enter dew point instead of humidity?

Yes. In section 03 Moisture input, switch the toggle from Rel. humidity to Dew point and enter values in °F or °C. The calculator derives relative humidity with a Magnus-Tetens relation, then runs the same Rothfusz equation shown in the results panel.

What do the NWS risk levels mean?

Caution (80–90°F heat index): fatigue with long exposure. Extreme Caution (91–103°F): heat cramps and exhaustion more likely. Danger (104–124°F): heatstroke possible. Extreme Danger (≥125°F): heatstroke highly likely. The colored banner above Feels like maps your computed value to these bands.

Why does heat index sometimes equal air temperature?

The Rothfusz formula is validated for warm, humid air. Below 80°F air or 40% relative humidity, the widget returns the thermometer reading unchanged—an info note appears under section 02 Air temperature when air is below 80°F.

What does No Advisory mean?

When computed heat index stays below 80°F, the results banner shows No Advisory with a green background. That usually means cool air, low humidity, or both—the model is not adding a humidity penalty on top of the thermometer.

When is outdoor work risky?

Occupational heat guidance often escalates near 104°F heat index (Danger band). Above 125°F (Extreme Danger), reschedule strenuous outdoor work when possible, increase rest breaks, and hydrate. This page is planning context, not an OSHA substitute.

Does wind or shade change the number?

The base equation assumes light wind and shade. Checking In direct sunlight adds 15°F (8.3°C) to the Feels like headline. Strong breeze can lower perceived heat slightly, but wind speed is not modeled here.

What happens at extreme air temperatures?

Air above 250°F triggers “Value out of range.” Between 150°F and that cap, the widget warns that inputs exceed meteorological norms and may show an EXTREME THERMAL HAZARD banner. Saunas, enclosed vehicles, and industrial heat need different safety standards than this outdoor regression.

Sources & citations

References used for the calculation method and definitions. Links open in a new tab when available.

[1]
Heat Index – National Weather Service

NWS heat index overview, risk categories, and direct-sun adjustment guidance.

[2]
Rothfusz LP. The Heat Index "Equation" (or, More Than You Ever Wanted to Know About Heat Index). NWS Technical Attachment SR 90-23

Original Rothfusz regression coefficients used in this calculator.

Mathematical Reference Note

Calculation Logic: This tool uses standard mathematical algorithms. While we strive for accuracy, errors in logic or user input can result in incorrect data.

Verification: Results should be cross-checked if used for important academic, professional, or personal calculations.

Standard Terms: This tool is provided free of charge and as-is. CalcRegistry provides no warranty regarding the accuracy or fitness of these results for your specific needs.

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