# Heat Index Calculator

Calculate the heat index using the NWS Rothfusz regression equation. Enter temperature and humidity to find the feels-like temperature and danger level.

## What this calculates

The heat index tells you how hot it actually feels when humidity is factored in with the air temperature. At 95°F with 55% humidity, it feels like 110°F because your body cannot cool itself as effectively through sweating when the air is already saturated with moisture. The NWS uses the heat index to issue heat advisories and warnings.

## Inputs

- **Air Temperature** (°F) — min 50 — Air temperature (80°F / 27°C minimum for heat index to apply).
- **Relative Humidity** (%) — min 0, max 100 — Relative humidity as a percentage (0-100).

## Outputs

- **Heat Index** (°F) — Feels-like temperature in Fahrenheit.
- **Heat Index** (°C) — Feels-like temperature in Celsius.
- **Danger Level** — formatted as text — NWS heat danger category.

## Details

The Rothfusz regression equation is the standard NWS formula for computing heat index:

HI = -42.379 + 2.049T + 10.143RH - 0.2248T*RH - 0.00684T2 - 0.0548RH2 + 0.00123T2*RH + 0.000853T*RH2 - 0.00000199T2*RH2

Where T is temperature in °F and RH is relative humidity in percent. Additional adjustments apply when humidity is very low (below 13%) or very high (above 85%).

**NWS Danger Categories:**
- **Caution (80-90°F HI):** Fatigue possible with prolonged exposure
- **Extreme Caution (91-103°F HI):** Heat cramps and heat exhaustion possible
- **Danger (104-125°F HI):** Heat cramps, exhaustion, and heat stroke likely
- **Extreme Danger (126°F+ HI):** Heat stroke highly likely

In the deadly 2023 Phoenix heat wave, air temperatures hit 119°F with low humidity, producing heat index values around 120°F. In more humid regions like the Gulf Coast, 95°F air temperature with 70% humidity produces a heat index of about 126°F, which is in the extreme danger zone.

## Frequently Asked Questions

**Q: When does the heat index apply?**

A: The heat index formula is designed for temperatures at or above 80°F (27°C). Below that threshold, the feels-like temperature is essentially the same as the actual air temperature. Humidity has little impact on comfort at lower temperatures.

**Q: Why does humidity make heat feel worse?**

A: Your body cools itself by evaporating sweat. When the air already holds a lot of moisture, sweat evaporates more slowly, so your body retains more heat. At 100% humidity, sweat basically does not evaporate at all.

**Q: What heat index is dangerous?**

A: The NWS considers a heat index of 104°F (40°C) or higher dangerous, with heat stroke likely during prolonged exposure. Above 126°F (52°C) is classified as extreme danger. Even the 91-103°F range (extreme caution) can cause heat cramps and exhaustion.

**Q: Is the heat index measured in the shade or sun?**

A: The heat index is calculated for shady, light-wind conditions. If you are in direct sunlight, the feels-like temperature can be up to 15°F higher. Always add 10-15°F to the heat index when accounting for full sun exposure.

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Source: https://vastcalc.com/calculators/physics/heat-index
Category: Physics
Last updated: 2026-04-08