Heat Index Calculator
Find the 'feels like' temperature from air temperature and humidity. Live weather for your country.
Health
Heat Index Calculator
Generated on April 25, 2026
Feels Like (Heat Index)
37°C
99 °F
Air: 32°C · 89.6°F · 60% RH
Heat cramps and heat exhaustion possible. Limit strenuous outdoor activity, especially midday.
Why does humidity make it feel hotter?
Heat-index estimates are based on shaded conditions and a light wind. Direct sun, calm air, or strenuous activity can push the felt temperature 5–10 °C higher.
Step-by-step calculation
Formula
HI ≈ Rothfusz regression (NOAA): HI = c₁ + c₂T + c₃R + c₄TR + c₅T² + c₆R² + c₇T²R + c₈TR² + c₉T²R²
- 1Temperature = 32°C = 89.6°F
- 2Relative humidity = 60%
- 3Within the Rothfusz regression's valid range — applying full NOAA polynomial.
- 4Heat Index = 98.7°F = 37.1°C → Extreme Caution
?What is the Heat Index Calculator?
The Heat Index — also called 'apparent temperature' or simply 'feels like' temperature — combines air temperature and relative humidity into a single number that better reflects how hot the body actually perceives the conditions. When humidity is high, sweat evaporates more slowly, so the body cannot cool itself as effectively. Two days at the same air temperature can feel completely different depending on humidity. This calculator uses the NOAA Rothfusz regression formula and live weather data from open-meteo.com so you can compare the air temperature reported by your forecast with what your body actually experiences.
The Formula
The Rothfusz regression is the polynomial used by the U.S. National Weather Service. It is valid for T ≥ 80 °F (≈ 27 °C) and R ≥ 40%. Below those thresholds the calculator falls back to a simpler Steadman approximation. NOAA also applies small corrections for very low humidity (RH < 13%) and very high humidity (RH > 85%) within specific temperature ranges. The output category — Caution, Extreme Caution, Danger, Extreme Danger — follows NOAA thresholds and aligns with widely published heat-safety guidance.
Heat Index Reference — Air Temperature × Humidity → 'Feels Like'
How the same air temperature feels at different humidity levels (NOAA Rothfusz regression). All values in °C.
| Air Temp | 30% RH | 50% RH | 70% RH | 90% RH |
|---|---|---|---|---|
| 27 °C / 80 °F | 26 °C | 27 °C | 29 °C | 31 °C |
| 29 °C / 84 °F | 28 °C | 30 °C | 32 °C | 35 °C |
| 32 °C / 90 °F | 31 °C | 34 °C | 38 °C | 44 °C |
| 35 °C / 95 °F | 34 °C | 39 °C | 47 °C | 57 °C |
| 38 °C / 100 °F | 38 °C | 44 °C | 57 °C | — |
| 40 °C / 104 °F | 41 °C | 51 °C | — | — |
| 43 °C / 110 °F | 45 °C | 59 °C | — | — |
Put It in Perspective
32 °C with 70% humidity (typical Karachi summer afternoon) feels like 38 °C — entering NOAA's 'Extreme Caution' band.
35 °C with 60% humidity in Lahore feels like 45 °C — beyond what most outdoor workers can sustain safely without breaks.
Dubai at 40 °C with low 25% humidity feels like only ~39 °C — dry desert heat is more tolerable than humid coastal heat at the same thermometer reading.
27 °C and 50% humidity is roughly room-comfort indoors — the heat-index formula barely moves above the air temp here.
A wet-bulb temperature of 35 °C (the survivability limit) corresponds to a heat index near 60 °C — beyond every NOAA category.
Saunas operate at 80–90 °C with very low humidity — the dry heat is tolerable; add steam (Turkish hammam) and the apparent heat skyrockets.
Practical Examples
32 °C (90 °F) air at 70% humidity feels like ≈ 41 °C (106 °F) — entering the 'Danger' band even though the thermometer hasn't crossed 35 °C.
40 °C (104 °F) air at 30% humidity feels like ≈ 39 °C — still extreme, but dry heat is more tolerable than the same number with moisture.
Karachi summer afternoons (35 °C, 65% RH) routinely produce a heat index above 47 °C, which explains why the city issues heatwave alerts even when the air temperature is 'only' 35–37 °C.
Indoor environments above 30 °C with 80%+ humidity (no AC, no fan) can produce heat-index readings near 38 °C — long enough exposure causes heat exhaustion in healthy adults.
Frequently Asked Questions
Popular Conversions
Jump to a ready-made conversion — useful for quick reference and sharing: