Heat Index Calculator
Compute heat index (apparent temperature — how hot the body feels) from air temperature and relative humidity, using the Rothfusz regression formula of the National Weather Service (NWS/NOAA). All computation runs on your device, no server.
Compute Heat Index
Results
- Input temperature
- —
- RH input
- —
- Jalur formula
- —
- Penyesuaian
- —
Verified worked examples
The numbers below were recomputed from a high-precision oracle (50-digit Decimal), not from memory.
- Diketahui: T = 90 °F, RH = 60%.
- Simple formula first: 0.5×{90 + 61.0 + [(90−68.0)×1.2] + (60×0.094)} = 91.52; average with T = 90.76 ≥ 80 → use the full Rothfusz regression.
- Regresi Rothfusz: HI = 99.68 °F ≈ 100 °F = 37.6 °C.
- RH 60% triggers no adjustment (not <13% or >85%).
- NWS category: Extreme Caution (91–103 °F) — kram panas & heat exhaustion mungkin terjadi.
- Pembanding ekstrem: T = 100 °F, RH = 55% → HI = 123.64 °F ≈ 124 °F = 50.9 °C, category Danger.
NWS danger categories
| Heat index | Category | Artinya |
|---|---|---|
| < 80 °F | Below threshold | No danger level set |
| 80–90 °F | Caution | Fatigue possible with prolonged exposure/physical activity |
| 91–103 °F | Extreme Caution | Kram panas & heat exhaustion mungkin terjadi |
| 104–124 °F | Danger | Heat exhaustion mungkin, heatstroke mungkin terjadi |
| ≥ 125 °F | Extreme Danger | Heatstroke very likely with continued exposure |
T in °F, RH in percent. Adjustments: RH < 13% & 80 ≤ T ≤ 112 °F → subtract ((13−RH)/4)·√((17−|T−95|)/17); RH > 85% & 80 ≤ T ≤ 87 °F → add ((RH−85)/10)·((87−T)/5). If the initial result is < 80 °F, the simple Steadman formula is used.
Pertanyaan umum
What is the heat index?
The heat index is the apparent temperature — the temperature the human body ‘feels’ when relative humidity is combined with the actual air temperature. This calculator uses the Rothfusz regression formula from the U.S. National Weather Service (NWS), the official standard used by U.S. weather services.
How is the heat index calculated?
Enter the air temperature and relative humidity (RH). If the initial result is below 80°F, the simple Steadman formula is used; at 80°F and above, the Rothfusz regression applies: HI = -42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH², plus two adjustments for very dry or very humid air.
Why is the heat index sometimes lower than the air temperature?
At very low humidity (RH below 13%), sweat evaporates efficiently so the body feels cooler than the thermometer reads. NWS reduces the regression result by ((13−RH)/4)·√((17−|T−95|)/17) for 80–112°F conditions. Conversely, RH above 85% raises the value because evaporation is inhibited.
Is the heat index the same as the ‘feels like’ in weather apps?
Not exactly. The NWS heat index is defined for shady conditions with a light breeze. Weather apps sometimes combine it with other factors. Honest label: the number here is the apparent temperature in the shade — in direct sunlight, NWS notes the felt value can be up to 15°F (±8°C) higher.
What are the NWS heat index danger categories?
NWS uses four levels: Caution 80–90°F (fatigue possible with prolonged exposure), Extreme Caution 91–103°F (heat cramps and heat exhaustion possible), Danger 104–124°F (heatstroke possible), and Extreme Danger 125°F and above (heatstroke very likely). Below 80°F no danger level is set.
Can I enter the temperature in Celsius?
Yes. Select the °C unit on the temperature field; the calculator converts to Fahrenheit (F = C×9/5 + 32) because the Rothfusz formula is defined in Fahrenheit, then shows the result in °F and °C together.
Why is the temperature limited to 70–110°F (21–43°C)?
That is the temperature-axis range of the official NWS heat index chart. Outside that range the Rothfusz regression becomes unreliable extrapolation — NWS itself states the formula is not valid for extreme conditions beyond the Steadman data. This limit keeps the displayed numbers defensible.
What is the difference between heat index and humidex?
Both measure ‘how hot it feels’, but humidex is a Canadian index based on the dew point with a different formula, while the NWS heat index is based on temperature + relative humidity. Their values cannot be compared directly; use one consistently.
When should I be genuinely worried about the heat index?
Starting at the Danger category (104°F / 40°C and above): limit strenuous outdoor activity, drink water before you feel thirsty, and know the signs of heatstroke — hot-dry skin, confusion, fainting — which need emergency medical care. At-risk groups (the elderly, children, outdoor workers) need protection earlier, starting at Extreme Caution.
Does this calculator replace official weather warnings?
No. This is an educational tool that computes the NWS formula from the numbers you enter. For safety decisions, follow heat advisories/warnings from official meteorological agencies such as BMKG or NWS, which consider duration, nighttime temperatures, and other local factors.
Free tool by NRTechStudio