{
  "slug": "heat-index-calculator",
  "title": "Heat Index Calculator",
  "heading": "Heat Index Calculator",
  "category": "other",
  "url": "https://www.revenuelab.fyi/toolbox/heat-index-calculator",
  "summary": "Feels-like temperature from air temp and humidity using the NWS Rothfusz regression.",
  "description": "Heat index is what your body actually experiences when humidity slows evaporative cooling from sweat. The National Weather Service uses the Rothfusz regression, a polynomial fit to Steadman's original biometeorological model, valid for temperatures at or above 80°F and relative humidity at or above 40%. Below that range the NWS just reports actual air temperature since the correction is negligible. This tool applies the full regression plus the two standard adjustments: a downward correction for low humidity with high heat, and an upward correction for high humidity with moderate heat. Heat index drives NWS heat advisories and warnings — 90-103°F is caution territory for heat exhaustion, 103-124°F triggers extreme caution to danger, and 125°F+ is extreme danger with heat stroke likely on continued exposure.",
  "formula": "HI = -42.379 + 2.049×T + 10.143×RH − 0.225×T×RH − 6.838e-3×T² − 5.482e-2×RH² + 1.229e-3×T²×RH + 8.528e-4×T×RH² − 1.99e-6×T²×RH², with low/high-humidity adjustments applied per NWS convention.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=heat-index-calculator",
  "inputs": [
    {
      "id": "temp",
      "label": "Air temperature",
      "kind": "number",
      "hint": null,
      "default": 95,
      "unit": "°F",
      "min": 40,
      "max": 130
    },
    {
      "id": "rh",
      "label": "Relative humidity",
      "kind": "number",
      "hint": null,
      "default": 55,
      "unit": "%",
      "min": 0,
      "max": 100
    }
  ],
  "outputs": [
    {
      "id": "HI",
      "label": "Heat index (feels like)",
      "format": "number",
      "hint": null,
      "primary": true
    },
    {
      "id": "category",
      "label": "NWS risk category",
      "format": "raw",
      "hint": null,
      "primary": false
    },
    {
      "id": "diff",
      "label": "Degrees hotter than air temp",
      "format": "number",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Air temperature: 95 °F",
      "Relative humidity: 55 %"
    ],
    "outputs": [
      "Heat index (feels like): 109",
      "NWS risk category: Danger",
      "Degrees hotter than air temp: 14"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter air temperature (°F).",
      "Enter relative humidity (%).",
      "Read your heat index (feels like) on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "temp": 57,
        "rh": 33
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "temp": 95,
        "rh": 55
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "temp": 130,
        "rh": 88
      }
    }
  ],
  "limitations": [
    "Estimates assume standard, average conditions — local rules, pricing, and materials vary.",
    "Results are rounded for readability; add a buffer before ordering, booking, or committing.",
    "Double-check anything with a real cost attached against a local quote."
  ],
  "faq": [
    {
      "q": "Why doesn't heat index apply below 80°F?",
      "a": "The Rothfusz regression was fit to data in the 80-110°F, 40-100% RH range, and below 80°F humidity's effect on perceived temperature is small enough that the NWS just reports the actual air temperature instead of a barely-different corrected number."
    },
    {
      "q": "Is heat index the same as the heat category used for outdoor sports?",
      "a": "No. Athletic and military heat categories (like WBGT — wet bulb globe temperature) also factor in direct sun load and wind, so they run hotter-feeling at the same air temp and humidity than NWS heat index, which assumes shade and light wind."
    },
    {
      "q": "What humidity makes the biggest difference?",
      "a": "Between 40-70% RH at temperatures near 90-100°F the curve is steepest — each 10-point jump in humidity can add 5-10°F to the feels-like number, which is why muggy 92°F days feel more dangerous than dry 100°F days."
    },
    {
      "q": "How accurate is this for direct sunlight?",
      "a": "Add roughly 10-15°F to the result if you're in direct sun rather than shade; the NWS heat index formula assumes shaded conditions with light wind around 5 mph."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/wind-chill-calculator",
    "https://www.revenuelab.fyi/toolbox/dew-point-calculator",
    "https://www.revenuelab.fyi/toolbox/wbgt-heat-stress"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Heat Index Calculator (https://www.revenuelab.fyi/toolbox/heat-index-calculator)"
}