{
  "slug": "relative-humidity-wet-bulb",
  "title": "Relative Humidity from Wet Bulb Calculator",
  "heading": "Relative Humidity from Wet Bulb Temperature",
  "category": "other",
  "url": "https://www.revenuelab.fyi/toolbox/relative-humidity-wet-bulb",
  "summary": "Estimate RH and dew point from dry bulb and wet bulb thermometer readings.",
  "description": "A sling psychrometer measures two temperatures: dry bulb (regular air temperature) and wet bulb (a thermometer wrapped in a wet wick, cooled by evaporation). The gap between them tells you humidity — a big gap means dry air evaporating water fast, no gap means saturated air. This calculator uses a standard psychrometric approximation (a form of the Sprung equation) to back out relative humidity and dew point from those two readings, which is how humidity was measured for over a century before capacitive humidity sensors became cheap. It's still the standard field method for calibrating electronic hygrometers and is used in industrial and agricultural settings where a simple, no-battery instrument is preferred.",
  "formula": "Vapor pressure e = es(Tw) − A×P×(T−Tw), where A≈0.000662/°C, P is station pressure, es is saturation vapor pressure (Tetens equation); RH = e / es(T) × 100.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=relative-humidity-wet-bulb",
  "inputs": [
    {
      "id": "dryBulb",
      "label": "Dry bulb temperature",
      "kind": "number",
      "hint": null,
      "default": 85,
      "unit": "°F",
      "min": 0,
      "max": 130
    },
    {
      "id": "wetBulb",
      "label": "Wet bulb temperature",
      "kind": "number",
      "hint": null,
      "default": 72,
      "unit": "°F",
      "min": 0,
      "max": 130
    },
    {
      "id": "pressure",
      "label": "Station pressure",
      "kind": "number",
      "hint": null,
      "default": 1013,
      "unit": "hPa",
      "min": 800,
      "max": 1050
    }
  ],
  "outputs": [
    {
      "id": "rh",
      "label": "Relative humidity",
      "format": "percent",
      "hint": null,
      "primary": true
    },
    {
      "id": "dpF",
      "label": "Dew point",
      "format": "number",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Dry bulb temperature: 85 °F",
      "Wet bulb temperature: 72 °F",
      "Station pressure: 1013 hPa"
    ],
    "outputs": [
      "Relative humidity: 53.4%",
      "Dew point: 66.2"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter dry bulb temperature (°F).",
      "Enter wet bulb temperature (°F).",
      "Enter station pressure (hPa).",
      "Read your relative humidity 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": {
        "dryBulb": 51,
        "wetBulb": 43,
        "pressure": 800
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "dryBulb": 85,
        "wetBulb": 72,
        "pressure": 1013
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "dryBulb": 130,
        "wetBulb": 115,
        "pressure": 1050
      }
    }
  ],
  "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 is wet bulb always lower than or equal to dry bulb?",
      "a": "Evaporation from the wet wick pulls heat from the thermometer, cooling it below ambient air temperature. In perfectly saturated air (100% RH) no net evaporation occurs, so the two readings converge and equal each other."
    },
    {
      "q": "Does elevation matter for this calculation?",
      "a": "Yes — station pressure appears directly in the psychrometric equation because evaporation rate depends on ambient air pressure. At high elevation (lower pressure) the same wet bulb depression corresponds to slightly higher humidity than at sea level."
    },
    {
      "q": "Is wet bulb temperature the same as wet bulb globe temperature (WBGT)?",
      "a": "No. Plain wet bulb temperature measures evaporative cooling only. WBGT, used for heat stress guidelines in sports and military settings, combines wet bulb, dry bulb, and a black globe thermometer to also account for radiant solar heat."
    },
    {
      "q": "How accurate is a sling psychrometer compared to digital sensors?",
      "a": "A well-maintained sling psychrometer with fresh distilled water on the wick is accurate to about ±2-3% RH, comparable to mid-range digital hygrometers, and it doesn't drift with age the way capacitive sensors can."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/dew-point-calculator",
    "https://www.revenuelab.fyi/toolbox/heat-index-calculator"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Relative Humidity from Wet Bulb Calculator (https://www.revenuelab.fyi/toolbox/relative-humidity-wet-bulb)"
}