{
  "id": "vpd",
  "version": "c0433fb95642",
  "status": "published",
  "name": "VPD Calculator",
  "question": "What is the VPD of my grow room?",
  "summary": "Computes the vapour pressure deficit (VPD) of the air from its temperature and relative humidity, with the saturation and actual vapour pressures, and the leaf VPD when you type a leaf temperature.",
  "category": "biology",
  "url": "https://www.acalculator.org/biology/vpd-calculator",
  "markdown": "https://www.acalculator.org/biology/vpd-calculator.md",
  "kind": "function",
  "method": "e°(T) = 0.6108 × exp(17.27 T ÷ (T + 237.3)) kPa; eₐ = e°(T_air) × RH ÷ 100; VPD = e°(T_air) − eₐ; leaf VPD = e°(T_leaf) − eₐ.",
  "assumptions": [
    "Temperatures are converted to °C before the formula. The formula is FAO-56’s Tetens form over water.",
    "The leaf VPD assumes the air inside the leaf is saturated at the leaf temperature.",
    "Temperatures run from −40 °C to 60 °C (−40 °F to 140 °F)."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "t": {
        "title": "Air temperature",
        "description": "The temperature of the air around the plants.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "temperature",
        "minimum": 233.14999999999998,
        "maximum": 333.15
      },
      "rh": {
        "title": "Relative humidity",
        "description": "The relative humidity of the air, from 0 to 100%.",
        "type": "number",
        "x-unit": "percent",
        "minimum": 0,
        "maximum": 100
      },
      "leaf": {
        "title": "Leaf temperature",
        "description": "The temperature of the leaves, from an infrared thermometer. Leave empty for the air VPD only.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "temperature",
        "minimum": 233.14999999999998,
        "maximum": 333.15
      }
    }
  },
  "outputs": {
    "vpd": {
      "label": "Vapour pressure deficit (air)",
      "description": "Saturation vapour pressure − actual vapour pressure.",
      "format": "quantity",
      "unit": "kPa"
    },
    "leafVpd": {
      "label": "Leaf VPD",
      "description": "Saturation vapour pressure at the leaf temperature − the actual vapour pressure of the air.",
      "format": "quantity",
      "unit": "kPa"
    },
    "saturation": {
      "label": "Saturation vapour pressure",
      "description": "0.6108 × exp(17.27 T ÷ (T + 237.3)) at the air temperature T in °C.",
      "format": "quantity",
      "unit": "kPa"
    },
    "actual": {
      "label": "Actual vapour pressure",
      "description": "The saturation vapour pressure × relative humidity ÷ 100.",
      "format": "quantity",
      "unit": "kPa"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "t": "77 F",
      "rh": 60
    },
    "outputs": {
      "vpd": 1267.1110870027428,
      "saturation": 3167.7777175068572,
      "actual": 1900.6666305041144
    },
    "text": "At 77 °F and 60% relative humidity, the vapour pressure deficit is 1.267 kPa."
  },
  "examples": [
    {
      "given": {
        "t": 297.65,
        "rh": 50
      },
      "expect": {
        "saturation": 3074.64905088159,
        "vpd": 1537.324525440795,
        "actual": 1537.324525440795
      },
      "source": "FAO Irrigation and Drainage Paper 56, Crop evapotranspiration, chapter 3, Air humidity (equation 11: e°(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa; equation 19: eₐ = e° × RH ÷ 100; the vapour pressure deficit is the saturation minus the actual vapour pressure; e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa), https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03); hand calculation in content.mdx: 3.0746 × (1 − 0.50) = 1.537 kPa"
    },
    {
      "given": {
        "t": 288.15,
        "rh": 80
      },
      "expect": {
        "saturation": 1705.3462321157722,
        "vpd": 341.06924642315437
      },
      "source": "FAO Irrigation and Drainage Paper 56, Crop evapotranspiration, chapter 3, Air humidity (equation 11: e°(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa; equation 19: eₐ = e° × RH ÷ 100; the vapour pressure deficit is the saturation minus the actual vapour pressure; e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa), https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03); hand calculation in content.mdx: 1.7053 × 0.20 = 0.341 kPa"
    },
    {
      "given": {
        "t": 298.15,
        "rh": 60,
        "leaf": 296.15
      },
      "expect": {
        "vpd": 1267.1110870027392,
        "leafVpd": 908.7709918929608
      },
      "source": "FAO Irrigation and Drainage Paper 56, Crop evapotranspiration, chapter 3, Air humidity (equation 11: e°(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa; equation 19: eₐ = e° × RH ÷ 100; the vapour pressure deficit is the saturation minus the actual vapour pressure; e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa), https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03); hand calculation in content.mdx: e°(23) − 0.60 × e°(25) = 2.8094 − 1.9007 = 0.909 kPa"
    }
  ],
  "sources": [
    "FAO Irrigation and Drainage Paper 56, Crop evapotranspiration (Allen, Pereira, Raes and Smith, 1998), chapter 3, Meteorological data, Air humidity: equation 11 for the saturation vapour pressure, equation 19 for the actual vapour pressure from relative humidity, and the vapour pressure deficit as their difference; Example 3 gives e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa. https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03)"
  ],
  "related": [
    "dew-point",
    "heat-index"
  ],
  "changelog": []
}