{
  "slug": "uv-exposure-burn-time",
  "title": "UV Exposure Burn Time Calculator",
  "heading": "UV Index Sunburn Time Calculator",
  "category": "fitness-health",
  "url": "https://www.revenuelab.fyi/toolbox/uv-exposure-burn-time",
  "summary": "Estimate minutes to sunburn based on UV index, skin type, and sunscreen SPF.",
  "description": "Time to sunburn depends primarily on UV index and skin phototype (the Fitzpatrick scale), then gets extended by sunscreen's SPF factor and shortened by reflective surfaces like snow, sand, or water. This calculator uses published baseline minimal erythema dose (MED) times for each Fitzpatrick skin type at UV index 1, scales inversely with the actual UV index, then multiplies by your sunscreen's SPF (properly applied sunscreen at SPF 30 roughly multiplies burn time by 30, though real-world application is usually thinner than the tested standard, cutting effective protection significantly). Use this as a planning tool for reapplication timing, not a guarantee — cloud cover, altitude, and surface reflection all shift real UV exposure.",
  "formula": "Burn time (min) = (base MED minutes for skin type ÷ UV index) × effective SPF factor × surface reflection adjustment.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=uv-exposure-burn-time",
  "inputs": [
    {
      "id": "skinType",
      "label": "Skin type (Fitzpatrick)",
      "kind": "select",
      "hint": null,
      "default": "3",
      "options": [
        {
          "value": "1",
          "label": "Type I — very fair, always burns"
        },
        {
          "value": "2",
          "label": "Type II — fair, burns easily"
        },
        {
          "value": "3",
          "label": "Type III — medium, sometimes burns"
        },
        {
          "value": "4",
          "label": "Type IV — olive, rarely burns"
        },
        {
          "value": "5",
          "label": "Type V — brown, very rarely burns"
        },
        {
          "value": "6",
          "label": "Type VI — dark brown/black, almost never burns"
        }
      ]
    },
    {
      "id": "uvIndex",
      "label": "UV index",
      "kind": "number",
      "hint": null,
      "default": 8,
      "unit": null,
      "min": 1,
      "max": 14
    },
    {
      "id": "spf",
      "label": "Sunscreen SPF (0 = none)",
      "kind": "number",
      "hint": null,
      "default": 30,
      "unit": null,
      "min": 0,
      "max": 100
    },
    {
      "id": "surface",
      "label": "Surface reflection",
      "kind": "select",
      "hint": null,
      "default": "1",
      "options": [
        {
          "value": "1",
          "label": "Normal ground / grass"
        },
        {
          "value": "0.7",
          "label": "Water (boosts UV ~30%)"
        },
        {
          "value": "0.5",
          "label": "Sand (boosts UV ~50%)"
        },
        {
          "value": "0.15",
          "label": "Fresh snow (boosts UV ~85%)"
        }
      ]
    }
  ],
  "outputs": [
    {
      "id": "withProtection",
      "label": "Est. burn time with sunscreen",
      "format": "duration",
      "hint": null,
      "primary": true
    },
    {
      "id": "noProtection",
      "label": "Burn time with no protection",
      "format": "duration",
      "hint": null,
      "primary": false
    },
    {
      "id": "reapply",
      "label": "Suggested reapplication interval",
      "format": "duration",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Skin type (Fitzpatrick): Type III — medium, sometimes burns",
      "UV index: 8",
      "Sunscreen SPF (0 = none): 30",
      "Surface reflection: Normal ground / grass"
    ],
    "outputs": [
      "Est. burn time with sunscreen: 266",
      "Burn time with no protection: 17",
      "Suggested reapplication interval: 120"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter skin type (fitzpatrick).",
      "Enter uv index.",
      "Enter sunscreen spf (0 = none).",
      "Enter surface reflection.",
      "Read your est. burn time with sunscreen 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": {
        "skinType": "3",
        "uvIndex": 5,
        "spf": 20,
        "surface": "1"
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "skinType": "3",
        "uvIndex": 8,
        "spf": 30,
        "surface": "1"
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "skinType": "3",
        "uvIndex": 13,
        "spf": 50,
        "surface": "1"
      }
    }
  ],
  "limitations": [
    "Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.",
    "Results do not account for medical conditions, medication, or pregnancy.",
    "Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it."
  ],
  "faq": [
    {
      "q": "Why does the calculator assume sunscreen performs below its labeled SPF?",
      "a": "SPF testing uses 2 mg/cm² of product, roughly a shot-glass amount for a full body, applied uniformly. Real-world users typically apply 20-50% of that amount, which cuts effective protection well below the labeled number — this tool applies a rough real-world discount instead of the lab-rated multiplier."
    },
    {
      "q": "How much does UV index change throughout the day?",
      "a": "UV index peaks within about 2 hours of solar noon and can be 3-4 times higher at midday than in early morning or late afternoon at the same air temperature, so the same activity at 10am vs. 1pm can have very different burn risk even on the same day."
    },
    {
      "q": "Why do reflective surfaces matter so much?",
      "a": "Fresh snow reflects up to 80-90% of UV back onto skin (which is why snow blindness and skiers' sunburns happen even in cold weather), water reflects roughly 10-100% depending on angle, and dry sand reflects around 15-25%, all adding to direct overhead exposure."
    },
    {
      "q": "Does cloud cover eliminate UV risk?",
      "a": "No — up to 80% of UV radiation passes through light cloud cover, and some cloud conditions can even intensify UV briefly through scattering effects. Overcast days still require sun protection at moderate-to-high UV index readings."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/air-quality-exposure-equivalent",
    "https://www.revenuelab.fyi/toolbox/heat-index-calculator"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — UV Exposure Burn Time Calculator (https://www.revenuelab.fyi/toolbox/uv-exposure-burn-time)"
}