{
  "id": "pack-year",
  "version": "742e0d6ac7b1",
  "status": "published",
  "name": "Pack Year Calculator",
  "question": "How many pack-years did I smoke?",
  "summary": "Works out a smoking history in pack-years, packs a day times years smoked, over one or more periods, and says whether it reaches the 20 pack-years in the USPSTF lung cancer screening advice.",
  "category": "health",
  "subcategory": "body",
  "url": "https://www.acalculator.org/health/pack-year-calculator",
  "markdown": "https://www.acalculator.org/health/pack-year-calculator.md",
  "kind": "function",
  "method": "pack-years = Σ (packs a day × years) over each period, where packs a day = cigarettes a day ÷ 20.",
  "assumptions": [
    "A pack holds 20 cigarettes.",
    "Each period has one steady amount a day; split your history into rows when it changed.",
    "Arithmetic is exact on the typed decimals; pack-years show to one decimal, rounded half up.",
    "Pack-years measure exposure only. They are one part of screening advice and not a diagnosis."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "unit": {
        "title": "I count",
        "description": "Whether you type how much you smoked a day in packs or in cigarettes (20 to a pack).",
        "type": "string",
        "enum": [
          "packs",
          "cigs"
        ]
      },
      "periods": {
        "title": "Smoking periods",
        "description": "One row for each stretch of time you smoked about the same amount: how much a day, and for how many years.",
        "type": "array",
        "items": {
          "type": "object",
          "properties": {
            "a": {
              "title": "Amount a day",
              "description": "Packs a day, or cigarettes a day, as chosen above.",
              "type": "number",
              "minimum": 0,
              "maximum": 200
            },
            "y": {
              "title": "Years",
              "description": "How many years you smoked that amount.",
              "type": "number",
              "minimum": 0,
              "maximum": 100
            }
          }
        }
      }
    }
  },
  "outputs": {
    "packYears": {
      "label": "Pack-years",
      "description": "The sum over all periods of packs a day × years.",
      "format": "number"
    },
    "cigarettes": {
      "label": "Cigarettes smoked",
      "description": "The total number of cigarettes over all periods: pack-years × 20 × 365.",
      "format": "integer"
    },
    "years": {
      "label": "Years smoked",
      "description": "The years of all periods added up.",
      "format": "number"
    },
    "screening": {
      "label": "Screening line",
      "description": "Whether the history reaches the 20 pack-years in the USPSTF lung cancer screening advice.",
      "format": "text"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "unit": "packs",
      "periods": [
        {
          "amount": 1,
          "years": 20
        }
      ]
    },
    "outputs": {
      "packYears": 20,
      "cigarettes": 146000,
      "years": 20,
      "screening": "At or over 20 pack-years: the history part of the USPSTF screening advice is met (it also needs age 50 to 80, and smoking now or quitting within 15 years)."
    },
    "text": "Your smoking history is 20 pack-years."
  },
  "examples": [
    {
      "given": {
        "unit": "packs",
        "periods": [
          {
            "amount": 1,
            "years": 20
          }
        ]
      },
      "expect": {
        "packYears": 20,
        "cigarettes": 146000
      },
      "source": "National Cancer Institute, NCI Dictionary of Cancer Terms, pack year (packs smoked per day × years smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year, retrieved 2026-10-02; hand calculation in content.mdx: 1 × 20 = 20 pack-years; US Preventive Services Task Force, Lung Cancer: Screening (2021): yearly screening for adults 50 to 80 with a 20 pack-year history who smoke now or quit within the past 15 years. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/lung-cancer-screening, retrieved 2026-10-02"
    },
    {
      "given": {
        "unit": "packs",
        "periods": [
          {
            "amount": 2,
            "years": 0.5
          }
        ]
      },
      "expect": {
        "packYears": 1
      },
      "source": "National Cancer Institute, NCI Dictionary of Cancer Terms, pack year (packs smoked per day × years smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year, retrieved 2026-10-02: 2 packs a day for half a year is 1 pack-year"
    },
    {
      "given": {
        "unit": "cigs",
        "periods": [
          {
            "amount": 10,
            "years": 30
          }
        ]
      },
      "expect": {
        "packYears": 15,
        "years": 30
      },
      "source": "hand calculation in content.mdx: 10 ÷ 20 = 0.5 packs a day × 30 years = 15 pack-years; National Cancer Institute, NCI Dictionary of Cancer Terms, pack year (packs smoked per day × years smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year). https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year, retrieved 2026-10-02"
    },
    {
      "given": {
        "unit": "packs",
        "periods": [
          {
            "amount": 1.5,
            "years": 10
          },
          {
            "amount": 0.5,
            "years": 12
          }
        ]
      },
      "expect": {
        "packYears": 21,
        "years": 22
      },
      "source": "hand calculation in content.mdx: 1.5 × 10 + 0.5 × 12 = 15 + 6 = 21 pack-years; US Preventive Services Task Force, Lung Cancer: Screening (2021): yearly screening for adults 50 to 80 with a 20 pack-year history who smoke now or quit within the past 15 years. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/lung-cancer-screening, retrieved 2026-10-02"
    }
  ],
  "sources": [
    "National Cancer Institute, NCI Dictionary of Cancer Terms, pack year: calculated by multiplying the packs of cigarettes smoked per day by the years the person has smoked; 1 pack-year is 1 pack a day for 1 year, or 2 packs a day for half a year. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/pack-year (retrieved 2026-10-02)",
    "US Preventive Services Task Force, Lung Cancer: Screening, final recommendation (March 9, 2021): annual low-dose CT for adults aged 50 to 80 years with a 20 pack-year smoking history who currently smoke or have quit within the past 15 years. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/lung-cancer-screening (retrieved 2026-10-02)"
  ],
  "related": [
    "life-expectancy",
    "ascvd-risk",
    "bmi"
  ],
  "changelog": []
}