{
  "slug": "linear-recording-density",
  "term": "Linear recording density",
  "aliases": [
    "linear bit density",
    "BPI",
    "bits per inch"
  ],
  "definition": "Linear recording density is the number of recorded bits per unit distance along a recording track.",
  "note": "Bits per inch (BPI) measures the along-track dimension; it is not track density, which counts tracks across the medium. BPI multiplied by TPI gives local areal density when their recording conditions and bit-count conventions match. [density-demo]\n\nSpecify whether the count describes user bits or encoded channel bits. Flux changes per inch are a distinct quantity and must not be substituted without the recording-code relationship. [density-demo]\n\nIn a 2018 HAMR demonstration, the displayed user areal density already allows for error-correction overhead, and raw written density is 13% higher. Recovering the user value requires division by 1.13. Subtracting 13% of the raw value uses the wrong denominator. [density-coding]\n\nFor a circular track at radius r inches, BPI x 2 x pi x r gives bits per revolution. Multiplying by RPM/60 gives idealized bits passing one head per second, using the same bit convention. This geometric result excludes workload effects and does not itself measure host throughput. [hdd-card]",
  "status": "shipping",
  "advantages": [],
  "limitations": [],
  "related": [
    "areal-density",
    "track-density",
    "hdd",
    "prml-encoding"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "citations": [
    {
      "id": "density-demo",
      "title": "Seagate: HDD Technology Update, Library of Congress DSA 2019, slide 14",
      "url": "https://digitalpreservation.gov/meetings/DSA2019/Day_1/05_trantham_LibraryOfCongress_2019_Trantham_Seagate.pdf",
      "accessed": "2026-10-19"
    },
    {
      "id": "density-manual",
      "title": "Seagate: Enterprise Capacity 3.5 HDD v4 SAS Product Manual, 100726011 Rev. A, May 2014, sections 3.5 and 4.1",
      "url": "https://www.seagate.com/www-content/product-content/enterprise-hdd-fam/enterprise-capacity-3-5-hdd/constellation-es-4/en-us/docs/100726011a.pdf",
      "accessed": "2026-10-19"
    },
    {
      "id": "hdd-card",
      "title": "Hesela: Hard Disk Drive, density and transfer-rate interpretation",
      "url": "https://hesela.com/tech/hdd/",
      "accessed": "2026-10-19"
    },
    {
      "id": "density-coding",
      "title": "Seagate: Disk Drive Technology Trends, Library of Congress DSA 2018, slide 10",
      "url": "https://www.digitalpreservation.gov/meetings/DSA2018/Day_1/5_TO_P2-2_Trantham_Disk_Trends.pdf",
      "accessed": "2026-10-19"
    }
  ],
  "updated": "2026-10-19",
  "provenance": [
    "Seagate's 2019 slide 14 distinguishes kBPI from kfci; the geometry is dimensional arithmetic, not a drive benchmark. [density-demo]"
  ]
}