linear-recording-density · shipping · also linear bit density, BPI, bits per inch

Linear recording density

Linear recording density is the number of recorded bits per unit distance along a recording track.

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. [1]

Specify 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. [1]

In 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. [4]

For 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. [3]

Relations

related

Areal density, Track density, Hard disk drive (HDD), PRML detection

Citations

1. Seagate: HDD Technology Update, Library of Congress DSA 2019, slide 14 (accessed 2026-10-19)
3. Hesela: Hard Disk Drive, density and transfer-rate interpretation (accessed 2026-10-19)
4. Seagate: Disk Drive Technology Trends, Library of Congress DSA 2018, slide 10 (accessed 2026-10-19)

Further reading

2. Seagate: Enterprise Capacity 3.5 HDD v4 SAS Product Manual, 100726011 Rev. A, May 2014, sections 3.5 and 4.1 (accessed 2026-10-19)

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