The Big Five Inventory (BFI-44; John, Donahue, & Kentle, 1991) measures five personality dimensions — Extraversion, Agreeableness, Conscientiousness, Neuroticism, and Openness — with 44 items total on a 5-point scale from disagree strongly to agree strongly. Several items are reverse-scored before subscale means are calculated, so double-check your reverse-coding before you compute anything.
The reliability line
Internal consistency across the five subscales averages around α = .83, with individual subscale alphas typically falling between .79 and .88. As with any multi-subscale instrument, report each dimension's alpha rather than only the average — a reader comparing your Neuroticism findings to another study needs to know your Neuroticism alpha specifically, not a blended figure across all five traits.
A sample paragraph
Personality traits were assessed using the Big Five Inventory (BFI-44; John, Donahue, & Kentle, 1991), a 44-item measure of Extraversion, Agreeableness, Conscientiousness, Neuroticism, and Openness, rated on a 5-point scale from disagree strongly to agree strongly. Subscale scores were computed as the mean of their constituent items after reverse-scoring the flagged items. Internal consistency was acceptable to good across the five subscales (α = .79–.88, M ≈ .83).
Swap in your own sample's per-subscale alphas once calculated — the figures above are from the validation literature, and citing them as your own sample's reliability (rather than the instrument's general track record) is a common and avoidable error.
Reference: John, O. P., Donahue, E. M., & Kentle, R. L. (1991). The Big Five Inventory—Versions 4a and 54. Berkeley, CA: University of California, Berkeley, Institute of Personality and Social Research.
Folio's BFI-44 scale page lists which items belong to which of the five dimensions and which are reverse-scored — worth a check before you score, not after.