WHO and NIOSH safe listening guidelines
The 85 dB / 8 h anchor
Both the WHO and U.S. NIOSH set 85 dB SPL for 8 hours as the safe daily exposure ceiling. Above that, the dose halves every 3 dB: 88 dB = 4 hours, 91 dB = 2 hours, 100 dB = 15 minutes, 109 dB = under 2 minutes.
Hearing damage is cumulative. A loud commute plus loud music at home stacks. The damage is usually painless and silent, surfacing years later as tinnitus, frequency notches, and reduced spatial hearing.
What this calculator gets right (and what it can't)
The math is the textbook headphone power-to-SPL formula: P = V² ÷ Z, then SPL = sensitivity + 10·log10(P_mW). It tells you the average SPL at a given volume - real music has 10-15 dB of crest factor on top, so transient peaks land higher.
Coupling to your ear (open vs closed cans, IEM seal quality, head-related transfer function) shifts the in-ear SPL by a few dB in either direction. For a precise number, use a coupler or a calibrated SPL meter at the ear cup.