Sound Pressure Level Calculator
Sound pressure level (SPL) is a logarithmic measure of the root-mean-square sound pressure relative to a reference value. In air, the standard reference pressure is 20 micropascals (20e-6 Pa), which corresponds to the threshold of human hearing at 1 kHz. The decibel scale is used because human hearing perceives loudness logarithmically: a 10 dB increase sounds roughly twice as loud, while the actual pressure increases tenfold. This calculator converts in both directions: enter a sound pressure in pascals to get dB SPL, or enter a dB SPL value to get the pressure in pascals.
Sound pressure level formula
SPL (dB) = 20 * log10(p / p0)
Where p is sound pressure in pascals and p0 = 20e-6 Pa (reference pressure in air). Inverse: p = p0 * 10^(SPL/20).
Sound pressure level reference points
- 0 dB SPL: Threshold of hearing (20 uPa).
- 30 dB SPL: Quiet bedroom at night (0.000632 Pa).
- 60 dB SPL: Normal conversation (0.02 Pa).
- 85 dB SPL: NIOSH 8-hour hearing damage threshold (0.356 Pa).
- 94 dB SPL: Standard sound level meter calibration point (approximately 1 Pa).
- 120 dB SPL: Threshold of pain (20 Pa).
Sound pressure level: frequently asked questions
What is the reference pressure for sound in air?
The standard reference sound pressure is 20 micropascals (20 uPa = 2e-5 Pa). This is approximately the threshold of human hearing at 1 kHz and is the international standard reference defined by IEC 61672 and used by NIST.
What does 0 dB SPL mean?
Zero dB SPL corresponds to the reference pressure of 20 uPa. It does not mean silence. It is the threshold of human hearing. A sound at 0 dB SPL is barely audible to a person with excellent hearing in an anechoic environment.
How do I calculate dB from sound pressure?
SPL (dB) = 20 * log10(p / p0), where p is the measured sound pressure in pascals and p0 is the reference pressure (20e-6 Pa). If p = 0.02 Pa, then SPL = 20 * log10(0.02 / 0.00002) = 20 * log10(1000) = 60 dB.
What is the formula to convert dB back to pascals?
p = p0 * 10^(dB/20). For example, 94 dB SPL = 20e-6 * 10^(94/20) = 20e-6 * 10^4.7 = 20e-6 * 50,119 = approximately 1.00 Pa. This is the standard sound level meter calibration point.
What is the loudness at 85 dB and why does it matter?
85 dB SPL is the NIOSH recommended exposure limit for an 8-hour workday. Above 85 dB, prolonged exposure causes permanent hearing loss. OSHA permissible exposure limits start at 90 dB for 8 hours, with the allowed time halving for every 5 dB increase.
Official sources
- NIST: Units outside the SI.
- CDC / NIOSH: Noise and Hearing Loss Prevention.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.