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Decibel Exposure Time Calculator

Occupational noise limits pair a criterion level (the steady A-weighted level allowed for a full 8-hour shift) with an exchange rate (how many decibels change the allowed time by half).

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the measured or rated noise level in A-weighted decibels (dBA).
  2. Optionally set the criterion level (85 dBA for NIOSH/EU, 90 dBA for U.S. OSHA) and the exchange rate (3 dB for NIOSH/EU, 5 dB for OSHA). Leave blank to use the NIOSH defaults of 85 dBA and 3 dB.
  3. Read the permissible exposure time before a full daily noise dose is reached; use it to plan job rotation, breaks, or hearing protection.

How it works

Occupational noise limits pair a criterion level (the steady A-weighted level allowed for a full 8-hour shift) with an exchange rate (how many decibels change the allowed time by half). The permissible exposure time is T = 8 ÷ 2^((L − criterion) ÷ exchange rate) hours, where L is the noise level in dBA. Because decibels are logarithmic, every step of one exchange-rate increment above the criterion halves the allowed time and every step below it doubles it.

The two common conventions give different answers for the same noise. NIOSH and most European (EU Directive 2003/10/EC) schemes use an 85 dBA criterion with a 3 dB exchange rate (equal-energy), which is more protective; U.S. OSHA uses a 90 dBA criterion with a 5 dB exchange rate. This calculator assumes a single steady level for the whole exposure — for varying noise you must combine partial doses using the same exchange rate. Peak/impulse noise limits and the mandatory action levels that trigger hearing protection are set separately by your regulations.

Worked example

How long is safe at 95 dBA (OSHA 5 dB rule)?. Enter a noise level of 95 dBA, a criterion of 90 dBA, and a 5 dB exchange rate (the U.S. OSHA method). T = 8 ÷ 2^((95 − 90) ÷ 5) = 8 ÷ 2^1 = 4 hours. So an unprotected worker reaches a full daily noise dose in 4 h at 95 dBA. Switch the criterion to 85 dBA and the exchange rate to 3 dB (the NIOSH/EU method) and the same 95 dBA level allows only 8 ÷ 2^((95−85)/3) = 8 ÷ 2^3.333 ≈ 0.794 h ≈ 47.6 min — far more protective.

Common mistakes

  • Mixing conventions — using the OSHA 90 dBA criterion with the NIOSH 3 dB exchange rate (or vice versa). Keep the criterion and exchange rate from the same standard.
  • Treating the result as a 'safe' time. It is the time to reach a 100% daily noise dose without protection; it does not account for impulse/peak limits, prior exposure that day, or the extra protection your regulations may require above an action level.
  • Entering a C-weighted or unweighted level. Occupational limits are based on A-weighted decibels (dBA); using the wrong weighting overstates or understates the allowed time.

Frequently asked questions

What criterion level and exchange rate should I use?

For NIOSH and most European workplaces use an 85 dBA criterion with a 3 dB exchange rate (the equal-energy method, the calculator's default). For U.S. OSHA compliance use a 90 dBA criterion with a 5 dB exchange rate. Always confirm the values required by your own jurisdiction's noise regulations.

Does this account for hearing protection?

No. It assumes an unprotected ear at the entered level. If hearing protectors are worn, subtract their rated attenuation (e.g. NRR/SNR, with a de-rating factor) from the noise level first, then enter the protected level. The result also ignores impulse/peak noise limits, which are assessed separately.

What happens if the level is at or below the criterion?

At or below the criterion (85 or 90 dBA), the formula permits a full 8-hour shift or longer, so the tool reports the calculated time and notes that a full shift is allowed. Quieter work is generally acceptable for noise dose, but action levels in your regulations may still require monitoring or hearing conservation measures.

Can I use this for noise that changes during the day?

Not directly — it assumes one steady level for the whole exposure. For varying noise, calculate the partial dose for each level and duration and add them (the total daily dose should not exceed 100%). A proper noise-dose or TWA assessment by a competent person is required for compliance.

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