Zerdly

Noise Exposure Calculator

Occupational noise limits are set as a daily dose against a reference 8-hour working day.

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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 noise level in dB(A) and how many hours the person is exposed to it.
  2. Optionally set the criterion level (default 85 dB(A)) and exchange rate (default 3 dB) to match your jurisdiction's WHS/OSH rules — the US OSHA PEL uses 90 dB(A) with a 5 dB exchange rate.
  3. Read the noise dose (%), the equivalent 8-hour TWA, and the maximum allowed time at that level; a dose of 100% is the full daily limit.

How it works

Occupational noise limits are set as a daily dose against a reference 8-hour working day. For a steady level L (dB(A)) held for T hours, the dose is Dose % = 100 × (T ÷ 8) × 2^((L − Lc) ÷ q), where Lc is the criterion level (the level allowed for a full 8 hours, e.g. 85 dB(A)) and q is the exchange rate — the number of decibels that halves or doubles the permitted time (3 dB under most ISO/EU/Australian rules, 5 dB under the US OSHA PEL). Because sound is logarithmic, every q dB increase doubles the dose.

From the dose, the equivalent 8-hour time-weighted average is TWA = Lc + q × log₂(Dose ÷ 100), which is the single steady level over 8 hours that gives the same exposure. The maximum allowed time at the entered level (the point where dose reaches 100%) is Allowed = 8 × 2^((Lc − L) ÷ q). All results are pure arithmetic on your inputs. This is guidance only — real assessments must account for varying levels through the day, peak/impulse noise, and hearing-protection attenuation, and must follow your jurisdiction's WHS/OSH regulations and a competent person or occupational hygienist.

Worked example

6 hours at 92 dB(A) (85 dB criterion, 3 dB exchange). A worker spends 6 hours near machinery measured at 92 dB(A). Using an 85 dB(A) criterion level and a 3 dB exchange rate: Dose % = 100 × (6 ÷ 8) × 2^((92 − 85) ÷ 3) = 378 %. The 8-hour TWA is 90.8 dB(A), and the permitted time at 92 dB(A) is just 1.59 h. At 378% of the daily dose the exposure is well over the limit — hearing protection and shorter exposure are needed.

Common mistakes

  • Mixing exchange rates — a 3 dB rate (ISO/EU/Australia) and a 5 dB rate (US OSHA PEL) give different doses for the same noise, so pick the one your regulation uses instead of assuming.
  • Confusing the criterion level with the action level — 85 dB(A) is a common criterion, but many rules set a lower exposure action value (e.g. 80–85 dB(A)) where controls are already required.
  • Treating this steady-level result as a full assessment — a real working day has different levels for different periods, and doses from each period add together; this tool covers one level at a time.

Frequently asked questions

What exchange rate and criterion should I use?

It depends on your jurisdiction. Australia, the EU and most ISO-based schemes use an 85 dB(A) criterion with a 3 dB exchange rate. The US OSHA permissible exposure limit (PEL) uses 90 dB(A) with a 5 dB exchange rate, while the OSHA action level uses 85 dB(A). Set the criterion and exchange rate fields to match the standard that applies to you.

What does a noise dose over 100% mean?

100% is the full daily exposure limit for the chosen criterion and exchange rate. A dose of 200% means twice the daily limit was received (equivalent to +3 dB on the TWA at a 3 dB exchange rate). Any dose at or above 100% means the exposure should be reduced — through shorter time, lower levels, or hearing protection — under a formal assessment.

Does this account for hearing protection?

No. The result is the exposure at the ear before any protector. Hearing protectors reduce the effective level by their rated attenuation (e.g. SNR or NRR, with a real-world derating), so a competent person should subtract that separately when checking protected exposure.

Is this a legal compliance assessment?

No — it is guidance only. A compliant noise assessment combines measured levels across the whole day, peak/impulse limits, instrument calibration and uncertainty, and must be carried out following your jurisdiction's WHS/OSH regulations by a competent person or occupational hygienist.

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Tip: Enter any known values to calculate the remaining results.

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