Noise dose and TWA calculator
Short answerNoise dose is the sum of each exposure time divided by the time allowed at that level, times 100. Under OSHA the allowed time is 8 hours at 90 dBA, halving for every 5 dB more; NIOSH uses 85 dBA and 3 dB. A 100% dose is the daily limit, and the TWA converts dose into one equivalent level.
What the noise dose calculator works out
Enter each part of the shift as a sound level in dBA and a duration in hours. Choose OSHA (90 dBA criterion, 5 dB exchange rate) or NIOSH (85 dBA criterion, 3 dB exchange rate). The calculator returns the daily noise dose as a percentage, the equivalent 8-hour TWA, and a verdict against the action level and limit.
The noise dose formula
L is the sound level in dBA. C is the hours actually spent at that level. T is the hours allowed at that level.
- OSHA allowed time: T = 8 / 2^((L minus 90) / 5)
- NIOSH allowed time: T = 8 / 2^((L minus 85) / 3)
- Dose (%) = 100 x (C1 / T1 + C2 / T2 + ... + Cn / Tn)
- OSHA TWA = 16.61 x log10(Dose / 100) + 90
- NIOSH TWA = 10 x log10(Dose / 100) + 85
Worked example
A fitter spends 2 hours at 100 dBA and 6 hours at 90 dBA.
OSHA: allowed time is 2 hours at 100 dBA and 8 hours at 90 dBA. Dose = 100 x (2/2 + 6/8) = 100 x 1.75 = 175%. TWA = 16.61 x log10(1.75) + 90 = 94.0 dBA.
NIOSH: allowed time is 0.25 hours at 100 dBA and about 2.52 hours at 90 dBA. Dose = 100 x (2/0.25 + 6/2.52) = about 1,038%. TWA = 10 x log10(10.38) + 85 = 95.2 dBA.
How to read the result
Both results above are over the limit by either method. The thresholds, with sources:
- OSHA action level: 85 dBA TWA or 50% dose. At or above it, a hearing conservation program is required (29 CFR 1910.95(c)(1)).
- OSHA PEL: 90 dBA TWA or 100% dose. Above it, feasible engineering or administrative controls are required (1910.95(b) and Table G-16).
- NIOSH REL: 85 dBA as an 8-hour TWA with a 3 dB exchange rate, and the dose should stay below 100% (NIOSH publication 98-126).
- NIOSH: no exposure above 140 dBA.
What the calculation does not tell you
- It counts every level you enter. OSHA integrates levels from 80 dBA up for hearing conservation but uses a 90 dBA threshold for PEL compliance, so a segment below 90 dBA can change the answer.
- It does not subtract hearing protector attenuation; it is the exposure at the unprotected ear.
- It does not assess impulse or peak noise.
- Hours you leave out count as no exposure.
- Spot readings can miss peaks. A dosimetry survey by a competent person, following site procedures, is the reliable measure.
Questions people ask
How do you calculate noise dose?
For each level, find the allowed time: under OSHA 8 hours at 90 dBA, halved for every 5 dB above it. Divide the hours actually spent at each level by its allowed time, add the fractions and multiply by 100. A total of 100% means the worker has reached the daily limit.
What is the OSHA noise dose limit?
The OSHA permissible exposure limit is a 100% dose, equal to a 90 dBA 8-hour TWA. At a 50% dose, an 85 dBA TWA, the action level is reached and a hearing conservation program is required under 29 CFR 1910.95(c).
Why do OSHA and NIOSH give different results?
They use different criteria. OSHA allows 8 hours at 90 dBA and halves the time every 5 dB. NIOSH allows 8 hours at 85 dBA and halves it every 3 dB, which follows the physics of sound energy. NIOSH is more protective, so the same exposure gives a much higher dose.
How do I convert noise dose to TWA?
Under OSHA, TWA = 16.61 x log10(dose / 100) + 90, using the dose as a percentage. Under NIOSH, TWA = 10 x log10(dose / 100) + 85. So a 50% OSHA dose is 85 dBA and a 200% OSHA dose is 95 dBA.
Does wearing earplugs change the dose?
Not in this calculation. Dose and TWA describe the noise at the ear without protection, which is what the limits are written against. Hearing protection is the last line of control and needs correct selection, fit and training; quieter equipment and engineering controls come first.
What if the shift is longer than 8 hours?
Enter every segment. The dose adds up all exposure time, so a 10 or 12 hour shift is captured in the percentage, and the TWA is still expressed as an 8-hour equivalent. Long shifts reduce recovery time, so a competent person may apply stricter criteria.
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