Noise Exposure TWA & Dose Calculator
Enter each sound level a worker is exposed to and how long it lasts. The calculator returns the noise dose and the 8-hour time-weighted average under OSHA's 5 dB exchange rate (29 CFR 1910.95 Appendix A) and, for comparison, under the NIOSH 3 dB exchange rate — so you can see the PEL, the action level and the REL at once.
OSHA 8-hour TWA
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Results are estimates for planning purposes and do not replace a competent person's evaluation or an engineer's design. Follow the OSHA standards that apply to your work and your employer's written program.
How to use it
- 1Break the shift into blocks. One line per task or area where the sound level is roughly steady — grinding bay, saw, cab, break room. Use a calibrated sound level meter, A-weighted, slow response.
- 2Enter the level and the time. Times are decimal hours; the total does not have to be 8 hours — a 10-hour shift or a 6-hour shift is computed exactly the same way.
- 3Read the three figures. The dose and TWA on the hearing conservation basis (80 dBA threshold), the same on the PEL compliance basis (90 dBA threshold), and the NIOSH figure with its 3 dB exchange rate.
- 4Act on it. At or above an 85 dBA TWA, a hearing conservation program is required. Above 90 dBA TWA, feasible engineering or administrative controls must be used, not hearing protectors alone.
Permissible durations and what they mean
| Sound level (dBA, slow) | OSHA permitted duration | NIOSH allowable duration |
|---|---|---|
| 85 | 16 h (by formula) | 8 h |
| 90 | 8 h | 2 h 31 min |
| 92 | 6 h | 1 h 35 min |
| 95 | 4 h | 47 min |
| 97 | 3 h | 30 min |
| 100 | 2 h | 15 min |
| 102 | 1 h 30 min | 9 min 27 s |
| 105 | 1 h | 4 min 43 s |
| 110 | 30 min | 1 min 29 s |
| 115 | 15 min (no longer permitted above this level) | 28 s |
OSHA durations come from Table G-16 (29 CFR 1910.95) and Table D-2 (29 CFR 1926.52) — identical values. NIOSH durations are computed with T = 8 / 2((L−85)/3) from NIOSH 98-126 and are shown for comparison only. Exposure to impulsive or impact noise should not exceed a 140 dB peak sound pressure level.
OSHA and NIOSH, side by side
| Setting | OSHA — PEL compliance | OSHA — hearing conservation | NIOSH |
|---|---|---|---|
| Criterion level (100% dose) | 90 dBA / 8 h | 90 dBA / 8 h (action level = 50% dose) | 85 dBA / 8 h |
| Exchange rate | 5 dB | 5 dB | 3 dB |
| Threshold | 90 dBA | 80 dBA | 80 dBA |
| Status | Enforceable limit | Enforceable trigger for the program | Recommendation, not law |
Dosimeter settings are described in the OSHA Technical Manual, Section III, Chapter 5. State Plan states may set stricter requirements — federal OSHA is the floor.
How it's calculated
- OSHA reference duration
T = 8 / 2^((L − 90) / 5)T is the hours of exposure permitted at sound level L (dBA). 5 dB exchange rate, 90 dBA criterion level.- Noise dose
D = 100 × (C₁/T₁ + C₂/T₂ + … + Cₙ/Tₙ)Cₙ = hours actually spent at level Lₙ; Tₙ = reference duration for that level. D is a percentage; 100% equals the PEL.- OSHA 8-hour TWA
TWA = 16.61 × log₁₀(D / 100) + 90The steady 8-hour level that produces the same dose. 100% dose = 90 dBA TWA (the PEL); 50% dose = 85 dBA TWA (the action level, 1910.95(c)(1)).- NIOSH allowable duration and TWA
T = 8 / 2^((L − 85) / 3) · TWA = 10 × log₁₀(D / 100) + 85NIOSH criteria: 85 dBA REL over 8 hours, 3 dB exchange rate, 80 dBA threshold. Shown for comparison — OSHA's PEL is the enforceable limit under federal law.- Threshold used
levels below the threshold are not counted in the doseThis page reports two OSHA figures: the hearing conservation basis (80 dBA threshold) and the PEL compliance basis (90 dBA threshold), which is the split OSHA describes for dosimeter settings.
Frequently asked questions
The time-weighted average is the single steady sound level that would give a worker the same total noise dose as the varying levels actually measured over the shift. OSHA computes it from the dose with TWA = 16.61 log10(D/100) + 90 (29 CFR 1910.95 Appendix A). A 100% dose equals a 90 dBA TWA, which is the permissible exposure limit.
For each sound level, find the reference duration T = 8 / 2^((L-90)/5) hours, divide the time actually spent at that level (C) by T, add the fractions, and multiply by 100: D = 100 × (C1/T1 + C2/T2 + ... + Cn/Tn). Four hours at 95 dBA gives C/T = 4/4 = 1.0, or 100% dose on its own.
The permissible exposure limit is a 90 dBA 8-hour TWA (100% dose). The action level is an 85 dBA 8-hour TWA (50% dose): at or above it the employer must put a hearing conservation program in place — monitoring, audiometric testing, hearing protectors and training (29 CFR 1910.95(c)(1)).
OSHA uses a 5 dB exchange rate with a 90 dBA criterion: every extra 5 dB halves the allowed time. NIOSH uses a 3 dB exchange rate with an 85 dBA REL, which matches equal-energy physics and is more protective. The same shift can be 85 dBA TWA under OSHA and 91 dBA TWA under NIOSH. Only the OSHA figure is enforceable under federal law.
Both, for different questions. OSHA's Technical Manual (Section III, Chapter 5) describes a 90 dBA threshold for measuring compliance with the PEL and engineering-control requirements, and an 80 dBA threshold for the hearing conservation program and the action level. This calculator shows both so you do not have to run the numbers twice.
Yes. OSHA's Table G-16 stops at 115 dBA for a quarter hour or less — there is no permitted continuous exposure above 115 dBA — and exposure to impulsive or impact noise should not exceed a 140 dB peak sound pressure level (29 CFR 1910.95(a) and its table footnote).
Not the measured TWA — that is the noise reaching the ear position without protection. Protectors reduce the level at the eardrum, and OSHA derates the labeled NRR to estimate that reduction. Use the hearing protector NRR derating calculator for the protected exposure, and keep the unprotected TWA for your monitoring records.
No. It reproduces the arithmetic of 1910.95 Appendix A from levels you supply. Compliance monitoring under 1910.95(d) needs a calibrated dosimeter or sound level meter and a representative sampling strategy, run by someone competent in noise measurement.
Sources & references
- 29 CFR 1910.95 App. A — Noise exposure computation (T, dose D and 8-hour TWA formulas)
- 29 CFR 1910.95 — Occupational noise exposure: Table G-16, 140 dB peak impulse limit, 85 dBA action level (c)(1)
- 29 CFR 1926.52 — Occupational noise exposure (construction): Table D-2 and the mixed-exposure fraction Fe
- OSHA Technical Manual, Section III Chapter 5 — Noise: dosimeter settings (90 dBA threshold for PEL compliance, 80 dBA threshold for the hearing conservation program)
- NIOSH Publication 98-126 — Criteria for a Recommended Standard: Occupational Noise Exposure (85 dBA REL, 3 dB exchange rate)
Content checked against these sources — last reviewed August 27, 2026.