How much noise dose does a shift add up to?
List the noise levels and how long you are exposed to each. Get the dose and the 8-hour average under OSHA and NIOSH.
This is a screening tool, not a safety assessment. It does not replace noise monitoring by a qualified person or advice from an industrial hygienist or occupational health professional. How we check our calculators.
Use A-weighted, slow-response readings from a sound level meter or dosimeter, one line per steady level. Both schemes are shown in the table.
What a noise dose tells you
A noise dose is the share of a day's allowed noise that you have used up. At 100% you have reached the limit; at 50% you have used half. Because the allowed time shrinks quickly as the level rises, you cannot average decibels like ordinary numbers: ten minutes at 110 dBA uses 33% of OSHA's daily budget, while a whole hour at 91 dBA uses only 14%. That is why the dose, not the average level, is what OSHA and NIOSH compute. The decibel calculator gives the safe time at one steady level; this page handles a real shift made of several.
The formula
Dose D = 100 × (C₁/T₁ + C₂/T₂ + … + Cₙ/Tₙ). C is the hours spent at one level and T is the hours allowed at that level. OSHA (29 CFR 1910.95, appendix A) sets T = 8 ÷ 2^((L − 90) ÷ 5), so every 5 dB halves the time, and TWA = 16.61 × log₁₀(D ÷ 100) + 90. NIOSH (publication 98-126) sets T = 8 ÷ 2^((L − 85) ÷ 3), halving the time every 3 dB, and TWA = 10 × log₁₀(D ÷ 100) + 85. The TWA is the single steady level that would give the same dose over exactly 8 hours.
Worked example
A machinist spends 4 hours at 90 dBA, 2 hours at 95 dBA and 30 minutes at 100 dBA. OSHA allows 8, 4 and 2 hours at those levels, so the dose is 100 × (4/8 + 2/4 + 0.5/2) = 125% and the TWA is 91.6 dBA: over the OSHA limit. The calculator's default values are exactly this shift. Under NIOSH's stricter rule the same day is 611%, a TWA of 92.9 dBA. For comparison, a plain 8 hours at 88 dBA, which no OSHA limit touches, is a NIOSH dose of 200%.
Limits of this method
- Which levels count. OSHA builds the PEL dose from 90 dBA and above, but the hearing conservation dose from 80 dBA and above (OSHA Technical Manual III:5). NIOSH counts 80 to 140 dBA. A shift that is all 85 dBA has a PEL dose of 0% and still reaches the action level.
- Hearing protectors are ignored. OSHA computes the action-level dose without any credit for earplugs or earmuffs (1910.95(c)(1)). This page does the same; it tells you the noise at the ear, not the noise under the protector.
- Steady levels only. Each line is one constant A-weighted, slow-response level. Impulse noise (a hammer strike, a nail gun) has its own limit: OSHA and NIOSH both say it should not exceed 140 dB peak, which a dosimeter or sound level meter must measure.
- Ceiling levels. Table G-16 stops at 115 dBA for 15 minutes or less. Table G-16 has no row above 115 dBA, and OSHA's Technical Manual calls 115 dBA the short-term level that is not to be exceeded, so the calculator warns about any level above it.
- Instrument error. Type-2 dosimeters read to about ±2 dBA, a spread of about 30% in dose under OSHA's 5 dB rule. Treat a result within 10% of a limit as unresolved.
Common questions
How do you calculate noise dose?
For each period, divide the time you spent at a level by the time OSHA or NIOSH allows at that level, add the fractions, and multiply by 100. OSHA allows 8 hours at 90 dBA and halves the time for every 5 dB: 4 hours at 95 dBA and 2 hours at 100 dBA. A shift of 4 hours at 90 dBA, 2 hours at 95 dBA and 30 minutes at 100 dBA is 4/8 + 2/4 + 0.5/2 = 1.25, a dose of 125%. A dose of 100% is the OSHA limit.
How do I turn a dose into a TWA?
OSHA's formula is TWA = 16.61 × log₁₀(D ÷ 100) + 90, with D in percent. A dose of 125% gives 91.6 dBA; a dose of 50% gives 85 dBA, the action level. NIOSH uses TWA = 10 × log₁₀(D ÷ 100) + 85. The same 125% shift scores 611% under NIOSH, because its 3 dB rule counts loud minutes more heavily, and a TWA of 92.9 dBA.
What is the OSHA action level?
The action level is an 8-hour TWA of 85 dBA, or a dose of 50%. At or above it an employer must run a hearing conservation program: noise monitoring, audiometric testing, hearing protector choices and training (29 CFR 1910.95(c) to (o)). For this program OSHA says the exposure is computed without any credit for hearing protectors, and counts every level from 80 dBA up.
Why does my 85 dBA shift show a dose of 0% under the OSHA limit?
The OSHA permissible exposure limit is built on levels of 90 dBA and above, so quieter noise adds nothing to the PEL dose (OSHA Technical Manual, section III, chapter 5). Eight hours at 85 dBA is 50% of the action-level dose and 100% of NIOSH's, which is why the table shows three rows for the same shift.
What does NIOSH recommend, and why is it stricter?
NIOSH recommends 85 dBA as an 8-hour average with a 3 dB exchange rate: every 3 dB more halves the time. At 100 dBA that is 15 minutes, against 120 minutes under OSHA, and at 94 dBA one hour. NIOSH estimates that a 40-year career at 85 dBA gives an 8% excess risk of occupational hearing loss, against 25% at OSHA's 90 dBA limit (NIOSH 98-126). OSHA's rule is the legal minimum; NIOSH's is a recommendation.
What if my shift is longer than 8 hours?
The dose adds up the real hours, so a long shift raises it, and the TWA is the 8-hour equivalent of that dose. Working 9.5 hours at 90 dBA is a dose of 119%, a TWA of 91.2 dBA. OSHA's Technical Manual also gives an adjusted action level for extended shifts, AL = 16.61 × log₁₀(50 ÷ (12.5 × hours)) + 90, which is 83.8 dBA for 9.5 hours.
Can I use this instead of a dosimeter?
No. Real shifts vary minute by minute, and a personal dosimeter on the worker's shoulder averages that correctly. This page is for planning (will this schedule stay under the limit?), for checking a dosimeter's reading, and for understanding what a sound level survey implies. Type-2 dosimeters are accurate to about ±2 dBA, so OSHA's Technical Manual treats 87 dBA (a 66% dose) and 92 dBA (132%) as the points where a compliance officer can say the action level and the PEL were exceeded.
This is a screening tool, not a safety assessment. A dose or average level is only as good as the readings behind it; have a qualified person measure real exposures and design any hearing conservation program.
Sources: OSHA, 29 CFR 1910.95, Occupational noise exposure, Table G-16, Table G-16a and appendix A (eCFR text, read October 27, 2026); OSHA Technical Manual, section III, chapter 5, Noise (osha.gov/otm); NIOSH, Criteria for a Recommended Standard: Occupational Noise Exposure, DHHS (NIOSH) Publication 98-126, sections 1.1 to 1.4 and chapter 3. Related: decibel calculator, sound distance calculator for how loud a source is at a distance, speed of sound calculator. How we check our calculators.