How do you calculate a noise dose?

For each level, divide the time spent at it by the time allowed at it, add the fractions and multiply by 100. OSHA allows 8 hours at 90 dBA and halves the time for every 5 dB more: T = 8 ÷ 2^((L − 90) ÷ 5). 4 hours at 92 dBA, 2 at 88 and 2 at 95 is 100 × (4 ÷ 6.06 + 2 ÷ 10.56 + 2 ÷ 4) = 134.9%. Convert to an 8-hour TWA with TWA = 16.61 × log10(D ÷ 100) + 90 = 92.2 dBA.

What is the difference between the OSHA and NIOSH noise limits?

OSHA's permissible exposure limit is 90 dBA over 8 hours with a 5 dB exchange rate, and a hearing conservation program starts at 85 dBA (a 50% dose). NIOSH recommends 85 dBA over 8 hours with a 3 dB exchange rate. OSHA's limits are the law for U.S. general industry; the NIOSH limit is a recommendation. The same noise gives a much higher NIOSH dose: 8 hours at 88 dBA is a 75.8% OSHA dose and a 200% NIOSH dose.

Why does the OSHA PEL count only levels from 90 dBA?

OSHA uses two dosimeter settings. For the action level, levels from 80 dBA are counted (1910.95(d)(2)(i)); for the permissible exposure limit, only levels from 90 dBA, where Table G-16 starts. The OSHA Technical Manual's example: 89 dBA for 8 hours reads 87% with an 80 dBA threshold and 0% with a 90 dBA threshold. That is why the calculator shows two OSHA doses.

Does a 10 or 12 hour shift change the limits?

The dose already counts every hour, so compare it with 50% and 100% as usual. OSHA lowers only the action level for longer shifts: a 50% dose over 10 hours is an average of 83.4 dBA, over 12 hours 82.1 dBA. Under a 1999 OSHA memorandum the PEL is not adjusted; it is judged on the worst 8 hours of the shift, which the calculator takes as the loudest 8 hours you enter.

My shift is shorter than 8 hours. Is the TWA still right?

Yes, as long as you entered the whole shift. Both TWAs are 8-hour values: time you did not enter counts as quiet. 4 hours at 95 dBA and nothing else is a 100% OSHA dose, a TWA of 90 dBA.

Can I use readings from a phone app?

Not for compliance. OSHA's Technical Manual says only properly calibrated Type 2 or Type 1 sound level meters and dosimeters document compliance; cheaper meters and apps are fine for finding loud spots and tracking a fix. NIOSH asks for at least a Type 2 meter set to slow response. Measure in the worker's hearing zone, at the level the worker actually hears.

How do hearing protectors change the result?

They do not change whether a hearing conservation program is needed: OSHA computes that exposure without the protector (1910.95(c)(1)). To check a protector, OSHA Appendix B subtracts 7 dB from the NRR and takes the rest off an A-weighted TWA; the protector must bring it to 90 dBA or below, or 85 dBA for someone with a standard threshold shift. OSHA's Technical Manual applies a further 50% correction when deciding whether engineering controls are needed. NIOSH derates the label by type instead: 25% off for earmuffs, 50% for formable earplugs, 70% for other earplugs (section 1.5).

Does OSHA allow for meter error?

In enforcement, yes. OSHA's Technical Manual treats a Type 2 instrument as accurate to ±2 dBA, so inspectors require a hearing conservation program at a measured 87 dBA TWA (66% dose) and cite overexposure above 92 dBA (132% dose). The limits in the regulation stay 85 and 90 dBA; use them to protect people and the corrected figures only to understand an inspection result.

What about impact noise from presses or hammers?

This calculator works with A-weighted slow levels and does not assess peaks. OSHA's Table G-16 says impulsive or impact noise should not exceed 140 dB peak sound pressure level, and NIOSH sets a 140 dBA ceiling. Peak levels need an instrument that measures them; ask a qualified person to measure presses, riveting and hammering.

Free tool

Noise dose and TWA calculator

Noise dose is the share of a day's allowed noise a worker has taken: 100% is the limit. Enter each sound level and the time spent at it to get the dose and the 8-hour time-weighted average (TWA) under OSHA's rules (85 dBA action level, 90 dBA permissible limit) and the NIOSH recommended limit (85 dBA), with which limits are reached and how much loud time to cut.

Dose, %equals100 × (C₁ ÷ T₁ + C₂ ÷ T₂ + … + Cₙ ÷ Tₙ)

T_OSHA, hoursequals8 divided by 2^{(L − 90) ÷ 5}

TWA_OSHAequals16.61 × log₁₀(D ÷ 100) + 90

T_NIOSH, hoursequals8 divided by 2^{(L − 85) ÷ 3}

TWA_NIOSHequals10 × log₁₀(D ÷ 100) + 85

C is the time at a level, T the time allowed at that level and L the A-weighted level in dBA. OSHA: 29 CFR 1910.95, Appendix A. NIOSH: Criteria for a Recommended Standard: Occupational Noise Exposure (1998), sections 1.1.1 and 1.1.3, which give T in minutes as 480 ÷ 2^{(L − 85) ÷ 3}. Levels under 80 dBA add nothing, and under 90 dBA nothing to the OSHA PEL dose.

Noise levels over the shift

A-weighted, slow response. One row per level, with the time at it. Include the quiet parts of the shift too.

Level, dBA
Hours
Minutes
Level 2, dBA
Hours at level 2
Minutes at level 2
Level 3, dBA
Hours at level 3
Minutes at level 3

Hearing protector NRR (optional)

The Noise Reduction Rating on the package. Leave blank to skip.

Pre-filled with the worked example (illustrative). Levels from a sound level meter or the readings of a dosimeter. The result is an estimate, not a measurement.

OSHA 8-hour TWA

92.2 dBA

Dose 134.9%, all levels from 80 dBA, over 8 h.

OSHA action level

Reached

85 dBA TWA or a 50% dose. Levels from 80 dBA count; 5 dB exchange rate.

134.9% dose, TWA 92.2 dBA

A hearing conservation program is required: monitoring, audiometric testing, hearing protectors, training and records, 1910.95(c) to (o).

Time to cut: 2 h at 95 dBA and 2 h 7 min at 92 dBA moved to work below 80 dBA, to bring the dose down to 50%, and a little more to get under it.

OSHA permissible exposure limit

Over

90 dBA TWA or a 100% dose. Levels from 90 dBA count; 5 dB exchange rate.

116.0% dose, TWA 91.1 dBA

Over the permissible exposure limit: feasible engineering or administrative controls are required, and hearing protectors if the controls do not bring it within the limit, 1910.95(b)(1).

Time to cut: 38 min at 95 dBA moved to work below 90 dBA, to bring the dose down to 100%.

NIOSH recommended limit

Over

85 dBA TWA or a 100% dose. Levels from 80 dBA count; 3 dB exchange rate.

554.0% dose, TWA 92.4 dBA

At or over the NIOSH recommended limit: NIOSH recommends a hearing loss prevention program and controls to bring it below 85 dBA.

Time to cut: 2 h at 95 dBA and 3 h 12 min at 92 dBA moved to work below 80 dBA, to bring the dose down to 100%, and a little more to get under it.

The working, step by step

For each level: the time allowed there (T, hours), then the time spent (C) divided by it. Dose = 100 × the sum of C ÷ T. Rows under the threshold add nothing.

OSHA, levels from 80 dBA (action level)

T = 8 ÷ 2^((L − 90) ÷ 5)

OSHA, levels from 80 dBA (action level): level, time, allowed time and fraction of the dose
L, dBA	C, h	T, h	C ÷ T
92	4.00	6.06	0.660
88	2.00	10.56	0.189
95	2.00	4.00	0.500

Dose = 100 × (0.660 + 0.189 + 0.500) = 134.9%
TWA = 16.61 × log10(134.9 ÷ 100) + 90 = 92.2 dBA

OSHA, levels from 90 dBA (PEL)

T = 8 ÷ 2^((L − 90) ÷ 5)

OSHA, levels from 90 dBA (PEL): level, time, allowed time and fraction of the dose
L, dBA	C, h	T, h	C ÷ T
92	4.00	6.06	0.660
88	2.00	10.56	not counted
95	2.00	4.00	0.500

Dose = 100 × (0.660 + 0.500) = 116.0%
TWA = 16.61 × log10(116.0 ÷ 100) + 90 = 91.1 dBA

NIOSH, levels from 80 dBA (REL)

T = 8 ÷ 2^((L − 85) ÷ 3)

NIOSH, levels from 80 dBA (REL): level, time, allowed time and fraction of the dose
L, dBA	C, h	T, h	C ÷ T
92	4.00	1.59	2.520
88	2.00	4.00	0.500
95	2.00	0.79	2.520

Dose = 100 × (2.520 + 0.500 + 2.520) = 554.0%
TWA = 10 × log10(554.0 ÷ 100) + 85 = 92.4 dBA

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Before you enter numbers

Getting levels and times you can trust

Use A-weighting and slow response. OSHA's limits are written for the A scale of a standard sound level meter at slow response (1910.95(a)), and NIOSH asks for the same. Measure in the worker's hearing zone, which OSHA's Technical Manual describes as a sphere about 2 feet across around the head.

A dosimeter beats a walk with a meter. The Technical Manual calls the noise dosimeter the primary instrument for compliance measurements. Worn for the whole shift, it records what the worker actually hears as they move. A meter reading per task works when each task is steady and you time each one honestly; NIOSH suggests a task-based approach for people who move between tasks.

Enter the whole shift. Include breaks and quiet work as rows below 80 dBA. They add no dose, but they show the full shift length. On shifts over 8 hours OSHA lowers only the action level and judges the PEL on the worst 8 hours (OSHA memorandum of 10 November 1999).

Use calibrated Type 2 or Type 1 instruments. The Technical Manual says other meters and phone apps can find noisy spots but cannot document compliance, and that instruments need a calibration check before and after each use.

This calculator does the arithmetic of Appendix A. It is not a substitute for a noise survey by a qualified person, it does not assess impact noise (the 140 dB peak limit), and it does not set up a hearing conservation program for you.

Worked example

Illustrative numbers, not a benchmark

An operator on an 8-hour shift: 4 hours at a press line measured at 92 dBA, 2 hours in the packing area at 88 dBA, and 2 hours at a grinding station at 95 dBA.

1OSHA time allowed: 92 dBA: 8 ÷ 2^0.4 = 6.06 h. 88 dBA: 8 ÷ 2^(−0.4) = 10.56 h. 95 dBA: 8 ÷ 2^1 = 4 h.
2OSHA dose, levels from 80 dBA: 100 × (4 ÷ 6.06 + 2 ÷ 10.56 + 2 ÷ 4) = 100 × (0.660 + 0.189 + 0.500) = 134.9%. TWA = 16.61 × log10(1.349) + 90 = 92.2 dBA. Over the 85 dBA action level.
3OSHA PEL dose, levels from 90 dBA: the 88 dBA time drops out. 100 × (0.660 + 0.500) = 116.0%, TWA 91.1 dBA. Over the PEL. To bring it to 100%: (116.0 − 100) ÷ 100 × 4 h = 0.64 h, about 38 minutes less at 95 dBA.
4NIOSH time allowed: 92 dBA: 8 ÷ 2^(7 ÷ 3) = 1.59 h. 88 dBA: 4 h. 95 dBA: 8 ÷ 2^(10 ÷ 3) = 0.79 h.
5NIOSH dose: 100 × (4 ÷ 1.59 + 2 ÷ 4 + 2 ÷ 0.79) = 100 × (2.520 + 0.500 + 2.520) = 554.0%. TWA = 10 × log10(5.540) + 85 = 92.4 dBA.
6Under the OSHA action level only by moving all 2 hours at 95 dBA and another 2 h 7 min of the 92 dBA work to quiet work. Not realistic, so the answer is quieter machines and a hearing conservation program.

OSHA: 134.9% dose, TWA 92.2 dBA, and 116.0% (91.1 dBA) against the PEL. NIOSH: 554.0%, TWA 92.4 dBA. The calculator above is filled in with this shift. The numbers are illustrative.

Same noise, different answers

The exchange rate. OSHA halves the time allowed for every 5 dB; NIOSH for every 3 dB, the rate it says is better supported by the evidence on hearing loss. At 95 dBA OSHA allows 4 hours and NIOSH 47 minutes 37 seconds.

The limit. 100% means 90 dBA for 8 hours under the OSHA PEL and 85 dBA for 8 hours under NIOSH. That is why the NIOSH dose is usually several times the OSHA dose: 554% against 134.9% in the example.

The TWAs end up close. The TWA is the steady level that would give the same dose in 8 hours, so a steady 8 hours at 88 dBA is a TWA of 88 dBA under both. When the level varies, the 3 dB rule gives the loud periods more weight, so the NIOSH TWA comes out a little higher (92.4 against 92.2 dBA in the example). Compare a TWA with the limit of the same system.

Which one applies. In U.S. general industry, OSHA's 1910.95 is the rule an inspector enforces. The NIOSH limit is a recommendation; OSHA's Technical Manual notes that NIOSH, EPA, ACGIH and most other countries use a 3 dB exchange rate. Many plants run their hearing program to the stricter number. Check your own country's rule outside the U.S.

What reaching a limit means. At the OSHA action level: noise monitoring, telling employees the results, baseline and annual audiograms, hearing protectors offered free, yearly training and records (1910.95(c) to (o)). Over the PEL: feasible engineering or administrative controls first, and hearing protectors when they are not enough (1910.95(b)(1)). Table G-16 stops at 115 dBA for a quarter hour or less, and OSHA reads it as a ceiling: no continuous noise above 115 dBA, whatever the dose. Over the NIOSH limit, NIOSH recommends a hearing loss prevention program.

Removing loud time is not the only way down. Moving people away from the source, enclosing it or fixing worn machines lowers the level itself. Record each fix like any other safety action, for example on a job safety analysis, and measure again afterwards.

Time allowed per day

Worked out from the two formulas above, the same way the calculator does, and rounded to the minute. They agree with OSHA's Table G-16a and NIOSH's Table 1-1.

Time allowed per day at each A-weighted level under OSHA and NIOSH
Level	OSHA	NIOSH
80 dBA	32 h	25 h 24 min
82 dBA	24 h 15 min	16 h
85 dBA	16 h	8 h
88 dBA	10 h 33 min	4 h
90 dBA	8 h	2 h 31 min
92 dBA	6 h 4 min	1 h 35 min
94 dBA	4 h 36 min	1 h
95 dBA	4 h	48 min
97 dBA	3 h 2 min	30 min
100 dBA	2 h	15 min
105 dBA	1 h	5 min
110 dBA	30 min	1 min
115 dBA	15 min	28 s

OSHA counts levels from 80 dBA for the action level and from 90 dBA for the PEL; NIOSH counts levels from 80 dBA. Sources: 29 CFR 1910.95 Appendix A and NIOSH Publication 98-126, sections 1.1 and 1.3.3. Checked October 9, 2026.

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FAQ

Noise dose and TWA calculator: common questions

OSHA's permissible exposure limit is 90 dBA over 8 hours with a 5 dB exchange rate, and a hearing conservation program starts at 85 dBA (a 50% dose). NIOSH recommends 85 dBA over 8 hours with a 3 dB exchange rate. OSHA's limits are the law for U.S. general industry; the NIOSH limit is a recommendation. The same noise gives a much higher NIOSH dose: 8 hours at 88 dBA is a 75.8% OSHA dose and a 200% NIOSH dose.
They do not change whether a hearing conservation program is needed: OSHA computes that exposure without the protector (1910.95(c)(1)). To check a protector, OSHA Appendix B subtracts 7 dB from the NRR and takes the rest off an A-weighted TWA; the protector must bring it to 90 dBA or below, or 85 dBA for someone with a standard threshold shift. OSHA's Technical Manual applies a further 50% correction when deciding whether engineering controls are needed. NIOSH derates the label by type instead: 25% off for earmuffs, 50% for formable earplugs, 70% for other earplugs (section 1.5).

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