How do you calculate noise exposure?

Short answer: To calculate noise exposure, convert a worker's day into an 8-hour time-weighted average (TWA) in dBA using OSHA's 90 dBA permissible exposure limit and 5 dB exchange rate. Add up the dose from each noise level and duration, express it as a percentage of the allowable daily dose, then convert that dose to a TWA. A TWA at or above 85 dBA triggers OSHA's hearing conservation action level.

How to calculate noise exposure: TWA, dose, and the OSHA action level (2026)

Learning how to calculate noise exposure is the foundation of every hearing conservation program, because OSHA does not regulate the loudest moment of a shift - it regulates the time-weighted average (TWA) of everything a worker hears across eight hours. A worker can stand next to a 100 dBA machine for an hour and still fall below the limit, or be overexposed at a steady 92 dBA all day. The governing rule, OSHA 29 CFR 1910.95, sets a 90 dBA permissible exposure limit, an 85 dBA action level, and a 5 dB exchange rate that you must apply correctly to get the math right.

Below we define the TWA, walk through dose percentage, explain the 5 dB exchange rate and where the NIOSH 3 dB rate differs, and run a full dosimeter-style worked example. We tie the result back to selecting protection from our hearing protection lineup so the number on your dosimeter leads directly to the right earplug or earmuff.

Why this matters.
Getting the noise-exposure calculation wrong has direct compliance and health consequences - underestimate the TWA and you skip required audiometric testing, training, and protection. OSHA 1910.95 mandates a hearing conservation program at an 85 dBA TWA, and NIOSH estimates about 22 million U.S. workers are exposed to hazardous noise annually, with noise-induced hearing loss being permanent and entirely preventable. The exchange rate you choose can swing the result by several decibels, which is the difference between a compliant program and an enforcement finding.

Part 1 - What the 8-hour TWA means

The time-weighted average (TWA) is the steady noise level that, over 8 hours, would deliver the same total energy as the varying noise a worker actually experienced. It collapses a whole shift of changing levels into a single dBA number you can compare against the limits. The TWA, not any single peak, is what OSHA enforces for continuous and intermittent noise.

• A TWA is always normalized to an 8-hour day, even if the shift is longer or shorter.
• It is energy-based, so loud short bursts count more than their clock time suggests.
• You usually obtain it directly from a noise dosimeter worn by the employee, or compute it from sound-level-meter readings and durations.

Once you have the TWA, you select protection that derates below 85 dBA from our hearing protection range and verify it with how to read the NRR.

Part 2 - The OSHA PEL, action level, and 5 dB exchange rate

OSHA fixes three numbers you must memorize to calculate noise exposure correctly:

• 90 dBA permissible exposure limit (PEL) - the maximum 8-hour TWA allowed; 90 dBA equals 100 percent dose for 8 hours.
• 85 dBA action level - at this 8-hour TWA (50 percent dose), the employer must enroll the worker in a hearing conservation program with testing, training, and protection.
• 5 dB exchange rate - for every 5 dB increase in level, the allowable exposure time halves. So 90 dBA is allowed for 8 hours, 95 dBA for 4 hours, 100 dBA for 2 hours, and 105 dBA for 1 hour.

The exchange rate is what makes loud noise so time-limited. Compare typical level-and-duration combinations in our decibel levels chart before you run the dose math.

Part 3 - Calculate noise exposure dose for each level

Dose is the heart of the calculation. For each distinct noise level a worker is exposed to, you find the permissible duration at that level, divide the actual time spent by that permissible time, and sum the fractions across the day. The result, multiplied by 100, is the percent dose.

The formula is: Dose (%) = 100 x (C1/T1 + C2/T2 + ... ), where C is the actual hours at a level and T is the permissible hours at that level. A dose of 100 percent equals a 90 dBA TWA (the PEL); a dose of 50 percent equals an 85 dBA TWA (the action level). The permissible time T at any level is derived from the 5 dB exchange rate, anchored at 8 hours for 90 dBA.

Part 4 - Convert dose to TWA

Regulators report exposure as a TWA, so the final step converts your percent dose into a single dBA number. OSHA provides the conversion formula in Appendix A of 1910.95:

• TWA = 90 + 16.61 x log10(Dose / 100), where Dose is the percent dose from Part 3.
• A 100 percent dose returns exactly 90 dBA, the PEL.
• A 50 percent dose returns 85 dBA, the action level.
• A 200 percent dose returns 95 dBA, which would require engineering or administrative controls.

If you use a dosimeter, it does this conversion automatically and displays the TWA directly. Either way, the TWA drives your protection choice - run it through how to choose hearing protection to land on the right device.

Part 5 - Why OSHA 5 dB and NIOSH 3 dB differ

There are two exchange rates in common use, and they give different answers, so you must state which you are using. OSHA enforces a 5 dB exchange rate for compliance, while NIOSH recommends a more protective 3 dB exchange rate based on the equal-energy principle used internationally.

• OSHA 5 dB - permissible time halves every 5 dB; this is the legal compliance standard in U.S. general industry.
• NIOSH 3 dB - permissible time halves every 3 dB, with an 85 dBA recommended exposure limit; this is more conservative and better matches hearing-damage data.

The practical upshot: NIOSH math flags more workers as overexposed than OSHA math at the same noise levels. Many safety programs design to the NIOSH 3 dB rate for a margin of safety even though they report compliance against OSHA. See the NIOSH basis in its Criteria for a Recommended Standard (Pub. 98-126).

Part 6 - From TWA to the right protection

The calculation only matters if it leads to action. Once you have a TWA, compare it to the limits and respond:

• Below 85 dBA - no program required, though protection may still be offered.
• 85 to 89 dBA - action level reached; enroll the worker, provide protection, and run baseline and annual audiograms.
• 90 dBA and above - the PEL is exceeded; feasible engineering or administrative controls are required, with protection used in the interim.

Then size the protector by derating its NRR below the TWA. For a steady 95 dBA line, a high-NRR plug from our NRR 33 ear plugs or a robust muff from our ear muffs collection is the starting point; very loud areas may need dual hearing protection.

OSHA permissible exposure durations by noise level (5 dB exchange rate)

Part 7 - Worked example: calculate noise exposure for a mixed-task shift with a dosimeter cross-check

To show how to calculate noise exposure end to end, here is a dosimeter-style hand calculation for a maintenance worker whose 8-hour day splits across three areas, leading to a protector choice from our hearing protection range:

1. Record level and time for each task. Log the shift: 3 hours at 95 dBA on the press line, 2 hours at 100 dBA at the grinder, and 3 hours at 85 dBA doing inspection. These are the C values - the actual time at each level.
2. Find permissible time at each level. Using the 5 dB exchange rate: 95 dBA is allowed 4 hours (T1), 100 dBA is allowed 2 hours (T2), and 85 dBA is allowed 16 hours (T3). These permissible durations come straight from the decode table above.
3. Compute the partial doses. Divide each actual time by its permissible time: 3/4 = 0.75, 2/2 = 1.00, and 3/16 = 0.1875. Each fraction is that task's share of the daily dose.
4. Sum to get total dose. Add the fractions: 0.75 + 1.00 + 0.1875 = 1.9375, or 193.75 percent dose. Anything over 100 percent means the worker exceeded the PEL before protection.
5. Convert dose to TWA. Apply TWA = 90 + 16.61 x log10(193.75 / 100). log10(1.9375) is about 0.2874, so TWA = 90 + 16.61 x 0.2874 = about 94.8 dBA - well above both the action level and the PEL.
6. Cross-check and select protection. A dosimeter worn for the shift should read close to 94.8 dBA TWA; if it does, the hand calc is validated. To get below 85 dBA you need at least 10 dB of derated attenuation, so pick a high-NRR plug such as the Howard Leight Max-1 and verify with the derating method.

With a 94.8 dBA TWA in hand, the worker needs protection that derates well below 85 dBA - start with the Howard Leight Maximum Max-1 NRR 33 cordless ear plugs or the 3M H10A Peltor Optime 105 ear muffs. Confirm the attenuation in how to read the NRR, choose the right category in how to choose hearing protection, and if a single device is not enough, follow dual hearing protection.

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Frequently asked questions

How do you calculate noise exposure?

To calculate noise exposure, record each noise level and its duration, find the OSHA permissible time at each level using the 5 dB exchange rate, divide actual time by permissible time, and sum the fractions for a percent dose. Then convert dose to an 8-hour TWA with TWA = 90 + 16.61 x log10(Dose/100). A dosimeter automates this.

What is an 8-hour TWA?

An 8-hour time-weighted average is the single steady dBA level that would deliver the same noise energy as a worker's varying shift, normalized to 8 hours. It is the value OSHA compares against the 85 dBA action level and 90 dBA PEL under 29 CFR 1910.95.

What is the OSHA action level for noise?

The OSHA action level is an 8-hour TWA of 85 dBA, equal to a 50 percent dose. At or above this level the employer must run a hearing conservation program with audiometric testing, training, and hearing protection. Selecting that protection starts with how to choose hearing protection.

What is the 5 dB exchange rate?

The 5 dB exchange rate is OSHA's rule that the permissible exposure time halves for every 5 dB increase in noise level. So 90 dBA is allowed 8 hours, 95 dBA 4 hours, and 100 dBA 2 hours. It is the conversion that links level and duration in every noise-dose calculation.

How is noise dose calculated?

Noise dose is the sum of each exposure's actual time divided by its permissible time, times 100: Dose = 100 x (C1/T1 + C2/T2 + ...). A 100 percent dose equals a 90 dBA TWA, and a 50 percent dose equals the 85 dBA action level. Compare your task levels against the decibel levels chart first.

What is the difference between OSHA and NIOSH exchange rates?

OSHA uses a 5 dB exchange rate with a 90 dBA PEL for compliance, while NIOSH recommends a more protective 3 dB exchange rate with an 85 dBA limit. NIOSH math flags more overexposure at the same levels, as explained in its Pub. 98-126.

How do you convert dose to TWA?

Use OSHA's Appendix A formula: TWA = 90 + 16.61 x log10(Dose / 100), where Dose is the percent dose. A 100 percent dose returns 90 dBA, 50 percent returns 85 dBA, and 200 percent returns 95 dBA. Dosimeters perform this conversion and display the TWA automatically.

What is the OSHA permissible exposure limit for noise?

The OSHA permissible exposure limit (PEL) is a 90 dBA 8-hour TWA, equal to a 100 percent dose. Exceeding it requires feasible engineering or administrative controls, with hearing protection used while controls are implemented under 1910.95.

How long can you be exposed to 100 dBA?

Under OSHA's 5 dB exchange rate, 100 dBA is permissible for only 2 hours per day. A full 8-hour shift at 100 dBA equals a 400 percent dose, far above the limit, so high-attenuation protection from our NRR 33 ear plugs or dual protection is required.

To calculate noise exposure, do I use the loudest noise or the average?

You measure both, but to calculate noise exposure you use the 8-hour TWA, an energy average of the whole shift, not a single peak. Brief peaks matter for impulse-noise and ceiling limits, but the TWA drives compliance and protection selection - see how to read the NRR.

What tool is used to calculate noise exposure?

A noise dosimeter worn on the worker for the shift will calculate noise exposure automatically, integrating all noise into a dose and TWA. A sound-level meter measures instantaneous levels you can hand-calculate into a TWA. Dosimetry is preferred for mobile workers whose noise varies by task and location.

What happens at a 200 percent dose?

A 200 percent dose converts to a 95 dBA TWA, double the PEL. This requires engineering or administrative controls plus hearing protection in the interim. Workers at this level typically need high-NRR plugs, muffs, or both - review dual hearing protection.

Does a longer shift change the noise exposure limit?

Yes. Limits are normalized to 8 hours, so a 12-hour shift compresses the permissible levels - the same daily dose is reached at a lower steady level. Many programs apply a more protective limit for extended shifts, often aligning with the NIOSH 3 dB approach.

Why does the exchange rate matter so much?

The exchange rate decides how fast permissible time drops as noise rises, so it can swing a TWA by several decibels at the same readings. Using OSHA's 5 dB rate yields a lower TWA than NIOSH's 3 dB rate, which is why you must always state which rate a calculation uses.

Is hearing protection counted in the noise exposure calculation?

The TWA is calculated at the ear before protection; protection is then applied separately by derating its NRR and subtracting from the TWA. OSHA compares the unprotected TWA to the action level to decide program enrollment, then checks that protection brings exposure below 85 dBA - see how to read the NRR.

What is the OSHA noise ceiling limit?

OSHA sets a ceiling of 115 dBA for continuous noise - no exposure to continuous, intermittent, or impact noise above this level is permitted, and impulse or impact noise must not exceed a 140 dB peak. These ceilings apply regardless of the calculated TWA.

How do I use the TWA to pick hearing protection?

Subtract the derated NRR of a protector from your TWA and confirm the result is below 85 dBA. If a single device falls short, combine plugs and muffs. Our how to choose hearing protection guide turns your TWA into a specific product category.

Further reading on this site

• Hearing protection — earplugs, earmuffs, and banded protectors to bring a measured TWA below 85 dBA.
• NRR 33 ear plugs — maximum-attenuation foam plugs for high-TWA environments.
• Decibel levels chart — common noise sources and OSHA limits to anchor your dose math.
• How to read the NRR — derate a protector's rating and subtract it from your TWA.
• How to choose hearing protection — convert a calculated TWA into the right earplug, earmuff, or dual setup.
• Dual hearing protection — combine plugs and muffs when a single device cannot reach 85 dBA.
• What is OSHA 29 CFR 1910.95? — the hearing conservation standard behind the action level and PEL.
• Best hearing protection — complete guide to industrial protection ranked by use case.