This noise exposure calculator applies two published formulas to the levels and durations you type in, and reports both answers rather than choosing between them. That choice is deliberate. The United States has two widely cited noise criteria — the OSHA permissible exposure limit and the NIOSH recommended exposure limit — and they are built on different exchange rates, so they give different allowed durations for the same sound level. A page that quietly picked one would hide the most important thing a reader needs to know.
Arb Digital publishes this within a free tools library of calculators that show their working. Everything below names its source. Where a number appears, it comes from OSHA 29 CFR 1910.95, Occupational noise exposure, from its Appendix A, Noise Exposure Computation, or from the NIOSH criteria document on occupational noise exposure (DHHS Publication 98-126). Nothing here is invented, and no table on this page is a table of our own making.
What This Noise Exposure Calculator Does
You enter up to three sound levels in A-weighted decibels along with the number of hours spent at each. The calculator returns four things under each standard: the maximum permitted duration at the highest level you entered, the combined daily noise dose as a percentage of the limit, and the equivalent eight-hour time-weighted average level.
A dose of 100 per cent means the day's exposure exactly reaches that standard's limit. Above 100 per cent it exceeds it. The two dose figures will usually differ substantially, and for levels above 85 dBA the NIOSH figure will be the larger one, because NIOSH treats noise as accumulating faster with level than OSHA does.
What the tool does not do is worth stating as plainly as what it does. It does not measure sound. It has no access to a microphone and makes no external requests. It does not know your workplace, your hearing protection, the accuracy of your instrument, or your hearing. It cannot tell you that a day was safe, and it cannot demonstrate compliance with any regulation to any authority.
For unit conversions between decibel scales and sound pressure, the site's sound level converter is the relevant tool; this page is about duration and dose rather than about converting between acoustic quantities.
How to Use It
- Enter the loudest sound level first. Use A-weighted decibels from a sound level meter or dosimeter reading. If you do not have a measured figure, you do not have an input for this tool.
- Enter how many hours that level lasted. Use the actual exposure time, not the length of the shift.
- Add up to two more level-and-duration pairs. Leave the hours at zero for any you do not need.
- Read both dose figures. They answer the same question under two different criteria, and the gap between them is real, not a rounding artefact.
- Take any figure that matters to a qualified professional. Exposure assessment is a measurement discipline, and a calculator is not a substitute for one.
The Formula: Two Standards, Two Exchange Rates
An exchange rate is how many decibels of increase halve the allowed exposure time. This is the single number the two standards disagree about, and every other difference follows from it.
OSHA uses a 5 dB exchange rate with a criterion level of 90 dBA for eight hours. Appendix A of 1910.95 gives the reference duration as T = 8 ÷ 2(L − 90) ÷ 5 hours, where L is the measured level in dBA. Table G-16 of the standard lists the same result in discrete steps: 90 dBA for 8 hours, 95 dBA for 4 hours, 100 dBA for 2 hours, 105 dBA for 1 hour, 110 dBA for half an hour, and 115 dBA for a quarter of an hour, with no exposure permitted above 115 dBA on that continuous scale.
NIOSH uses a 3 dB exchange rate with a recommended exposure limit of 85 dBA for eight hours, giving T = 8 ÷ 2(L − 85) ÷ 3 hours. NIOSH also recommends that no exposure be permitted above 140 dBA.
Where multiple levels occur in one day, both standards combine them the same way. Appendix A of 1910.95 defines the daily noise dose as D = 100 × (C1/T1 + C2/T2 + … ), where each C is the time actually spent at a level and each T is that level's reference duration. The equivalent eight-hour average follows from the dose: OSHA computes TWA = 16.61 × log10(D ÷ 100) + 90, and the NIOSH 3 dB equivalent is TWA = 10 × log10(D ÷ 100) + 85.
Worked example, which is the default on this page. Four hours at 95 dBA. Under OSHA, T = 8 ÷ 2(95−90)÷5 = 8 ÷ 2 = 4 hours, so the dose is 100 × (4 ÷ 4) = 100 per cent and the eight-hour TWA is 16.61 × log10(1) + 90 = 90 dBA, exactly at the permissible limit. Under NIOSH, T = 8 ÷ 2(95−85)÷3 = 8 ÷ 10.079 = 0.794 hours, which is about 48 minutes. Four hours against a 0.794-hour allowance is a dose of 504 per cent, and the equivalent average is 10 × log10(5.04) + 85 = 92 dBA. The same four hours are exactly at one limit and five times over the other.
Why the Two Standards Disagree, and What That Means
The 3 dB exchange rate used by NIOSH follows the equal-energy principle: a 3 dB increase doubles sound energy, so it halves the time. The 5 dB rate in the OSHA standard is a historical criterion adopted when the regulation was written, and it allows longer exposures at high levels than the equal-energy rule would.
The practical consequence is the one shown in the worked example. At 95 dBA the two standards differ by a factor of about five in allowed time. At 100 dBA the gap is wider still. The higher the level, the more the two answers diverge, and at levels below the OSHA criterion of 90 dBA the NIOSH limit is the binding one simply because its criterion level is 85 dBA rather than 90.
Neither figure is a safety guarantee. The NIOSH criteria document is explicit that its recommended limit is intended to reduce the risk of hearing loss, not to eliminate it, and that some people will experience hearing damage at exposures below any published limit. Treat the numbers as regulatory and advisory thresholds, not as a line below which nothing happens.
Both are also occupational criteria, written for an eight-hour working day with recovery time in between. NIOSH has separately noted that occupational limits are not designed to be read across to general environmental or recreational noise, where the exposure pattern is different. Applying a workplace number to a concert or a pair of headphones is outside what either standard covers.
What A-Weighting Means, and Why It Is on Every Figure
The unit throughout is dBA — decibels with A-weighting applied. A-weighting is a filter that reduces the contribution of very low and very high frequencies to approximate the sensitivity of human hearing at moderate levels. It is what both standards specify, and a figure measured without it is not comparable.
This matters because instruments report several things that all look like a decibel number. There is the A-weighted level, the C-weighted level used mainly for assessing hearing protector performance and peak impulses, the unweighted or Z-weighted level, and the peak level, which is not the same as a maximum average level. Feeding a C-weighted or peak reading into a formula written for A-weighted averages produces a number with no meaning.
Decibels are also logarithmic, which is why none of this arithmetic can be done by simple averaging. Two machines each producing 90 dBA do not produce 180 dBA; they produce about 93 dBA, because the scale measures a ratio of energies rather than a linear quantity. If you need to work with logarithmic quantities directly, the logarithm calculator handles the base-10 arithmetic that sits underneath every formula on this page.
Impulse Noise Is a Separate Problem
Everything above concerns continuous or intermittent noise averaged over time. Sudden impulsive sounds — a gunshot, a nail gun, a press, a dropped steel plate — are handled differently, because the damage mechanism is different. OSHA 1910.95 addresses impulsive or impact noise with a separate limit on peak sound pressure level rather than through the duration table, and NIOSH recommends that no exposure exceed 140 dBA.
A dose calculation built from average levels will systematically understate a day that contained a small number of very loud impulses, because the average smooths them away. This is one of several reasons why a real exposure assessment uses a dosimeter worn for the shift rather than a spot reading and a calculation. If impulse noise is present in the work you are thinking about, the arithmetic on this page is not the right description of it.
Hearing Protection Is Not a Number You Can Assume
People often want to subtract a hearing protector's rating from the measured level and enter the result here. This page deliberately does not offer that field. Manufacturer ratings are derived under laboratory conditions, real-world attenuation is typically lower and highly variable with fit, and the derating methods differ between the standards and between countries. A protector that is fitted badly, worn intermittently, or unsuitable for the frequency content of the noise does not deliver its rated figure.
The result is that any "protected exposure" number computed casually is likely to be optimistic in exactly the situation where being optimistic is most harmful. Selecting hearing protection is a task for a qualified occupational health or safety professional working from measured data, not an arithmetic step. Enter measured levels here, read the unprotected figures, and take the protection question to someone qualified to answer it.
Arb Digital maintains a large library of free calculators and converters, and our team is happy to talk through anything the tools cannot answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Quoting one standard's allowed time as "the" allowed time — OSHA and NIOSH give materially different answers, and which one applies to a given workplace is a legal and professional question.
- Entering a peak or C-weighted reading — both formulas are written for A-weighted average levels, and mixing scales makes the output meaningless.
- Averaging decibels arithmetically — the scale is logarithmic, so an eight-hour average is computed from energy, not from adding levels and dividing.
- Subtracting a hearing protector rating from the measured level — real-world attenuation is lower and more variable than the laboratory rating, and derating is a professional judgement.
- Treating a dose under 100 per cent as safe — published limits reduce risk, they do not eliminate it, and individual susceptibility varies.
Related Free Tools From Arb Digital
Use the sound level converter for conversions between decibel scales and sound pressure, the logarithm calculator for the base-10 arithmetic behind the formulas, the frequency converter for hertz and its multiples, the time duration calculator to work out how long a shift segment actually lasted, and the work hours calculator for totalling hours across a week. Everything else is in the free online tools hub.
Frequently Asked Questions
Both, shown side by side. The OSHA permissible exposure limit in 29 CFR 1910.95 uses a 90 dBA criterion with a 5 dB exchange rate; the NIOSH recommended exposure limit in publication 98-126 uses an 85 dBA criterion with a 3 dB exchange rate. They give different answers for the same input.
Because of the exchange rate. NIOSH halves the allowed time for every 3 dB increase, following the equal-energy principle, while the OSHA standard halves it every 5 dB. At 95 dBA that difference produces four hours under OSHA and roughly 48 minutes under NIOSH.
It means the day's exposure exactly reaches that standard's limit, as defined by the dose formula in Appendix A of 1910.95. Above 100 per cent the exposure exceeds the limit. It does not mean an exposure below 100 per cent carries no risk.
Not here. Laboratory attenuation ratings are usually higher than what a protector delivers in real use, derating methods vary, and fit dominates the result. Hearing protection selection should be handled by a qualified occupational health or safety professional using measured data.
No. It makes no measurement of any kind and has no access to a microphone. Every figure it reports is calculated from levels and durations you type in, so the output is only as good as the measured inputs you supply.
No. It certifies nothing and is not a compliance record. Regulatory exposure assessment requires calibrated instruments, documented methods and qualified personnel, and the governing standard text is the authority.
They are occupational criteria written for a working day with recovery between shifts. NIOSH has noted that occupational limits are not designed to be applied directly to general environmental or recreational noise exposure, which follows a different pattern.
Separately. OSHA 1910.95 sets a peak sound pressure limit for impulsive or impact noise rather than using the duration table, and NIOSH recommends no exposure above 140 dBA. An average-based dose calculation understates days containing loud impulses.
This page is an arithmetic aid that applies published formulas to numbers you enter. It does not measure sound, assess anyone's hearing, diagnose any condition, or certify compliance with any regulation, and it is not medical, occupational health or legal advice. Noise-induced hearing loss is permanent. Any question about real exposure, hearing protection or a hearing conservation programme should go to a qualified occupational health or safety professional, and the text of the governing standard is the authority in every case.
