Noise dose & TWA calculator

Real days are not a single noise level. You might spend two hours on a loud shop floor, an hour commuting, and the rest in a quiet office. This calculator adds those segments together the way an occupational dosimeter does, and tells you two things that matter: your total daily noise dose as a percentage of the limit, and your 8-hour time-weighted average (TWA) in dBA. Switch between OSHA and NIOSH rules to see both the legal minimum and the health-based recommendation.

Standard
ActivityLevel (dBA)Hours
Total daily dose 0%
8-hour TWA - dBA
Status Within limit

How the calculation works

For each activity, the calculator works out the maximum time you are allowed at that level, using the selected criterion level and exchange rate. The formula for the permitted time T at level L is T = 8 / 2^((L − criterion) / exchange). Your time in that activity divided by the permitted time is the dose fraction for that segment. Adding the fractions for every segment gives your total daily dose. A dose of 100% is the full day's limit.

The 8-hour time-weighted average is then derived from the total dose with TWA = criterion + exchange × log2(dose ÷ 100). In plain terms, it is the single steady level that would have produced the same dose over a standard 8-hour shift, which is exactly the number occupational limits are written against.

NIOSH's 85 dBA criterion with a 3 dB exchange rate is the health-based recommendation and is more protective. OSHA's 90 dBA criterion with a 5 dB exchange rate is the US legal permissible exposure limit. It is common for a day to be within the OSHA limit yet over the NIOSH recommendation, which is why both are shown.

This tool is for awareness and planning. It is not a substitute for a calibrated dosimeter or a professional occupational-noise assessment. To estimate the level of each activity, you can use the sound meter, and for a quick single-level check see the noise exposure calculator.

Frequently asked questions

What is noise dose?
Noise dose is the amount of noise you accumulate over a day expressed as a percentage of the daily limit. 100% means you have reached the maximum permissible exposure for one working day. Above 100% means the day exceeded the limit.
What is an 8-hour TWA?
The time-weighted average (TWA) converts a day of varying noise levels into the single steady level that would give the same total dose over 8 hours. It is the number occupational limits are written against, so comparing your TWA to 85 or 90 dBA tells you where you stand.
What is the difference between OSHA and NIOSH here?
OSHA uses a 90 dBA criterion with a 5 dB exchange rate (the US legal permissible exposure limit). NIOSH recommends a more protective 85 dBA criterion with a 3 dB exchange rate. This calculator lets you switch between the two so you can see both the legal minimum and the health-based recommendation.
What is an exchange rate?
The exchange rate is how many extra decibels halve the allowed time. With a 3 dB exchange rate (NIOSH), every 3 dB increase cuts the safe time in half. With a 5 dB exchange rate (OSHA), it takes 5 dB. A lower exchange rate is more protective.
Are these readings suitable for legal compliance?
No. This is an educational planning tool. A browser microphone is not a calibrated instrument, and a formal OSHA assessment must be performed with a calibrated Class 1 or Class 2 sound level meter or dosimeter by a qualified professional. Use this to understand and plan, not to certify.
How do I get the dB level for each activity?
You can measure each activity with the sound meter on this site, or use typical values: normal office around 60 dBA, busy restaurant around 75 dBA, lawn mower around 90 dBA, nightclub around 100 dBA. Enter your best estimate for each segment.