Calculator · 886 substances · federal OSHA and Cal/OSHA
8-Hour TWA Calculator
Find your 8-hour time-weighted average from air samples, and whether it is over the legal limit for that substance.
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Where do these limits come from?
Federal limits are read off OSHA's own Table Z-1 and California's off Cal/OSHA's Table AC-1, each quoted line by line on the substance's page here. The two agencies genuinely differ on many substances, and OSHA's own side-by-side comparison page is out of date on some of them where our record is current.
How do you know it's right?
Every limit is checked back against its own table row: the number has to appear in the line we quote, and that line has to appear on the agency's page. The average is computed in your browser and written out in full, so you can check the arithmetic against the formula in 29 CFR 1910.1000(d)(1)(i). More on how that works.
What does this not tell you?
It does not decide compliance for you. Ceiling limits, short-term (15-minute) limits, skin absorption and respirator use all sit outside an 8-hour average, and substances with their own OSHA standard — asbestos, silica, lead, benzene — are not in Table Z-1 at all. It computes one number, the one the regulation defines, against the limit we hold for that substance.
About this calculation
The regulation defines the 8-hour time-weighted average as the sum of each sample's concentration multiplied by its duration, divided by eight — always eight, whatever the shift length. Dividing by hours actually worked is the common mistake, and on a 10- or 12-hour shift it understates the exposure by a quarter or more. Look up any substance's limit.