Chemical TWA calculator
Combine concentration readings and durations for one substance across a complete eight-hour period.
Enter your values
Check this tool fits your task
Account for exactly eight hours using measured or justified values. One substance and one unit only. No built-in exposure limit, mixture, peak, STEL, ceiling or extended-shift adjustment.
Your result
Enter your values, or try the example to see how it works.
Calculation breakdown
What to do next
Method and sources
Eight-hour TWA = Σ(concentration × hours) ÷ 8. This implementation requires the entire eight-hour period to be accounted for.
OSHA 1910.1000(d)(1): cumulative exposure formula and example ↗
How to use this tool
- Check the scope and collect measured or verified input values.
- Enter values using the displayed units, then calculate.
- Read the result, calculation breakdown and next steps together.
Worked example
2 h at 150 ppm + 2 h at 75 ppm + 4 h at 50 ppm → 81.25 ppm eight-hour TWA.
The example button fills illustrative values. Where a scope confirmation is shown, check it yourself before calculating.
Method and references
Eight-hour TWA = Σ(concentration × hours) ÷ 8. This implementation requires the entire eight-hour period to be accounted for.
Account for exactly eight hours using measured or justified values. One substance and one unit only. No built-in exposure limit, mixture, peak, STEL, ceiling or extended-shift adjustment.
Common questions
Can I leave unsampled hours blank?
No. Obtain representative data or an appropriately justified assessment. The calculator will not treat missing periods as zero.
Can I combine different chemicals?
No. Enter one substance. Mixture assessment requires a separate method.
Does the result show that exposure is acceptable?
No limit is assumed. A hygienist must check the applicable limit, sampling quality and other relevant exposure criteria.
Public beta · Check the method and inputs with a competent reviewer before operational use.
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