US general industry · single substance
Eight-hour exposure TWA calculator
Combine same-substance exposure periods over a complete eight-hour assessment window.
YOUR INPUTS
Set the scope. Enter the data.
Calculator inputs stay in this page’s browser memory and clear on reload. No sign-in required.
YOUR RESULT
A result you can trace.
Enter your data and select Calculate. Your result will show the formula, units and interpretation alongside the definitions and sources below.
No automatic “safe” rating or compliance certificate.
UNDERSTAND THE METRIC
Definition
A concentration TWA weights each concentration by the time spent at it. It is different from the logarithmic sound-level calculation used for noise.
When to use it
Use representative measured or otherwise justified concentrations for one substance in one consistent unit. Account for the full eight-hour window and optionally compare with a verified, applicable eight-hour limit in the same unit.
Formula
Eight-hour TWA = Σ(concentration × duration in hours) ÷ 8. Optional comparison (%) = TWA ÷ entered eight-hour limit × 100.
USE IT CONSISTENTLY
Inputs and interpretation
- Enter periods totalling exactly eight hours. Do not fill unknown or unmeasured periods with zero. This tool does not assess extended shifts.
- Use the same substance and unit in every row. Selecting ppm or mg/m³ changes the label only; this tool does not convert between them.
- Any entered limit must be the verified applicable eight-hour limit for that substance. Ceiling limits, short-term limits, mixtures and substance-specific requirements need separate assessment.
- A result below an entered TWA limit does not establish that short-term peaks, skin exposure or other applicable conditions are acceptable. The tool does not select a PEL or certify compliance.
Sources
Technical references checked on 31 October 2026. OSHA sources establish the methods or guidance described above; they do not constitute OSHA approval or endorsement of this calculator.