TWA / STEL Calculator

An 8-hour time-weighted average (TWA) spreads the concentrations you were exposed to across a full 8-hour shift. Each period contributes its concentration multiplied by its duration in minutes; the total is divided by 480 — the number of minutes in an 8-hour shift.

• TWA(8h) = Σ(Cᵢ × Tᵢ) / 480
• STEL(15min) = Σ(Cᵢ × Tᵢ) / 15

The divisor is always the reference period (480 minutes for the 8-hour TWA, 15 minutes for the STEL), not the sum of the durations you enter. That is the standard convention: any time you were not exposed counts as zero toward the average. So 4 hours at a concentration and 4 hours at zero average out to half that concentration over the shift. Because of that fixed basis, the 8-hour TWA here applies only to exposures within a single 8-hour shift; extended shifts (>8 h) need a separate shift-adjustment model that this tool does not cover, so it will not return an 8-hour TWA for entered time over 480 minutes.

A STEL (short-term exposure limit reference) uses a 15-minute period instead of a full shift — useful for short, higher exposures.

This tool computes exposure magnitudes only. It does not include and does not compare against any occupational exposure limit values — no ACGIH TLVs or STELs, no OSHA PELs, no NIOSH RELs. Compare the number it gives you against your own applicable limit (e.g. the OSHA Exposure Limit Lookup). This version uses a fixed 480-minute (8-hour) basis.