Time Weighted Average Exposure Calculator

Work out time weighted average exposure instantly with clear inputs, formula shown and shareable results.

About Time Weighted Average Exposure Calculator

A time weighted average multiplies each measured concentration by the time it applied, sums the products, and divides by eight hours. Dividing by eight rather than by the sampled time is deliberate: unsampled parts of the shift are treated as zero exposure only if that is genuinely the case, otherwise the average over sampled time is the more honest figure.

Formulas

Eight hour TWA

TWA = sum(Ci x Ti) / 8

Sampled average

Average = sum(Ci x Ti) / sum(Ti)

Worked example

With the default values below, Time Weighted Average Exposure Calculator returns Eight hour time weighted average of 38.125. Change any input above and every figure here is recalculated in your browser.

Inputs used

Concentration in the first periodminimum 0 ppm: 60 ppm
Duration of the first periodaccepts 0–24 h: 3 h
Concentration in the second periodminimum 0 ppm: 25 ppm
Duration of the second periodaccepts 0–24 h: 5 h

Results

Eight hour time weighted average: 38.125 ppm
Sum of concentration times time: 305.00 ppm·h
Total sampled time: 8.00 h
Average over the sampled period: 38.125 ppm

Step by step

1Values used
2Eight hour TWA
3Sampled average
4Eight hour time weighted average= 38.125 ppm
5Sum of concentration times time= 305.00 ppm·h
6Total sampled time= 8.00 h
7Average over the sampled period= 38.125 ppm

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or fees or charges into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Exposure assessment must be performed by a qualified occupational hygienist to a recognised sampling standard.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Exposure assessment must be performed by a qualified occupational hygienist to a recognised sampling standard.

Frequently Asked Questions

How is the Time Weighted Average Exposure figure worked out?

TWA = sum(Ci x Ti) / 8. A time weighted average multiplies each measured concentration by the time it applied, sums the products, and divides by eight hours.

Which denominator should I use?

Divide by eight when the unsampled time is genuinely exposure free. If exposure continued unsampled, divide by sampled time and apply that concentration to the rest of the shift.

What values do I need to enter?

This calculator takes 4 inputs: Concentration in the first period, Duration of the first period, Concentration in the second period, Duration of the second period. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.