8-hour TWA calculator: time-weighted average exposure, 15-minute STEL, excursion check and Brief and Scala long-shift adjustment

Work out a worker's 8-hour time-weighted average from sampled periods (unsampled time as zero or at the sampled average), the highest 15-minute average for a STEL, an excursion check, and a reduced limit for long shifts by Brief and Scala or the OSHA model, against limits you type in.

Keep it one tap away for everyday use.

Samples
C × t
TWA, STEL
Limits

Nothing is uploadedWorks offline once open

For study and teaching, not medical advice. Check with a qualified professional.

Sampled periods

One per line: start, end and concentration (07:00 09:30 12), or minutes and concentration (150 12). A task name at the end is optional.

Unit Shift length (hours) Time not sampled

The 8-hour TWA adds up concentration × time over the whole shift and divides by 8 hours (480 minutes). Count unsampled time as zero only when there really was no exposure, such as a lunch break away from the work area.

Your limits

8-hour limit 15-minute limit (STEL, optional) Long-shift adjustment Hours per week

Excursion rule (for substances with no STEL)

Above this many × the limit for no more than (min a day) and never above × the limit

Type the limits that apply to you; the example limits are made up and no limit tables are included. The excursion defaults (3 × for at most 30 minutes, never 5 ×) follow the ACGIH TLV excursion guidance, checked 2026-11-09; check the official source for your country.

Result

Working (check the sums)

Download

What it does

Workplace air limits are usually written as an 8-hour time-weighted average (TWA), often with a 15-minute short-term exposure limit (STEL). Air sampling gives a series of periods, each with its own measured concentration. This page adds up concentration × time for each period, works out the 8-hour TWA, the average over the whole shift and the highest 15-minute average, checks the excursion rule, and for shifts longer than 8 hours reduces the limit by the Brief and Scala model or the OSHA model. It compares everything with limits you type in, and shows the working so you can check every sum. It is for study and teaching, not medical advice or a compliance report.

How to use it

Paste the sampled periods, one per line, either as clock times and a concentration (07:00 09:30 12) or as minutes and a concentration (150 12). A task name at the end of the line is optional. You can also open a CSV or Excel file. Periods that cross midnight work.
Choose the unit and type the shift length in hours.
Choose what happens to time not sampled: counted as zero exposure (only if there truly was none, such as a break away from the work) or assumed to be at the average of the samples.
Type your 8-hour limit and, if there is one, your STEL. Choose a long-shift adjustment and the hours per week for the weekly models.
The excursion rule is for substances without a STEL. Change the three numbers if your guidance differs.
Download the working as CSV, or the report as an image, PDF or Word file.

The formulas

8-hour TWA = Σ(Cᵢ × tᵢ) ÷ 480 minutes, over the whole shift (so a 10-hour shift is still divided by 8 hours).
Shift average = Σ(Cᵢ × tᵢ) ÷ shift length.
Highest 15-minute average: the largest average over any 15-minute window, with gaps between samples counted as zero. It is exact for the stepped profile the samples give.
Brief and Scala: daily reduction factor = (8 ÷ h) × (24 − h) ÷ 16; weekly = (40 ÷ h) × (168 − h) ÷ 128. OSHA model: 8 ÷ h daily, 40 ÷ h weekly. The adjusted limit (limit × factor) is compared with the shift average. The factor is never above 1.

Worked example

The example is a made-up 10-hour day from 07:00 to 17:00 with a 30-minute lunch break, with an 8-hour limit of 20 ppm and a STEL of 50 ppm (both made up). The five samples cover 570 minutes and add up to 6000 ppm·min, so the 8-hour TWA is 6000 ÷ 480 = 12.5 ppm (63 % of the limit) and the shift average is 10 ppm. The 15-minute tank cleaning at 60 ppm gives a highest 15-minute average of 60 ppm from 09:30, which is above the 50 ppm STEL. For a 10-hour shift the Brief and Scala daily factor is 0.8 × 14 ÷ 16 = 0.7, so the adjusted limit is 14 ppm, and the 10 ppm shift average is within it.

Tips

Take short (15-minute) samples during peak tasks; long samples hide peaks.
Brief and Scala is simple and cautious. Models based on how the body clears a substance need its half-life, which this page does not ask for.

FAQ

Why divide by 8 hours on a 10-hour shift? The 8-hour TWA is defined as the total daily exposure spread over 8 hours, so long shifts are counted in full. The long-shift adjustment then asks whether the limit itself should be lower.

Are limit values included? No. Type the limits that apply to you from the official source.

Is anything uploaded? No. Everything is worked out in your browser.

Where the numbers come from

8-hour TWA = Σ(C × t) ÷ 8 h: the standard definition, as in US OSHA 29 CFR 1910.1000(d) and UK HSE EH40/2005. Excursion guidance (3 × the TWA limit for at most 30 minutes a day, never 5 ×): ACGIH TLV documentation, given as editable defaults (checked 2026-11-09; check the official source).
Long-shift adjustment: R. S. Brief and R. A. Scala, Occupational exposure limits for novel work schedules, Am Ind Hyg Assoc J 36:467 (1975). OSHA model (8 ÷ h daily, 40 ÷ h weekly) as described by D. J. Paustenbach in Patty's Industrial Hygiene and Toxicology. Formulas only.
No exposure limit tables are included; the example limits and measurements are made up. Checked in the tests by hand and against an independent Python calculation (minute-by-minute profile) on 30 random sampling days.

Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.

Last updated 2026-11-09.