Determining Activity Durations
How long will it really take? Durations turn quantities, crews and productivity into honest, defensible time estimates — the last input the CPM needs. Estimate first, then read the date; fix a missed target by changing the plan, not the numbers. Built on AACE 32R-04.
What a duration is — and isn't
Durations are the most commonly fudged number in scheduling. Padded "just in case," or shrunk to make a deadline, they quietly turn a CPM model into fiction. A duration should answer one honest question: given this quantity of work and this crew's real output, how many working days does it take?
Quantity, productivity, and crews
For production work, duration comes from three things: how much work there is, how fast a crew does it, and how many crews you run. The core relationship:
Worked example. Install 1,200 m² of formwork. One crew places 80 m²/day. With one crew: 1,200 ÷ 80 = 15 working days. Add a second crew (160 m²/day): 1,200 ÷ 160 = 7.5 → 8 days. The work didn't change — the capacity did. That's the lever you pull to hit a date, and it's visible and honest in the schedule.
Methods for estimating durations
The formula needs a productivity rate — and there are several ways to get a credible one, in rough order of reliability:
| Method | How it works | Best when |
|---|---|---|
| Productivity-based | Quantity ÷ a known output rate (from estimate, norms, or historical data) | Quantities and rates are known — the default for production work |
| Historical / benchmarking | Use actuals from your own past similar work | You have reliable records of comparable activities |
| Published norms | Industry productivity tables (RS Means, trade associations) | No project-specific data; corroborate, don't blindly copy |
| Expert judgment | Experienced estimator/superintendent's call | Non-repetitive or unusual work where formulas don't fit |
| Parametric / three-point | Optimistic–likely–pessimistic blended (PERT) | High uncertainty — captures a realistic range |
Duration calculator
Set the quantity, one crew's daily output, and how many crews. See the duration — and watch the crew lever.
Duration calculator
Try it yourselfSet the quantity, one crew's daily output, and how many crews. Duration = quantity ÷ (output × crews), rounded up to whole working days.
Calendars turn duration into dates
A duration is a count of working days — but the schedule has to place it on the calendar, and that's where the calendar comes in. A 15-working-day activity on a 5-day calendar spans 3 weeks; the same 15 days on a 6-day calendar finishes sooner. Weekends, holidays, shift patterns, and weather-shutdown days all live in the calendar, not the duration.
Ten things to remember
- Duration is an estimate of effort against capacity — not a deadline or a target-fit guess.
- Estimate durations first, then read the date — fix a missed target by changing the plan, not the numbers.
- Duration = quantity ÷ (daily crew output × crews) — the core production relationship.
- Crews are the honest lever to shorten a duration — visible in the schedule, unlike padding.
- Adding crews has diminishing returns — crowding a work front lowers productivity.
- Productivity-based estimating is the default; use norms, history, judgment, or PERT as fits.
- Cross-check with a second method — reconcile differences before baselining.
- Round up to whole working days — you can't schedule a fraction of a crew-day.
- Calendars turn working days into dates — weekends, holidays, shifts live there, not in the duration.
- Assign the right calendar per activity (cure = 7-day, crew = 5-day) — a wrong calendar is a silent error.
Glossary
- Benchmarking
- Estimating from actuals of comparable past work.
- Calendar
- The working/non-working pattern that converts working-day durations into calendar dates.
- Crew
- The resource team performing an activity; more crews can shorten duration, within limits.
- Dilution
- The productivity loss when too many crews crowd one work front.
- Duration
- The estimated working time an activity needs, in the schedule's time unit.
- Productivity norm
- A standard output rate from published or in-house tables.
- Productivity rate
- Units of work installed per crew per working day (or labour hours per unit).
- Three-point / PERT
- Blending optimistic, most-likely, and pessimistic estimates for uncertain work.