Working out activity durations from quantities and crew rates
The project manager wants a duration for the drywall board on level 3, so the look-ahead can show when taping starts. The drywall foreman says "a week, maybe less". The last job's schedule says three days. Neither number came from level 3.
Divide the quantity by what the crew installs in an hour, the number of workers times each one's rate, and turn the hours into working days: duration = quantity ÷ (crew × rate). A copied or guessed number belongs to another floor, crew or set of conditions, and every activity after it inherits the error. A university construction-planning text uses the same relationship, with quantities from the take-off and rates from the firm's records, and calls it an approximation: difficult access lowers productivity, crews speed up as they learn, and more crews do not always shorten the work in proportion.
- Take the quantity for this activity and area from the drawings or the take-off, in your rate's unit: square metres of board, metres of wall, tonnes of rebar.
- Choose a rate from your own records for similar work, per worker-hour. If your records give hours per unit, divide one by it: 0.08 hours per m² is 12.5 m² an hour. Once work starts, switch to the first-run rate.
- Use the crew the trade will send, if the area can hold it; see sizing a crew.
- Calculate crew hours, then divide by the hours worked a day.
- Add what the rate leaves out: stocking, layout, moves, cleanup and inspection waits. Curing and drying go in as separate activities or lags.
- Test the answer against a past job and with the foreman who will run it; if they disagree, find the wrong assumption first.
- Write the basis in the activity's notes, so the next update can show which assumption failed.
Common mistakes
- Using a rate from easier conditions, such as open walls instead of bulkheads.
- Assuming twice the crew takes half the time.
- Mixing square metres of floor and of board.
- Taking the crew's best week as its average.
Checklist
Before you enter a duration
- Quantity in the rate's unit
- Rate from your records for similar work
- Crew size the trade has confirmed
- Setup, moves and inspection waits added
- Checked with the foreman who will run it
- Basis written in the activity notes
Check your understanding
Your calculated duration is 6 working days, and the project manager asks you to enter 4 so the look-ahead meets a milestone. What do you say?
