Build Project Timeline Calculator

How the build project timeline calculator works

Lay the job out as a chain of tasks: strip out, first fix, plaster, second fix, decorate, and so on. Give the number of tasks and how long an average one takes your crew. If every task waited for the last to finish, the job would take the tasks times the days each. Trades rarely wait that long. The second can usually start once the first is part done, and that overlap is where schedules gain time.

D = [d + (n − 1) × d × (1 − o)] × (1 + b)

Here n is the number of tasks, d the average days per task, o the overlap and b the weather allowance, both as decimals. The first task always runs in full. Each of the others adds only the part that does not overlap the task before it. The weather allowance goes on top.

Overlap is a lead in a critical path schedule

Professional programmes are built with the critical path method, CPM. Tasks are linked by relationships: finish-to-start, where the next task waits for the last to finish, or start-to-start, where it can begin once the last has started. CPM training given at Florida DOT's Construction Academy sets out the four relationship types, with the example of sodding starting a day after finish grading starts. The Tennessee DOT guide to reading CPM schedules describes lag as an offset between a task and the one after it, and calls a negative lag a lead. The overlap here is exactly that: a start-to-start link with a lag of (1 − o) × d, so a 25% overlap on 3-day tasks lets each start 2.25 days after the one before.

It assumes one chain of similar tasks. A real job has several paths running side by side, and the longest of them, the critical path, sets the finish date. If your job splits into independent streams, run the calculator for the longest one.

It also uses one average task length. The last task is the one that runs in full at the end, so if it is much longer or shorter than the average, the finish moves by the overlap share of the difference: o × (last task − average). Six tasks averaging 3 days with 25% overlap take 14.25 days before weather, but if the last of them is a 7-day decorate, the chain takes 0.25 × (7 − 3) = 1 day longer.

A small job, worked through

With the defaults, six tasks of 3 days each would take 18 working days end to end. With a 25% overlap the first task runs its full 3 days and the other five add 2.25 days each: 3 + 5 × 2.25 = 14.25 days, a saving of 3.75. A 10% weather allowance adds 1.43, for about 15.7 working days. That is 3.1 working weeks, or roughly 22 calendar days on a five-day week. With two people on site throughout, it is 28.5 person-days of labour before weather.

A bigger job shows how the pieces scale. Say an extension breaks into 15 tasks averaging 6 days, with 20% overlap. End to end that is 90 days; overlapped, 6 + 14 × 4.8 = 73.2 days. Built through a wet season with a 15% weather allowance, it becomes about 84.2 working days: 16.8 working weeks, or around 118 calendar days, close to four months. Four people on site for the 73.2 working days is 292.8 person-days. Notice that the overlap saved 16.8 days while the weather cost 11. On a long outdoor job the allowance matters about as much as the sequencing.

Choosing a weather allowance

Contracts often write the weather allowance down month by month. One Arizona transit contract lists the anticipated adverse weather days for Flagstaff from National Weather Service records: 3 days in June, 12 in July and 7 in January, counting days with more than 0.01 in of rain or a high below freezing. That is 10% of June but 39% of July. Outdoor work in a wet or cold season needs a far bigger allowance than the 10% default; internal work after the building is weathertight needs less. Local climate records for the months you will be on site are the best guide.

Crew size and why it does not shrink the time

Enter the task durations as your crew will actually achieve them. The calculator does not divide the time by the crew, because extra hands rarely cut a task in proportion: access, supervision and the order of work get in the way. A study of 54 US mechanical and sheet metal projects by Hanna and colleagues measured productivity losses of 0% to 41% from overmanning. Crew size is used for the labour instead: people on site times working days, a figure to take into a labour cost estimate.

What this tool does not do

It does not build a network of tasks, find the true critical path, or allow for long-lead materials, inspection hold points, deliveries, or the drying and curing time that has to pass between some trades. Permits and inspections vary by country and by job. Treat the result as a first check on whether a target date is realistic, then compare it with the programme from whoever is running the work.

Using this calculator with other BuildMetricLab tools

The labour cost calculator prices the person-days, the contingency calculator sizes the money buffer to match the time buffer, and the tiling and drywall time calculators give better durations for those tasks. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.