Construction Duration Calculator
Quantity × labour norm ÷ crew gives the days you need.
Quick answer
With the default inputs: Quantity 1000 · Labour norm 0.5 man-days/unit · Crew size 20 persons etc.Calendar duration 32 days
Edit the inputs below to recalculate.Inputs
Result
Calendar duration
32days
| Total man-days | 500.0man-days |
| Daily man-days | 19.00man-days/d |
| Working days | 27days |
| Calendar days | 32days |
| In months | 1.1months |
| Output per day | 37.0units/d |
| Crew for 30 days | 18persons |
| With 20% float | 39days |
Two figures must not be confused: working days are days actually worked, calendar days include rain, stoppages and rest — and only the latter goes to the client and the programme. Calendar = working ÷ workable ratio. Note that adding people is not linear: work fronts are limited, efficiency collapses past a sensible crew size, and night shifts run at only 70-85% of day output. This tool is linear, so add 10-20% in practice.
Formula
| Total man-days | T = quantity × labour norm |
| Daily capacity | C = crew × shifts × attendance |
| Working days | D = ⌈T ÷ C⌉ |
| Calendar days | D = ⌈working days ÷ workable ratio⌉ |
| Crew for a target | N = T ÷ target ÷ (shifts × attendance) |
1000 units at 0.5 man-days, 20 workers, one shift, 95% attendance and 85% workable: 500 man-days, 19 per day, 27 working days, 32 calendar days — 18 workers to finish in 30 days.
FAQ
Why are calendar days so much longer than working days?
Because sites do not work every day. Rain, high wind that idles cranes, heat, late materials and unapproved preceding work all stop production. The workable-day ratio captures this: 75-85% for general building work, lower in the wet season, and over 90% for interior finishes and services. Hence 27 working days become 32 calendar days. Quoting working days as a duration guarantees overrun.
Does adding people shorten the programme?
Only within limits. Duration follows the critical path, and if a work front only fits ten people, thirty is overcrowding that lowers output. There are practical caps per square metre of front (about 0.5 workers/m² for bricklaying, 0.3 for plastering); beyond that you need zoned or multi-level flow. Real acceleration comes from more fronts, a different method such as precasting, or longer daily hours.
Where do labour norms come from?
From the national or provincial building consumption and pricing norms, which list man-days per unit for each work item. Company history works too: how many cubic metres a gang lays or square metres it plasters in a day gives the real norm. Published values are industry averages, so a well-run crew achieves 1.1-1.3 times them — adjust to your own productivity.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.