Gantt Duration Calculator

Work out gantt duration instantly with clear inputs, formula shown and shareable results.

About Gantt Duration Calculator

Converting a working-day duration into a calendar finish date requires stepping over weekends and public holidays. The calendar-to-working-day ratio of roughly 1.4 explains why a 45-day task takes over nine calendar weeks, which is the most common source of schedule surprise.

Formulas

Finish date

Step forward from the start date, counting only working days, until the duration plus holidays is consumed

Ratio

Ratio = Calendar days elapsed / Working days consumed

Worked example

With the default values below, Gantt Duration Calculator returns Finish date of March 11, 2026. Change any input above and every figure here is recalculated in your browser.

Inputs used

Start date: 2026-01-05
Duration in working daysmaximum 100,000: 45
Public holidays within the periodmaximum 10,000: 3
Working days per week: 5

Results

Finish date: March 11, 2026
Calendar days elapsed: 66
Working days consumed: 48
Calendar days per working day: 1.375

Step by step

1Values used
2Finish date
3Ratio
4Finish date= 2026-03-11
5Calendar days elapsed= 66
6Working days consumed= 48
7Calendar days per working day= 1.375

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or fees or charges into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Should holidays be added to duration?

They extend the calendar span but do not add effort, which is why they are counted as days to step over rather than extra work.

What about part-time resources?

Convert the effort to elapsed working days at the actual availability before using this calculator, otherwise the finish date will be optimistic.