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Working Days and Construction Programme Calculator

How to Calculate Working Days and Finish Dates for a Construction Programme

A 20-day task rarely finishes in 20 calendar days. Learn how to turn working days into a finish date, count working days between two dates, and allow for weekends, UK bank holidays and site shutdowns, with worked examples and a free calculator.

Last updated 7 November 2026

Working days are not calendar days

A task that takes 20 working days does not take 20 calendar days. Weekends, bank holidays and site shutdowns all stretch the calendar. Forgetting this is one of the most common reasons programmes slip before work even starts.

A reliable way to turn durations into dates keeps your programme honest and makes it easier to agree handover dates. KaelSite’s free Working Days and Construction Programme Calculator finds a finish date from a start date and duration, or counts the working days between two dates.

How it works

Start date plus working days gives a finish date. Starting from the start date, the calculator moves forward day by day and counts only valid working days until it reaches the number you entered. The last one is the finish date. If the start date is not a working day, counting begins on the next working day.

Start date plus finish date gives working days. The calculator counts the working days between the two dates, including both ends when they are working days.

A day is skipped if it is:

  • Not in your week pattern, such as a weekend on a Monday to Friday week
  • An England and Wales bank holiday, if you choose that option
  • An extra bank holiday you add
  • An excluded date, such as a site shutdown

Choosing a week pattern

  • Mon–Fri (5 days): the usual office and many site patterns.
  • Mon–Sat (6 days): common on busy sites and groundworks.
  • Every day (7 days): continuous working, for example on night shifts or specific operations.
  • Custom weekdays: for example Tuesday to Saturday.

The pattern has a large effect. The same 30 working days finish a week earlier on a six-day week than on a five-day week.

Worked examples

The dates below are for illustration. Run your own start date through the tool.

1. A task across Christmas
Start Monday 7 December 2026, 20 working days, Monday to Friday, with England and Wales bank holidays:

  • Days 1 to 5: 7 to 11 December
  • Days 6 to 10: 14 to 18 December
  • Days 11 to 14: 21 to 24 December
  • Skipped: Friday 25 December (Christmas Day) and Monday 28 December (Boxing Day substitute)
  • Days 15 to 17: 29, 30 and 31 December
  • Skipped: Friday 1 January (New Year’s Day)
  • Days 18 to 20: 4, 5 and 6 January

Finish: Wednesday 6 January 2027. That is 31 calendar days from start to finish for 20 working days.

2. Six-day week versus five-day week
Start Monday 11 January 2027, 30 working days, with no bank holidays in the period:

  • On a Monday to Saturday pattern, 30 days is five full weeks, finishing Saturday 13 February 2027.
  • On a Monday to Friday pattern, 30 days is six full weeks, finishing Friday 19 February 2027.

The six-day week saves six calendar days.

3. Counting working days between two dates
From Monday 22 March 2027 to Friday 9 April 2027, Monday to Friday, with England and Wales bank holidays:

  • There are 15 weekdays in the period.
  • Good Friday (26 March) and Easter Monday (29 March) are bank holidays.
  • Result: 13 working days, counting both the start and finish dates.

4. Start date on a weekend
Start Saturday 12 December 2026, 5 working days, Monday to Friday:
Counting begins on the next working day, Monday 14 December, so the finish is Friday 18 December 2026.

Bank holidays across the UK

The calculator includes England and Wales bank holidays: New Year’s Day, Good Friday, Easter Monday, Early May, Spring, Summer, Christmas Day and Boxing Day, with weekend substitute days. Scotland and Northern Ireland have different public holidays, for example in Scotland 2 January and St Andrew’s Day, and in Northern Ireland St Patrick’s Day and the Twelfth of July. If your site is there, add those dates as extra bank holidays. Check the official list on GOV.UK for the current year.

Excluded dates

Use excluded dates for non-working days that are not public holidays, such as:

  • Christmas shutdowns
  • Industry shutdown weeks
  • Client closures
  • Possession or access restrictions
  • Planned shutdowns of services or utilities

They are treated exactly like bank holidays when counting.

Common mistakes

  • Treating working days as calendar days. The programme ends too early.
  • Forgetting substitute bank holidays. When a holiday falls on a weekend, the following Monday or other day is usually the holiday.
  • Missing the Christmas and Easter periods. These cut a lot of working time.
  • Using the wrong region’s holidays. Scotland and Northern Ireland differ from England and Wales.
  • Not allowing for weather and delays. A programme with no float misses dates on the first bad week.
  • Confusing inclusive and exclusive counting. Decide whether the start day counts, and check that your contract agrees.

UK contract and programme points

Contracts define completion dates and time in different ways. Some use a fixed date, others a number of days, and the definition of “working day” varies. Always check what your contract and programme requirements say before relying on a calculated date. Guidance on programming and delay analysis is available from the Chartered Institute of Building and the Society of Construction Law. A calculated date is a planning tool, not a contractual extension of time.

Frequently asked questions

How do working days differ from calendar days?
Calendar days count every day. Working days skip weekends or other non-working days, bank holidays and excluded dates. A 20-working-day task on a Monday to Friday week usually spans more than 20 calendar days.

How does the finish-date calculation work?
It moves forward from the start date, counting only valid working days until it has the number you entered. The last one is the finish date.

Which bank holidays are included?
England and Wales bank holidays, including weekend substitutes. Add Scotland, Northern Ireland or site-specific days as extra holidays.

What are excluded dates for?
Site shutdowns, client closures and any other non-working dates that are not public holidays.

Can I count working days between two dates?
Yes. Switch to the start-and-finish mode. Both dates are counted when they are working days.

Can I run a six-day site week?
Yes. Choose Mon–Sat, every day or custom weekdays.

Related calculators

A programme needs durations, and durations come from quantities and crews. These tools help:

See all of them in our complete guide to free construction calculators.

Keep your programme and site records in one place

A programme is only useful if progress is tracked against it. KaelSite helps UK site teams manage inspections, safety and reporting from one mobile-first app. Explore KaelSite features.

References