The bank holidays are not the same list
Nearly every working-days calculator quietly assumes England and Wales. If you are in Glasgow or Belfast that gives you the wrong answer several times a year, and it gives it confidently.
Scotland has no Easter Monday. It has 2 January and St Andrew's Day instead, and its summer bank holiday is the first Monday in August rather than the last. That is three differences in one year, and the August one catches out anybody planning a shutdown across both countries.
Northern Ireland has everything England and Wales has, plus St Patrick's Day on 17 March and the Battle of the Boyne on 12 July. Ten days rather than eight.
Substitute days
When a fixed-date holiday lands on a weekend, a substitute is given on the next available weekday. Christmas Day on a Saturday moves to the Monday, and Boxing Day, which would want the same Monday, gets pushed to the Tuesday. This works that chain out rather than just adding a day, which is where hand-built holiday lists usually go wrong.
Easter moves every year, which is why a hardcoded table of dates goes stale. Good Friday and Easter Monday are computed here, so the tool keeps working for years nobody has typed in yet.
Counting between dates includes both ends
Monday to Friday is five working days, not four. That is what the question means when somebody asks how many working days there are in a period: they are counting the days people turn up, and both the first and last are days people turn up.
Adding working days does not count day zero
The other mode works the other way, because that is what the phrase means. "Within 5 working days" of a Monday is the following Monday, not the Friday. The start date is day zero and the count begins the next working day.
That convention runs through payment terms, statutory deadlines, notice provisions and adjudication timetables. Getting it a day out is the difference between being in time and being out of time.
Your working week may not be five days
Plenty of sites work six days. Plenty of offices are on a four-day week. Some trades work Tuesday to Saturday. The working week is set here rather than assumed, and the calculation follows whatever is switched on.
Bank holidays can be turned off entirely too. If you need a plain business-day count without public holidays, or you are working somewhere the UK list does not apply, switch them off and only the weekend rule applies.
What it cannot know
One-off bank holidays are announced by the government and cannot be worked out from a rule. The ones already announced are in here: the Platinum Jubilee and the State Funeral in 2022, the Coronation in 2023, VE Day 75 in 2020. Anything announced after this page was written will not be, and neither will your own company shutdown days.
This also does not know about local holidays. Several Scottish towns and cities keep their own trades holidays, which are local custom rather than statutory bank holidays, and no calculator can guess them.
Common questions
Are UK bank holidays the same everywhere?
No. Scotland has no Easter Monday, has 2 January and St Andrew's Day instead, and its summer bank holiday is the first Monday in August rather than the last. Northern Ireland has everything England and Wales has, plus St Patrick's Day and the Battle of the Boyne, which is ten days a year rather than eight.
What happens when a bank holiday falls on a weekend?
A substitute day is given on the next available weekday. If Christmas Day falls on a Saturday it moves to the Monday, and Boxing Day, which would also want that Monday, gets pushed to the Tuesday instead. The tool works that chain out rather than simply adding one day.
How many working days are there between Monday and Friday?
Five, not four, because counting between two dates includes both ends. That is what the question means when someone asks how many working days fall in a period: both the first and the last day are days people turn up.
If something is due within 5 working days of a Monday, what is the deadline?
The following Monday, not the Friday. When adding working days to a date, the start date counts as day zero and the count begins on the next working day, which is the convention used in payment terms, statutory deadlines and notice provisions.
Can I use this if my working week is not Monday to Friday?
Yes. The working week can be set to match a six-day site, a four-day week, or a Tuesday-to-Saturday pattern, and the calculation follows whichever days are switched on. Bank holidays can also be turned off entirely for a plain business-day count.
Does the calculator know about every bank holiday, including one-off ones?
Only the ones already announced at the time it was built, such as the Coronation bank holiday in 2023 and the State Funeral bank holiday in 2022. It cannot know about holidays announced later, your own company shutdown days, or local Scottish trades holidays, which are a matter of local custom rather than statutory bank holidays.