Week Number Calculator

How to use the week number calculator

Enter a date and the tool returns its ISO 8601 week number, the ISO year that week belongs to, and the Monday to Sunday range the week covers. The example loaded is 31 July 2026, which falls in week 31 of 2026, running from Monday 27 July to Sunday 2 August.

There are no options to set, because ISO 8601 admits none: the standard fixes both the start of the week and the rule for numbering it, which is precisely what makes week numbers useful for coordinating across countries. What a wall calendar in your country shows may differ, and that difference is the reason to check rather than a reason to distrust the answer.

1. 1

   An ISO week runs Monday to Sunday

   The weekend sits at the end of the week rather than split across two, whatever a wall calendar in your country shows.

2. 2

   Week 1 is the week containing the first Thursday of the year

   Which is the same as saying it is the week containing 4 January, and the same again as saying it is the first week with at least four days in the new year. Those three phrasings are equivalent and each is easier to apply in a different situation.

Two rules define everything, and the Thursday version of the second is the one to remember, because it explains every strange case at once: whichever year owns the Thursday owns the whole week. The consequence is that the ISO year and the calendar year do not always agree, and the tool flags it when they diverge.

Why some years have 53 weeks

Most ISO years contain 52 weeks and roughly one in five contains 53, which is a real planning fact rather than a curiosity. Between 2015 and 2040 the long years are 2015, 2020, 2026, 2032 and 2037.

A 53-week year adds a whole reporting period to anything measured in weeks: retail calendars, weekly payroll runs, rotas and week-by-week budgets all gain a week that the previous year did not have, and comparing week 52 of one year against week 52 of another is not comparing equal periods.

The formula, worked line by line

The calculation uses the Thursday trick, which is the standard way to resolve ISO week numbering without special cases. Step to the Thursday of the week the date falls in, by adding four minus the ISO weekday.

That Thursday calendar year is the ISO year by construction, because week 1 is defined as the week holding the first Thursday, so every Thursday belongs to the year printed on it. Count whole weeks from 1 January of that ISO year to the Thursday, add one, and you have the week number.

Nothing in that procedure treats New Year as a special case, which is the point. The straddle behaviour that surprises people is not an exception bolted on; it falls out of the same three lines that handle a date in July. The tool also reports the Monday and Sunday that bound the week, found by stepping back to the Monday and forward six days, so the abstract number is always accompanied by the concrete dates it stands for.

ISO weekday: Monday = 1, Tuesday = 2, ... Sunday = 7
thursday = date + (4 - ISO weekday)
ISO year = the calendar year of that Thursday
week = floor(days from 1 January of the ISO year to that Thursday / 7) + 1
week start = date + (1 - ISO weekday)   ... the Monday;   week end = week start + 6
weeks in an ISO year = the ISO week number of 28 December that year

The straddle cases run through the identical arithmetic. Take Friday 1 January 2021: Friday is ISO weekday 5, so the Thursday of that week is 31 December 2020, which sits in calendar 2020, making the ISO year 2020 rather than 2021. From 1 January 2020 to 31 December 2020 is 365 days, 365 divided by 7 is 52 with a remainder, and adding one gives week 53.

Going the other way, Tuesday 31 December 2019 has its Thursday on 2 January 2020, so the ISO year is 2020 and the week is 1. Neither result needs a rule of its own.

A 53-week ISO year happens when 1 January falls on a Thursday, or on a Wednesday in a leap year, because either arrangement leaves 53 Thursdays inside the year. Between 2015 and 2040 the long years are 2015, 2020, 2026, 2032 and 2037, and 2026 being one of them is worth noticing for any plan measured in weeks.

The general test the tool uses is shorter than the rule: the number of weeks in an ISO year is the week number of 28 December, since 28 December is always in the last week of its own ISO year. That is not a shortcut, it is a direct consequence of the definition, and it is the check the engine tests itself against on every year from 1995 to 2035.

Questions people ask