Week Number Calculator

How to use this tool

Choose what to find. “The week number of a date” takes one date and returns the ISO 8601 week, the US week that Excel’s WEEKNUM gives by default, and the plain count of seven day blocks from January 1, together with the day of the year and the days left. “The dates of an ISO week” takes an ISO year and a week number and returns the Monday to Sunday dates with each day listed. “Every ISO week of a year” prints all 52 or 53 weeks of a year with their dates and says why the year has that many. The calendar under the result shows the month with its ISO week numbers, and the year view draws the year as one block per week.

The input people most often get wrong is the year in the ISO week boxes. ISO weeks belong to a week based year, which is not always the calendar year of the dates inside them. Week 1 of 2026 starts on December 29, 2025, and January 1, 2027 sits in week 53 of 2026. So when a supplier promises “week 1 of 2027”, enter 2027 and 1 and read the dates, January 4 to 10, 2027, rather than assuming the week begins on New Year’s Day. The other trap is the system. A week number with no system named is ambiguous close to the turn of the year, which is why the tool shows all three side by side.

What the calculation is doing

ISO 8601 weeks run Monday to Sunday, and week 1 is the week that holds the first Thursday of the year, which is the same week that holds January 4. The number comes from the day of the year and the ISO weekday, Monday 1 to Sunday 7: week = (day of year − weekday + 10) ÷ 7, rounded down. A result of 0 means the date belongs to the last week of the previous year, and a result of 53 in a year that only has 52 weeks means week 1 of the next year. A year has 53 ISO weeks when January 1 falls on a Thursday, or on a Wednesday in a leap year, which is another way of saying the year contains 53 Thursdays. December 28 is always in the last week, so its week number is the number of weeks in the year.

The US system, which Excel’s WEEKNUM uses when the second argument is left out, starts weeks on Sunday and calls whichever week holds January 1 week 1, however short that week is. Week = (day of year − 1 + offset) ÷ 7 rounded down, plus 1, where the offset is the number of days January 1 sits after the Sunday that starts its week. WEEKNUM with 2 as the second argument does the same with Monday. The simple count ignores weekdays altogether: (day of year − 1) ÷ 7 rounded down, plus 1, so days 1 to 7 are week 1, days 8 to 14 are week 2, and December 31 ends up in a week 53 that is one day long, or two in a leap year.

The ISO and US counts agree more often than the turn of the year suggests, but never completely. In a year that starts between Sunday and Thursday they give the same number from Monday to Saturday, and the US number is one higher on every Sunday because its week has already turned over. In a year that starts on a Friday or Saturday, such as 2027 and 2028, ISO week 1 does not begin until January 3 or 4, so the US count runs one ahead of ISO on every weekday for the whole year and two ahead on Sundays.

Why the ISO year and the calendar year part company

The ISO rule exists so that every week is a whole seven days and every year is a whole number of weeks. That is only possible if the year boundary moves to the nearest Monday, so up to three days at either end of the calendar year get handed to the neighbouring ISO year. The Thursday rule decides which way: a week goes with whichever year holds four or more of its days. December 29, 2025 to January 4, 2026 has four days in 2026, so it is week 1 of 2026. December 28, 2026 to January 3, 2027 has four days in 2026, so it stays week 53 of 2026 even though it runs into January.

The practical consequence is that a date and its week number can carry different years. Payroll runs, factory plans and delivery schedules across Europe use the ISO number and write it as 2026-W53, year first, so the reader knows which year the week belongs to. Anyone passing a bare week number without that year risks being 52 weeks out. Excel’s default never has this problem, because it restarts on January 1 and simply tolerates a short week at each end, but in exchange it produces a week 53 every year and a week 54 in a leap year that starts on a Saturday, as in 2000 and 2028.

A 53rd ISO week arrives on average every 5.6 years: 71 times in the 400 year Gregorian cycle, or five times in most 28 year runs. The recent and coming ones are 2020, 2026, 2032, 2037, 2043 and 2048. If a weekly budget divides an annual figure by 52, those years come up short by one week, about 1.9%, and anyone paid weekly collects 53 pay days. The US count does not flag these years at all, since it shows 53 weeks in every year, which is one reason finance and manufacturing teams prefer the ISO count.

Which years have 53 ISO weeks

Every year from 2020 to 2040, from the calculator, with the weekday of January 1 that decides the count. The last column is the largest number Excel’s WEEKNUM returns for that year in its default Sunday start system.

A worked example you can check on paper

The tool opens on today’s date. On the day this was written that was Tuesday, September 22, 2026, day 265 of 365 (January to August hold 243 days, plus 22), with 100 days left. Tuesday is ISO weekday 2, so the ISO week is (265 − 2 + 10) ÷ 7 = 39.00, rounded down to 39. That week runs from Monday, September 21, 2026 to Sunday, September 27, 2026. January 1, 2026 is a Thursday, four days after the Sunday that starts its week, so the US week is (265 − 1 + 4) ÷ 7 = 38.29, rounded down to 38, plus 1, which gives 39. The simple count is (265 − 1) ÷ 7 = 37.71, rounded down to 37, plus 1, so week 38. Three systems, two answers: the US count is one ahead of the simple count because its week 1 began on Sunday, December 28, 2025, four days before the year did, so every later week starts four days earlier as well.

Now the awkward case, Friday, January 1, 2027: day 1, ISO weekday 5, so (1 − 5 + 10) ÷ 7 = 0.86, rounded down to 0. Zero means the last week of the previous year, and 2026 has 53 weeks because it started on a Thursday, so the answer is week 53 of 2026. The US week is (1 − 1 + 5) ÷ 7 rounded down, plus 1, which is week 1. Going the other way, week 1 of 2027 must contain January 4, a Monday, so it runs January 4 to 10, 2027; the calculator gets the same dates by adding 7 × (week − 1) days to that Monday.

Mistakes that catch people out

Quoting a week number without saying which system. Week 1 in an American spreadsheet and week 1 on a German production plan can be different weeks. Name the system, or better, give the Monday date as well.

Assuming week 1 starts on January 1. Under ISO it can start as early as December 29 or as late as January 4. In 2027 it starts on January 4, 2027, and the first three days of the year are still week 53 of 2026.

Planning 52 weeks in a 53 week year. 2026 has 53 ISO weeks and its last week ends on January 3, 2027. Budgets, rotas and forecasts built on 52 come up a week short, and a 52 week rota that simply repeats will also drift a day a year against the calendar.

Comparing Excel WEEKNUM with an ISO planner. The default WEEKNUM is the US system. Use ISOWEEKNUM, or WEEKNUM with 21 as the second argument, before comparing numbers with anything printed in Europe.

Dropping the ISO year on the last week. “Week 53” on its own could be 2026 or 2032, and “week 1” written on December 31, 2024 means week 1 of 2025, not 2024. Write the ISO year with the week number, as the week date format does.

Popular calculations

Each address opens the calculator with that date, week or year filled in, and the inputs can be changed once it is open.

Week numbers for dates people look up

ISO week number to dates

Every ISO week of a year

How this was checked

The engine was walked day by day through every date from 1990 to 2060, 25,933 days in all. A running ISO counter that resets on the Monday of week 1 and a running US counter that resets on January 1 matched the engine on every day, as did a second ISO method that counts whole weeks from the Monday of week 1. Every ISO week of every year from 1990 to 2060 was converted to its seven dates and back with no errors. Week 1 was confirmed to contain January 4 and the first Thursday for every year from 1900 to 2100, and the day of the year was checked against a month length sum on 3,000 random dates from 1600 to 9999.

A separate Python script then compared 5,017 dates from 1900 to 2199, including the edge dates around every New Year quoted above, with Python’s isocalendar() and with an independent WEEKNUM written from the Microsoft description, and found no mismatches. The Microsoft WEEKNUM documentation example, March 9, 2012, gives 10 by default and 11 with a Monday start, and ISOWEEKNUM gives 10; the engine returns the same three numbers. The Python documentation example, December 29, 2003 as day 1 of week 1 of 2004, is reproduced too. The list of 53 week years from the day by day walk matches Python and the published table of years with 53 weeks.

Common questions

What week number is it right now?

Open the calculator with no address parameters and it fills in today from your device clock and shows three answers: the ISO 8601 week, the US week that Excel gives by default, and the simple count from January 1. In a year that starts between Sunday and Thursday the ISO and US numbers agree from Monday to Saturday and the US number is one higher every Sunday, because its week has already turned over. In a year that starts on a Friday or Saturday, such as 2027 and 2028, the US number runs one ahead of ISO on every weekday and two ahead on Sundays, so say which one you are quoting.

How many weeks are there in a year?

A calendar year is 52 weeks and one day, or 52 weeks and two days in a leap year, so it can never be a whole number of weeks. The ISO 8601 calendar fixes that by giving each year exactly 52 or 53 whole weeks: 53 when January 1 is a Thursday, or a Wednesday in a leap year. That happens 71 times in every 400 years, roughly once every five or six years. The US system always counts 53 weeks and reaches 54 in a leap year that starts on a Saturday, because its first and last weeks are short.

Why does Excel give a different week number from my planner?

Excel’s WEEKNUM uses the US system unless you say otherwise: weeks start on Sunday and the week holding January 1 is week 1. Most planners printed in Europe use ISO 8601: weeks start on Monday and week 1 is the week holding the first Thursday. Use ISOWEEKNUM, or WEEKNUM with 21 as the second argument, to get the ISO number in Excel. Google Sheets has the same two functions and the same default.

What does the ISO week date format mean?

A week date such as 2026-W53-5 is the ISO 8601 way of writing a date by week: the ISO week based year, the letter W, the two digit week number and the weekday, where Monday is 1 and Sunday is 7. 2026-W53-5 is Friday of week 53 of 2026, which is January 1, 2027. The year in the week date is the ISO year, so it can differ from the calendar year of the day itself, and that is the whole reason the format carries its own year.

Which years have 53 ISO weeks?

From 2015 to 2050 the 53 week years are 2015, 2020, 2026, 2032, 2037, 2043 and 2048. The rule is that January 1 falls on a Thursday, or on a Wednesday in a leap year, which is the same as the year containing 53 Thursdays. The year view in the calculator lists every week of any year from 1600 to 9999 and states which rule applied.

Does the week start on Monday or Sunday?

ISO 8601 says Monday, and that is what the ISO week number and the week date format assume. The United States, Canada and Japan commonly print calendars with Sunday first, which is why Excel’s WEEKNUM defaults to a Sunday start. The calculator shows both, and the table under the result gives the Excel Monday start variant too, so you can match whatever your spreadsheet or diary is doing.

Related Calculators and Reference

• Date Duration Calculator: the days, weeks and months between two dates once you know which weeks they fall in.
• Days From Today Calculator: the date a set number of days or weeks ahead, for a delivery promised in so many weeks.
• Add or Subtract Days Calculator: move a date forward or back by calendar or working days.
• Working Days Calculator: business days between dates with Canadian and US holidays removed.
• Sunrise and Sunset Calculator: daylight hours for any date and place, for scheduling outdoor work by the week.
• ISO 8601 at iso.org: the standard behind the week date format and the Monday start.