Case 1
- Which date?
- 20621
25
Open with these valuesISO Week Number Calculator
25
Result: 25Enter a date and the calculator returns its ISO 8601 week number. Weeks run Monday to Sunday, and week 1 is the one holding the year's first Thursday — which is why 1 January 2021 still belongs to week 53 of 2020. A year has either 52 or 53 such weeks.
Worked examples
Case 2
- Which date?
- 18628
53
Open with these valuesCase 3
- Which date?
- 20453
1
Open with these valuesHow it's calculated
week = floor(day of year of that week's Thursday ÷ 7) + 1
- StepTake the Monday-to-Sunday week that contains the date, and find its Thursday.
- StepThat Thursday's calendar year is the week year — which is why early January can still belong to the old year.
- ResultCount how many whole weeks the Thursday is into its year, then add 1.
What this number means
The page returns the ISO 8601 week number of a date. The convention hangs on the Thursday rather than on New Year: every week runs Monday to Sunday and belongs to the year its Thursday falls in. Work through the preset, 17 June 2026. That day is a Wednesday, so its week's Thursday is 18 June 2026. The first day of 2026 is itself a Thursday and therefore sits in week 1; between it and 18 June lie 168 days, which is 24 whole weeks. 24 plus 1 gives week 25. The same three lines need no branch for years of 52 or 53 weeks — that difference drops out of the Thursday rule by itself. What the page does not do: it reports neither the day of the year nor the days left in it, and it will not run backwards from a week number to a date. The limit that matters most is the missing year. What comes out is the bare number, not the week year, so 1 January 2021 reads as 53 when what is meant is week 53 of 2020. In full an ISO week is written 2020-W53, and only with that year is the number unambiguous around New Year.
Week 1 hangs on the Thursday, not on 1 January
Week 1 is the week holding the year's first Thursday, which is the same as the week containing 4 January. Every week belongs to the year its Thursday falls in.
Early January can still belong to the old year
If 1 January is a Friday, Saturday or Sunday, that week's Thursday still lies in December. This is how 1 January 2021 ends up in week 53 of 2020.
A year has either 52 or 53 weeks
A year has 53 ISO weeks when it starts on a Thursday, and also when it is a leap year starting on a Wednesday. That happens roughly every five to six years — 2015, 2020 and 2026 are examples.
US week numbers are counted differently
Many US systems start the week on Sunday and call the week containing 1 January week 1. Around New Year that gives a different number for the same date; this calculator always follows the ISO rule.
Commonly misread
The week containing 1 January is always week 1.
That is the US convention. Under ISO 8601 the week belongs to the old year unless it holds the year's first Thursday, which is why 1 January 2006 falls in week 52.
Weeks start on Sunday.
ISO weeks run Monday to Sunday. Sunday-start weeks are the US convention and give a different number around New Year.
31 December always belongs to the last week of that same year.
It belongs to the week its Thursday is in. 31 December 2025 is a Wednesday and already counts as week 1 of 2026.
Every year has 52 weeks, so a week 53 must be a bug.
A year starting on a Thursday, or a leap year starting on a Wednesday, has 53 ISO weeks. 2020 was one, and 31 December 2020 falls in week 53.
Reference table
| Date | Why it is interesting | ISO week |
|---|---|---|
| 2005-01-01 | Saturday — the week's Thursday is in 2004 | 53 |
| 2006-01-01 | Sunday — the week's Thursday is in 2005 | 52 |
| 2020-12-31 | Thursday — 2020 is a 53-week year | 53 |
| 2021-01-01 | Friday — still counted in 2020 | 53 |
| 2022-01-03 | Monday — first day of week 1 | 1 |
| 2025-12-31 | Wednesday — already week 1 of 2026 | 1 |
| 2026-06-17 | an ordinary Wednesday in June | 25 |
Questions
What is an ISO week number?
An ISO week number counts the weeks of a year as defined by ISO 8601. Weeks run Monday to Sunday, and week 1 is the week containing the year's first Thursday — equivalently, the week that contains 4 January. A year has either 52 or 53 ISO weeks.
Why can 1 January fall in week 52 or 53 of the previous year?
Because week 1 is fixed by the Thursday rule, not by the calendar year. If 1 January is a Friday, Saturday or Sunday, its week's Thursday still lies in December, so the whole week belongs to the previous year. That is how 1 January 2021 ends up in week 53 of 2020.
Does every year have 52 weeks?
Most do, but a year has 53 ISO weeks when it starts on a Thursday, and also when it is a leap year starting on a Wednesday. That happens roughly every five to six years — 2015, 2020 and 2026 are examples.
How does ISO week numbering differ from the US week number?
ISO 8601 starts weeks on Monday and puts week 1 where the first Thursday is. Many US systems start weeks on Sunday and call the week containing 1 January week 1, which gives a different number for the same date around New Year. This calculator always uses the ISO rule.
Sources and last check
Information, not professional advice.