ISO week number calculator
ISO 8601 week numbering starts the week on Monday and puts week 1 as the one containing the first Thursday.
1 is Monday, 7 is Sunday.
Week number
29
- Day of the week 1 is Monday.
- 6
- Weeks in this year
- 52
- This year has 53 ISO weeks
- 0
- Weeks left in the year
- 23
- Days left in the year
- 165
Show the working
- Day of the year = 200
- Weekday of 1 January = 3
- Leap year = 0
- offset = mod(3 - 1, 7) = 2
- adjusted_day = 200 + 2 = 202
- Week number = ceil(202 / 7) = 29
- Day of the week = mod(200 + 2 - 1, 7) + 1 = 6
- This year has 53 ISO weeks = 3 == 4 || (0 > 0 && 3 == 3) ? 1 : 0 = 0
- Weeks in this year = 0 > 0 ? 53 : 52 = 52
- Days left in the year = (0 > 0 ? 366 : 365) - 200 = 165
- Weeks left in the year = 52 - 29 = 23
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- ISO week 1 is the week containing the first Thursday of January
- a year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year
The ISO week year can differ from the calendar year at the very start and end of a year, which catches out a lot of reporting.