Week Number Calculator (ISO Week)
Pick a date to see its ISO week number and day of the year.
How are ISO week numbers calculated?
ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of the year. This means 1 January can fall in week 52 or 53 of the previous year.
To add time to a date, use the Date Add / Subtract Calculator.
Frequently asked questions
What week number is it today?
Leave the date as today to see the current ISO week.
How many weeks are in a year?
52 or 53 ISO weeks, depending on the year.
Is the UK tax year weeks-based?
HMRC uses its own tax weeks starting 6 April, not ISO weeks.
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