Week Number Calculator (ISO 8601)
Find the week number for any date under the ISO 8601 standard — the numbering used across European business, manufacturing schedules and project plans. Also shows the week's Monday–Sunday range.
How to use this calculator
- Pick a date, or leave it empty for today.
- Press the button to see the ISO week number and the week's Monday–Sunday span.
- Note the ISO year: dates around New Year can belong to the previous or next year's week numbering.
Formula used
Equivalently, Week 1 contains January 4th. This means Dec 29–31 can fall in Week 1 of the next year, and Jan 1–3 can fall in Week 52/53 of the previous one. Years have 52 or 53 ISO weeks.
Example calculation
1 January 2027 falls on a Friday. The first Thursday of 2027 is Jan 7, so Jan 1 belongs to the previous ISO year: it's in Week 53 of 2026. Meanwhile 4 January 2027 (Monday) opens Week 1 of 2027.
How ISO week numbering works
ISO 8601 numbers weeks Monday-to-Sunday and defines Week 1 as the week containing the year's first Thursday — a rule that guarantees Week 1 always holds a majority of its days in the new year. The side effect surprises everyone eventually: a few days each January can carry last year's week numbers, and late December can already be in Week 1.
Week numbers are the default planning currency in much of European industry — deliveries land "in KW 34", projects report by week — so being able to translate a date to its week (and back) avoids real scheduling mistakes.
Why use this calculator?
- Translate "week 34 delivery" into actual dates — and vice versa.
- Get the correct ISO answer near New Year, where guessing fails.
- See the exact Monday–Sunday range for planning and reporting.
Frequently asked questions
Why does my date show a week from a different year?
ISO weeks belong to the year containing their Thursday. If Jan 1 falls Friday–Sunday, those days finish the old year's last week; if Dec 29–31 fall Monday–Wednesday, they start the new year's Week 1.
How many weeks are in a year?
52 or 53 ISO weeks. A year has 53 weeks when it starts on a Thursday, or starts on a Wednesday in a leap year — roughly once every 5–6 years.
Does the US use the same week numbers?
Not always — American calendars often number weeks starting from the week containing Jan 1, with Sunday as the first day, which can differ from ISO by one. For international work, state the standard ("ISO week 34") to avoid ambiguity.