Week Number Calculator

What is a week number?

A week number tells you where a date sits in the running count of weeks for a year — Week 1, Week 23, Week 52 and so on. The Week Number Calculator finds that number for any date using the ISO 8601 standard, the same week numbering printed on calendars and used by ERP systems, factories and logistics teams across Europe and much of the world. It also shows the Monday-to-Sunday dates that make up the week, how many weeks the year has in total, and how the result compares with the looser US convention.

ISO 8601 week rules

ISO 8601 fixes two things that simpler counters get wrong. First, a week always starts on Monday and ends on Sunday. Second, Week 1 is the week that contains the year's first Thursday — equivalently, the week that contains 4 January. Everything else follows from those two rules, including the slightly surprising cross-year behaviour:

• The last days of December can belong to Week 1 of the next year. For example 31 December 2025 falls in ISO Week 1 of 2026.
• The first days of January can belong to Week 52 or 53 of the previous year. For example 1 January 2027 falls in ISO Week 53 of 2026.

Because a week is never split across two years, ISO uses a separate week-numbering year, which is why this tool always prints the ISO year next to the week number. There is no guessing about which year a January or December week really belongs to.

ISO vs US week numbering

The United States and parts of Canada often use a different convention. In that system a week starts on Sunday, and Week 1 is simply the week that contains 1 January, so 1 January is always in Week 1 and partial weeks at the start and end of the year are counted. The ISO system starts weeks on Monday and defines Week 1 by the first Thursday instead. The two can disagree by one number on the same date, which is exactly why the calculator shows both the ISO week and the US week side by side — so you never quote the wrong one to a colleague who uses the other standard.

How to use it

1. Open the page and read the banner at the top — it shows today's ISO week number, the ISO year and today's weekday, taken from your device clock.
2. To look up another date, stay on the By date tab and pick a day; the ISO week, US week, weeks-in-year total and the Monday–Sunday range update instantly.
3. To go the other way, switch to By year + week, type a year and a week number, and the tool returns the exact start and end dates of that week.
4. Hit Copy result to drop the answer into an email, ticket or spreadsheet, then close the tab.

Worked examples

• 1 June 2026 is a Monday and falls in ISO Week 23 of 2026; that week runs Monday 1 June to Sunday 7 June.
• 31 December 2025 is a Wednesday but belongs to ISO Week 1 of 2026 — a textbook cross-year edge.
• Year 2026, Week 53 reversed gives Monday 28 December 2026 to Sunday 3 January 2027, because 2026 is one of the years that has 53 ISO weeks.

Why a year can have 53 weeks

Most years have 52 ISO weeks, but some have 53. That happens when 1 January falls on a Thursday, or when the year is a leap year and 1 January falls on a Wednesday. In those years an extra Thursday is pulled into the count, so Week 53 exists. The calculator reports how many weeks the selected year holds, which matters for anyone building a year-long schedule or planning "Week 52 vs Week 53" production runs.

Common use cases

• Business and email — agreeing on "Week 23" instead of a full date when scheduling meetings or releases.
• Logistics and supply chain — delivery and shipping windows are routinely quoted as a calendar week (KW / CW number).
• Manufacturing and production planning — factory schedules and batch codes are organised by week.
• Project management — sprints and milestones tracked as week numbers across a year.
• Payroll and reporting — weekly pay periods and weekly KPI reports keyed to the ISO week.

Why use this one

Most "what week is it" pages show only the current week and stop there. This one does the whole job on a single screen: today's week up top, a lookup for any date with the full Monday–Sunday range, a reverse lookup that turns a year and week back into dates, the count of weeks in the year, and a direct ISO-versus-US comparison so you always quote the right standard. It handles the awkward cross-year edges correctly, runs entirely in your browser using your device clock, keeps your date on your device, and needs no account.

It belongs to a small cluster of everyday date tools. To measure the span between two dates, use the Date Difference Calculator; to count down to a future day, use Days Until Date; and to turn a birth date into an exact age, use the Age Calculator.

Frequently asked questions