Week Number Today: Week 40 of 2026 — ISO Week Calculator

All 53 ISO weeks of 2026

Each row shows the Monday the week starts on and the Sunday it ends on. The current week is highlighted.

Other years: 2025 · 2027 · 2028 · all week-number tables.

Week numbers of key dates in 2026

ISO week vs. US week number

Two conventions are in everyday use, and they can differ by one:

• ISO 8601 (this tool's main result, used across Europe and in most software): weeks start on Monday, and week 1 is the week that contains the first Thursday of the year — equivalently, the week containing January 4. A few days at the very start or end of a year can belong to the previous or the next ISO year, and some years (2026 among them) have 53 weeks.
• US / "simple" week numbers (used by many US planners, Excel's default WEEKNUM, and the broadcast calendar): weeks start on Sunday, and week 1 is whichever week contains January 1. Every year has a week 1 starting in the previous December unless January 1 is a Sunday, and the last week is week 53 or 54.

Today's numbers side by side: ISO week 40, US week 40. If a colleague's "week 37" and yours don't match, this is almost always why.

How week numbers are used

• Manufacturing, logistics and retail in Europe plan by calendar week ("delivery in CW 32"); the German abbreviation KW is the same thing.
• Sprint planning, OKR cadences and quarterly reporting often reference ISO weeks because they don't move with holidays.
• School and university timetables in Germany, the Nordics and the UK label weeks by ISO number.
• Spreadsheets: Excel's ISOWEEKNUM() and Google Sheets' ISOWEEKNUM() match this tool; plain WEEKNUM() gives the US-style number unless you pass 21 as the second argument.

Related

• What is an ISO week number? — the rules in detail, with examples
• All ISO week numbers for 2027
• Days between two dates · Add or subtract days