Week Number Calculator

You're scheduling a project sprint, the spec says "deliver by week 14," and you're staring at the calendar trying to figure out which week of the year week 14 actually is. ISO week numbering is the standard most European countries and many businesses use, and it has its own quirks — week 1 is the week containing the first Thursday of January, which means a year can have 52 or 53 ISO weeks.

Punch in a date, get the ISO week number. If you're working with American calendars that start the week on Sunday rather than Monday, the result still applies because ISO weeks are anchored to Monday and Thursday by definition. Useful for Agile sprint planning, manufacturing schedules, and any cross-team coordination where "week 14" is less ambiguous than "the week of April 1."

The ISO 8601 rules: weeks start on Monday and end on Sunday. Week 1 of any year is the week containing the year's first Thursday — equivalently, the week containing January 4. This sounds arbitrary but it's the only definition that produces a consistent system where every week belongs to exactly one year. Years can have either 52 or 53 weeks. A year has 53 ISO weeks if it starts on a Thursday (or if it's a leap year and starts on a Wednesday). The pattern recurs roughly every 5-7 years. Both 2020 and 2026 are 53-week years; 2024 is a 52-week year despite being a leap year because it starts on a Monday.

A worked example: April 1, 2026 is a Wednesday. Find the Monday of that week: March 30. Find the Thursday of that week: April 2. April 2 is in the year 2026, so this is a 2026 ISO week. To find the week number, count weeks from January's week 1. January 1, 2026 is a Thursday, which means week 1 of 2026 is the week containing Thursday January 1 — which is the week of December 29, 2025 (Monday) through January 4, 2026 (Sunday). Counting forward: April 1 falls in week 14. The calculator does all this with a formula but the manual check matches.

Where ISO weeks confuse people: dates near January 1. January 1, 2024 is a Monday and falls in week 1 of 2024. January 1, 2023 is a Sunday and falls in week 52 of 2022 (because the Thursday of that week was December 29, 2022). January 1, 2027 is a Friday and falls in week 53 of 2026 (because 2026 is a 53-week year and that week's Thursday, December 31, is in 2026). The calculator uses the ISO year (the year that owns the week) which can differ from the calendar year for the first few or last few days of January. For US business contexts that use "week 1 starts on the first Sunday of January," the result will differ — that system is sometimes called "US weeks" or "broadcast weeks" and isn't ISO-compatible. Always verify which system your spec uses before scheduling around a week number.