Week Numbers
Find the ISO week number for any date, or browse all weeks in a year.
Current Week
Look Up
All Weeks in Year
| Week # | Start Date | End Date |
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The week numbers tool shows the ISO 8601 week number for any date and lists every week of any year with its Monday start date and Sunday end date. ISO weeks are the standard used by European business calendars, software projects, manufacturing schedules, and any system that needs an unambiguous way to refer to a week of the year. The tool also shows the current week, the day of the year, and how many days remain in the year, all updating live.
How ISO Week Numbers Work
ISO 8601 defines a precise way to number the weeks of a year so that "week 17 of 2026" means the same thing in every system. The rules are simple but a bit unusual compared to informal week counting:
- Weeks start on Monday. Not Sunday. This matches European business convention but differs from the US convention where calendars often show Sunday as the first day.
- Week 1 contains the first Thursday of the year. Equivalently, week 1 is the week that contains January 4. This rule means a year can have 52 or 53 ISO weeks, and the first few days of January can belong to the previous year's week 52 or 53.
- The ISO year may differ from the calendar year. January 1, 2023 is part of week 52 of ISO year 2022, because that Sunday was the tail of the previous week. December 31, 2024 is part of week 1 of ISO year 2025 for the same reason in reverse.
This explains why some tools display dates like "2025-W01-1" (ISO year 2025, week 1, day 1 = Monday) - that compound notation removes all ambiguity. The week numbers tool computes the ISO week for any input date and shows both the ISO year and the calendar year when they differ.
What the Tool Shows
The page is divided into three live sections that work independently:
- Current Week: the ISO week number for right now, plus the day of the year (1-366) and the days remaining in the current year. Useful as a quick reference when filling out forms, naming files (project-w17-status.pdf), or aligning a deadline with a week-based sprint.
- Look Up: enter any date and the tool returns its ISO week number, the Monday that starts that week, and the Sunday that ends it. Works for past, present, and future dates.
- Year Table: pick a year and the tool generates a full table of every week with the start date, end date, and ISO week number. Useful for planning recurring schedules, sprints, or pay periods for an entire year at once.
For computing the gap between two specific dates, use the date difference calculator. To project a date forward by a number of weeks (for example "8 weeks from today"), use the date add/subtract calculator. To set up a recurring schedule by week, the cron expression builder handles weekly cron expressions.
Common Uses for ISO Week Numbers
ISO weeks come up in business, engineering, and personal scheduling more often than people realize:
- Sprint planning: agile teams often label sprints by their starting week (Sprint 17 = the sprint that started in week 17). Knowing which week a given date falls in is the first step in planning sprint boundaries.
- Manufacturing and supply chain: production schedules and shipping windows in Europe are routinely referred to by ISO week (delivery in week 23) because the format is unambiguous across countries and translates cleanly between calendars.
- Sales forecasting: weekly revenue reports are easier to compare week-over-week when each week is referred to by its ISO number rather than its date range.
- Project naming and filing: reports, backups, and snapshots that follow a weekly cadence often use the ISO week in their filenames (2026-W17-snapshot.zip), which sorts naturally and is easy to parse.
- Payroll periods: some payroll systems pay every two ISO weeks (biweekly), which means the calendar may include 26 or 27 pay periods in a single year depending on which week pay starts.
Why ISO Weeks Sometimes Cross Years
Because week 1 is defined as the week containing January 4, the first three days of some years can belong to the previous year's week 52 or 53. For example, January 1-3, 2027 will be week 53 of ISO year 2026, even though the calendar says 2027. This sounds awkward but is the price of having every week be exactly seven days that always start on Monday and end on Sunday.
If you need a date that uses the calendar year instead of the ISO year, the tool shows both side by side so you can pick the convention that matches your system. Most databases and APIs that expose week numbers (PostgreSQL's date_part('week', ...), JavaScript's getWeek libraries, Python's isocalendar()) follow the ISO convention. Some legacy systems use the US convention (week 1 starts January 1, weeks start on Sunday), so always confirm which convention your source data uses.
Frequently Asked Questions
An ISO week number is a value from 1 to 52 (occasionally 53) that identifies a week of the year according to the ISO 8601 standard. Weeks always start on Monday and end on Sunday. Week 1 is defined as the week containing the first Thursday of the year, which is equivalent to saying it is the week containing January 4. This makes week numbering unambiguous across countries and software systems.
The Current Week section at the top of the page shows today's ISO week number, updated live. The value is computed from your browser's local time, so it matches whatever week your calendar would show. The same section also shows the day of year (1-366) and the days remaining in the current calendar year.
52 weeks of 7 days equals 364 days, while a calendar year has 365 (or 366 in a leap year). That extra day or two accumulates over the years and eventually requires a 53-week ISO year to keep the calendar aligned. A 53-week year happens when the year starts on a Thursday or, for a leap year, when it starts on a Wednesday. Recent and upcoming 53-week years include 2015, 2020, 2026, and 2032.
Because ISO week 1 is defined as the week containing the first Thursday of the year. If January 1 falls on a Friday, Saturday, or Sunday, then the first Thursday is in the previous year (or rather, the week containing the first Thursday started before January 1), so those first days of January belong to the previous ISO year's week 52 or 53. This is by design: every ISO week is a full 7 days, no partial weeks at year boundaries.
Monday. The ISO 8601 standard explicitly defines Monday as the first day of the week and Sunday as the seventh. This matches European business convention but differs from the common US convention where calendars often display Sunday as the first day. If a tool says weeks start on Sunday, it is using the US convention, not ISO.
ISO weeks start on Monday and week 1 contains the first Thursday of the year. US weeks start on Sunday and week 1 contains January 1. The two conventions can differ by one for any given date, especially near the year boundary. Always check which convention a system uses before comparing week numbers; ISO is the international standard, US is mostly a North American convention.
Python's datetime.isocalendar() returns a (year, week, weekday) tuple. PostgreSQL's date_part('week', date) returns the ISO week. PHP's date('W') returns the ISO week as a two-digit string. JavaScript does not have a built-in helper, but libraries like date-fns expose getISOWeek() or you can compute it from getDay() and getTime(). The week numbers tool computes the same value as these helpers.
The day of year (also called ordinal date or Julian day in some contexts) is a number from 1 to 365 or 366 that represents which day of the year a date is. January 1 is day 1; December 31 is day 365 in a normal year or 366 in a leap year. Useful for tracking percentage of year complete, computing milestones (50% of the year = day 183), or simple ordinal references.
Yes. The Year Table section lets you pick any year and lists every week with its number, Monday start date, and Sunday end date. Useful for sprint planning, scheduling biweekly pay periods, or generating filename templates for an entire year's worth of weekly reports. The table is generated entirely in your browser so it loads instantly even for far-future years.
Because ISO weeks always span Monday to Sunday and week 1 is anchored to the first Thursday of the year. A date in early January can belong to the previous ISO year if its week started before January 1; a date in late December can belong to the next ISO year if its week extends into the next year. The tool shows the ISO year separately when it differs from the calendar year so the distinction is always clear.