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Select a date and click Calculate// find the iso week number for any date instantly
Calculate the ISO week number for any date instantly. Find what week of the year a date falls in, with first/last day of the week and year info.
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Select a date and click CalculateUse the date picker or click a quick-select button (Today, Week 1, etc.)
Click the Calculate button or press Enter to get the ISO week number instantly.
See the week number, week range, day of year, quarter, and full year calendar.
The Week Number Calculator determines the ISO 8601 week number for any date. Weeks run Monday–Sunday and Week 1 is the week containing the first Thursday of the year. Results include the week's start/end dates, day of week, day of year, quarter, and a full-year calendar.
An ISO 8601 week number is a standard way to identify weeks in a year. Weeks run from Monday to Sunday. Week 1 is defined as the week containing the year's first Thursday, which means it always has at least 4 days in the new year.
Most years have 52 ISO weeks, but some years have 53. A year has 53 weeks when January 1st falls on a Thursday (or on a Wednesday in a leap year), because week 1 then starts in the previous calendar year.
Yes. If December 29, 30, or 31 falls in the week containing January's first Thursday, those days are considered part of week 1 of the following ISO year. This is why results show an ISO year that may differ from the calendar year.
ISO 8601 weeks always start on Monday and end on Sunday. This differs from some locales (like the US) where weeks start on Sunday. This tool strictly follows the ISO 8601 standard.
Simple week numbers just divide the year into 7-day blocks from January 1st, which can vary by locale. ISO week numbers follow a strict international standard (ISO 8601), ensuring consistency across countries and software systems.
Yes, the Week Number Calculator is completely free, runs entirely in your browser, and requires no sign-up or account. Your input data is never sent to a server.
A week number calculator is an online tool that determines which numbered week of the year a specific date falls into, following the ISO 8601 international standard. Instead of thinking in months and days, many businesses, governments, and software systems refer to weeks by number — such as "deliver the report in Week 12" or "the sprint runs from Week 20 to Week 21." This tool makes it instantly clear what week any date belongs to, along with supporting details like the week's start and end dates, the day of the week, the day of the year, and the current quarter.
The ISO 8601 standard is an international date and time format published by the International Organization for Standardization. It defines how weeks are counted in a consistent, unambiguous way used worldwide. Under this standard:
This definition ensures that every week has exactly seven days and that each week belongs unambiguously to one year — eliminating the ambiguity that arises when a week spans two calendar years.
When you use this calculator, you'll receive several pieces of information:
Week numbers are used extensively across industries. In software development, agile sprints are often scheduled by week number. In finance and accounting, weekly reporting cycles and payroll periods frequently reference ISO week numbers. In logistics and supply chain management, shipment schedules and delivery windows are communicated using week numbers. In project management tools like Jira, Asana, and Monday.com, timelines and burndown charts often display week numbers alongside calendar dates.
For international teams, ISO week numbers provide a common language that transcends locale-specific calendar conventions. A team in Germany, the US, and Japan can all agree on "Week 35" without any ambiguity about which days are included.
Most years have exactly 52 ISO weeks, but some years — called "long years" — have 53 weeks. A year has 53 weeks when January 1st falls on a Thursday, or when it falls on a Wednesday in a leap year. This happens approximately 71 times per 400-year cycle. Years with 53 weeks include 2004, 2009, 2015, 2020, and 2026. If you're building scheduling software or annual planning tools, correctly identifying whether a year has 52 or 53 weeks is critical for avoiding off-by-one errors in week-based iterations.
The most counterintuitive aspect of ISO weeks involves dates near the year boundary. December 29, 30, and 31 may belong to Week 1 of the next ISO year if they fall within the week containing January's first Thursday. Similarly, January 1, 2, and 3 may belong to Week 52 or 53 of the previous ISO year. For example, January 1, 2016 was a Friday and belonged to Week 53 of ISO year 2015, not Week 1 of 2016. This calculator correctly handles all such edge cases and always shows you the correct ISO year alongside the week number.
Most programming languages and date libraries support ISO week number extraction. In PHP, you can use date('W', $timestamp) for the week number and date('o', $timestamp) for the ISO year. In JavaScript, there is no built-in method, so libraries like date-fns (getISOWeek()) or Luxon (DateTime.fromISO().weekNumber) are commonly used. In Python, datetime.date.isocalendar() returns a tuple of (ISO year, ISO week, ISO weekday). When storing week numbers in databases, always store both the ISO week number and ISO year together — never the week number alone — to avoid year-boundary ambiguity.
Some of the most common queries people use this tool for include: finding what week number today is, determining which week a project deadline falls in, checking whether a specific quarter starts in a particular week, verifying sprint start and end dates, and auditing whether payroll dates align with expected week boundaries. The year calendar view in this tool makes it easy to see all weeks at a glance, with the selected week highlighted for quick visual reference.