Week Number Finder
Week Number Finder
Find Any Week Number
Week numbers are commonly used in business planning, project management, and scheduling. This tool shows both ISO (used internationally) and US week numbers.
ISO weeks always start on Monday, while US weeks start on Sunday.
How ISO Week Numbers Work
The ISO 8601 standard defines a week as running Monday through Sunday, and numbers the weeks of the year from W01 to W52 or W53. Week 1 is the week that contains the year’s first Thursday — equivalently, the week containing January 4. This rule means the first few days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. For example, Friday January 1, 2027 falls in ISO week 53 of 2026, because that week’s Thursday is December 31, 2026.
52 or 53 weeks?
Most ISO years have 52 weeks, but a year has 53 weeks when January 1 falls on a Thursday, or when it is a leap year starting on a Wednesday. This happens roughly every 5–6 years — 2020 and 2026 are both 53-week years. Payroll and reporting systems that assume exactly 52 weeks can drift in these years, which is one reason to check with a proper week-number tool.
ISO vs. US Week Numbering
The US convention starts weeks on Sunday and calls the week containing January 1 week 1, regardless of which weekday it is. As a result, the US week number for a given date is often one higher or lower than its ISO week number, especially early in the year and around weekends. This tool shows both side by side, so you can match whichever convention your calendar, ERP system, or client uses. Microsoft Outlook and most European business calendars default to ISO weeks; many US paper calendars use the Sunday-based count.
What Else the Tool Shows
- Week boundaries: the Monday start date and Sunday end date of the selected ISO week.
- Quarter: which calendar quarter (Q1–Q4) the date falls in — useful for aligning weekly plans with quarterly targets.
- Days into / left in the week: how far through the Monday-based week the date sits.
Who Uses Week Numbers
- Manufacturing and logistics: European factories schedule production, deliveries, and vacations almost entirely by calendar week (“delivery in CW 38”).
- Project management: Sprint and milestone planning by week number avoids ambiguity across time zones and date formats.
- Retail: Year-over-year sales comparisons are made week-to-week to align weekdays.
- Date labels: Products and documents are often stamped with a year-week code like 2026-W27.
Frequently Asked Questions
Why do the ISO and US week numbers differ for the same date?+
ISO weeks start on Monday and week 1 is the week containing the year's first Thursday, while US weeks start on Sunday and week 1 simply contains January 1. Those two rules assign the first days of the year differently, so the two numbers can disagree by one for much of the year.
Can a year really have 53 weeks?+
Yes. An ISO year has 53 weeks whenever January 1 falls on a Thursday, or on a Wednesday in a leap year. Recent and upcoming examples include 2015, 2020, and 2026. All other years have exactly 52 ISO weeks.
What week number do dates around New Year get?+
Under ISO rules, December 29-31 can belong to week 1 of the following year, and January 1-3 can belong to week 52 or 53 of the previous year, depending on where the Thursday falls. The tool applies the standard correctly, so edge dates get the week (and week-year) that international calendars use.
Which convention should I use for business planning?+
If you work with European partners, manufacturing, or logistics, use the ISO week number - it is the international standard and what 'calendar week' (CW) refers to. Use the US number only when matching a Sunday-start calendar. When in doubt, state the week's Monday-Sunday date range, which this tool displays.
Is the week number finder free, and does it work on mobile?+
Yes. It is completely free with no sign-up, and the week calculation runs entirely in your browser - no data is sent to any server. It works on phones, tablets, and desktops alike.
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