Week Number of the Year Calculator
Find the week number for any date under the ISO-8601 or US convention, with the weeks left in the year.
How it works
ISO: week 1 is the week containing the first Thursday (Mon-start) · US: week = ⌊(day-of-year − 1 + Jan-1 weekday) ÷ 7⌋ + 1
The two systems disagree because they define week 1 differently. ISO-8601, used across Europe and in business and manufacturing, starts weeks on Monday and puts week 1 as the one containing the year’s first Thursday — so early January can belong to the previous year’s week 52 or 53. The common US convention starts weeks on Sunday and simply calls the week containing 1 January week 1, giving up to 54 partial weeks. Picking the right system matters when a payroll or planning tool expects one specific numbering.
Worked example
15 Jun 2026 is a Monday, and both conventions place it in week 25. The systems diverge near the year’s edges: 4 Jan 2026 (a Sunday) is ISO week 1 — because ISO week 1 holds the year’s first Thursday — but US week 2, since the US scheme opens week 2 on the first Sunday after 1 January.
Sources: ISO week date — Wikipedia
Frequently asked questions
Which week numbering should I use?
Use ISO-8601 if you are in Europe or your software follows the international standard; use the US convention if your calendar or payroll system counts the week of 1 January as week 1. When in doubt, check what your existing reports say.
Can a year have 53 weeks?
Yes. Under ISO-8601 a year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year. Most years have 52.
Why can early January show week 52 or 53?
Under ISO rules a week belongs to the year that holds most of its days. If 1–3 January fall before the year’s first Thursday, those days are counted in the last week of the previous year.
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This calculator is for educational purposes. Double-check important results before acting on them.