ISO Week Number Calculator

Frequently Asked Questions

Can January 1 be in the previous year's ISO week?

Yes. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last ISO week of the previous year, because that week's Thursday is still in the previous year.

How do I know if a year has 52 or 53 ISO weeks?

A year has 53 ISO weeks if it starts on Thursday, or if it is a leap year starting on Wednesday. All other years have 52 ISO weeks.

What is the ISO 8601 notation for a week date?

The format is YYYY-Www-D, for example 2026-W21-1 meaning ISO year 2026, week 21, Monday. The two-digit week number is always zero-padded.

Why does this tool use UTC for calculations?

Using UTC avoids daylight-saving transitions and local-offset ambiguities that could shift a date near midnight by one day and therefore into a different ISO week.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

ISO 8601 defines weeks so that every week belongs to exactly one year. Week 1 is the week containing the first Thursday of January - equivalently, the week containing January 4th. Weeks start on Monday (weekday 1) and end Sunday (weekday 7). To find a date's ISO week: shift to the Thursday of its week; that Thursday's calendar year is the ISO year, and the week number = ⌈((Thursday − Jan 1) ÷ 7⌉ + 1. The reverse uses the fact that the Monday of ISO week 1 is January 4 minus its weekday offset, then adds (week − 1) weeks and (weekday − 1) days.

This tool supports both directions: Date → ISO Week and ISO Week → Date.

Worked Example

Take 2026-05-18 (a Monday). Its week's Thursday is 2026-05-21, which falls in calendar year 2026, so the ISO year is 2026. The number of days from 2026-01-01 to that Thursday, divided by 7 and rounded up, gives week 21. Reversing - ISO 2026-W21-1 - lands right back on 2026-05-18, confirming the round trip. Day-of-year for May 18 in a non-leap year is 138.

Common Mistakes

Confusing ISO year with calendar year. January 1, 2027 may belong to ISO week 53 of 2026; December 31 may belong to week 1 of the following year.

Assuming weeks start Sunday. US convention often uses Sunday; ISO is strictly Monday-first.

Off-by-one weekday numbering. ISO uses Monday = 1 through Sunday = 7, not 0-indexed.

Forgetting week 53. Long years (those starting Thursday, or Wednesday in a leap year) have 53 ISO weeks, not 52.

Real-World Use

ISO week numbers are the backbone of business and manufacturing calendars, especially in Europe. Production schedules, payroll periods, fiscal reporting, and project plans are routinely labeled "W21" rather than by date. Databases such as PostgreSQL (using isoyear and week functions) and standards-based logging rely on ISO weeks so that week boundaries never split across years ambiguously. If you receive a report labeled "W38 2026," this tool tells you the exact start and end calendar dates.

Limitations

This tool works in UTC to avoid time-zone and daylight-saving drift; a local date near midnight could differ by one calendar day from its local-time week. It assumes the proleptic Gregorian calendar and does not account for historical calendar reforms. An out-of-range week (such as week 53 of a 52-week year) will roll forward into the next ISO year - the verification fields show the true normalized values. For elapsed time between two dates, use a date-difference calculator.

ISO Week Number Calculator

Frequently Asked Questions

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