ISO Week Number Calculator

Frequently Asked Questions

What is an ISO week number?

ISO-8601 numbers the weeks of a year 1–52/53, with Monday as the first day. Week 1 is the week containing the year's first Thursday (equivalently, the week with January 4th).

Why can a date's ISO year differ from its calendar year?

Early January dates can belong to the last ISO week of the previous year, and late December dates to week 1 of the next - so Dec 31 can be "2026-W01". The tool reports the ISO year separately.

When does a year have 53 weeks?

When the year starts on a Thursday, or on a Wednesday in a leap year. Most years have 52 ISO weeks.

Where are ISO weeks used?

Manufacturing, logistics, payroll, and project planning - especially in Europe - because every week has exactly 7 days and a stable Monday start.

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Estimates for informational purposes only.

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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 precisely 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 is ⌈((Thursday − Jan 1) ÷ 7 days) + 1⌉. The reverse uses the fact that the Monday of week 1 is Jan 4th minus its weekday offset, then adds (week − 1) weeks and (weekday − 1) days.

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. Day-of-year for May 18 in a non-leap year is 138. Reversing - ISO 2026-W21-1 - lands right back on 2026-05-18, confirming the round trip.

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 (PostgreSQL's isoyear/week, ISO formats in spreadsheets) and standards-based logging rely on this scheme so that week boundaries never split across years ambiguously.

Common Mistakes

Confusing ISO year with calendar year. Jan 1 2027 may belong to ISO week 53 of 2026; Dec 31 may belong to week 1 of the next year.

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

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

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

Limitations & Related

This tool works in UTC to avoid timezone 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. Inputs are validated for week 1–53 and weekday 1–7 but an invalid week (e.g. 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 instead.

ISO Week Number Calculator

Frequently Asked Questions

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