How is the ISO week number defined?

Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday. This means the first few days of January can fall in the last week of the previous year, and late December can fall in week 1 of the next year.

Why does the ISO year sometimes differ from the calendar year?

Because a week belongs to the year that contains its Thursday. For example, 1 January 2027 is a Friday, so it belongs to ISO week 53 of 2026, and its ISO year is 2026 even though the calendar year is 2027.

How many weeks are in an ISO year?

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

Does the ISO week start on Monday or Sunday?

ISO 8601 weeks always start on Monday and end on Sunday. This differs from the US convention where calendar weeks often begin on Sunday, which is why ISO and US week numbers can disagree.

What week number is Christmas?

It depends on the year, but 25 December usually falls in ISO week 51 or 52. Use the date picker to get the exact ISO week and the Monday-to-Sunday range for any specific year.

Is my date sent anywhere?

No. The calculation runs entirely in your browser and nothing is uploaded.

What is the ISO Week Number Calculator?

Find the ISO 8601 week number (year-Wnn) for any date, plus the weekday and the Monday-to-Sunday range of that week. Runs entirely in your browser — nothing is uploaded. It runs free in your browser on Gera Tools, with nothing uploaded.

ISO Week Number Calculator

Name: ISO Week Number Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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ISO 8601 week number for any date

Enter a date and get its ISO 8601 week number in the standard year-Wnn format, along with the weekday and the Monday-to-Sunday span of that week. ISO week numbers are widely used in manufacturing, payroll, project planning and European business, where “week 14” is an unambiguous way to refer to a period.

How it works

ISO weeks always start on Monday, and week 1 is the week containing the year’s first Thursday (equivalently, the week containing 4 January). To find a date’s week number, the calculator takes the Thursday of that date’s week and counts how many weeks it is from the first Thursday of that ISO year. Because a week belongs to whichever year holds its Thursday, the ISO year can differ from the calendar year at the boundaries of January and December.

Worked examples

29 May 2026 falls in 2026-W22 (Thursday of that week is 28 May, which is solidly in 2026). The week runs Monday 25 May to Sunday 31 May.
1 January 2027 is a Friday. Its week’s Thursday falls on 31 December 2026, which belongs to 2026, so the ISO year is 2026 and the week is 2026-W53 — even though the calendar year reads 2027.
31 December 2024 is a Tuesday. Its week’s Thursday is 2 January 2025, which belongs to 2025, so this date is 2025-W01. The calendar year is 2024 but the ISO week year is 2025.
25 December 2026 falls in 2026-W52. Christmas falls in week 51, 52, or 53 depending on the year — use the calculator for any specific year.

When ISO week numbers are used

Manufacturing and supply chains. European production schedules almost universally use ISO weeks. A purchase order or delivery commitment stated as “W34 delivery” is unambiguous to any supplier who follows the standard. Mixing ISO and US week numbers causes planning errors.

Payroll and time reporting. Weekly payroll systems often store time entries by ISO week. This matters most in December and January when the ISO year can differ from the calendar year — week 53 of 2026 may contain days from January 2027.

Project management. Sprint planning and Gantt charts in European and international teams frequently express deadlines as ISO week references. The notation 2026-W22 identifies both the week and the year unambiguously, unlike simply saying “week 22.”

Database and reporting. SQL functions like WEEK() in MySQL and DATE_PART('week', ...) in PostgreSQL have ISO mode options. The ISO week is the safest cross-database representation because it is unambiguous regardless of the locale setting.

ISO weeks vs. US calendar weeks

The key difference is the start day:

Standard	Week starts	Week 1 definition
ISO 8601	Monday	Week containing the year’s first Thursday (= week containing 4 January)
US convention	Sunday	Week containing 1 January

Because of this, the ISO week number for a given date can differ from the US week number by 1, and the ISO week year can differ from the calendar year by 1 in early January or late December. Always confirm which convention applies when working with international partners.

Everything is computed locally in your browser; nothing is uploaded.