Week Number Calculator

Find the ISO week number of a date.

Date

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ISO week number

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Day of the year—

How it works

A week number calculator tells you which numbered week of the year a date falls in under the ISO-8601 standard, and also how many days into the year that date is. ISO weeks run Monday to Sunday, and week 1 is defined as the week that contains the year's first Thursday, which is the same as the week holding 4 January. That definition is what businesses, ERP systems and calendars mean when they refer to "week 32" or "CW14." This matters because week numbers are a common planning currency, especially in Europe and in manufacturing, logistics and retail. Production schedules, shipping windows, sprint plans and financial calendars are frequently quoted by week number rather than by date, and everyone involved has to agree on the same numbering. The ISO rule removes ambiguity about which week a date near a year boundary belongs to, where informal counting tends to disagree. The day-of-year figure, from 1 to 365 or 366, is the ordinal date and is handy for logs, sequence numbering and quick gap estimates. Because the ISO definition anchors on Thursdays, the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next, which a week number calculator resolves correctly so you never mislabel a cross-year week.

Formula

ISO-8601 week: week 1 is the week containing the year's first Thursday; weeks run Monday–Sunday

The date is shifted to the Thursday of its own Monday–Sunday week, because ISO ties each week to its Thursday. The number of whole weeks between that Thursday and the first Thursday of its year, plus one, is the ISO week number. Anchoring on Thursday is what makes 1–3 January sometimes fall in the previous year's week 52 or 53, and 29–31 December sometimes fall in week 1 of the next year. Day of the year is the date minus 1 January.

Source:ISO 8601 / Gregorian calendar

We review all our tools from time to time to ensure accuracy.

Worked example

Date:2026-06-15

15 June 2026 is a Monday in ISO week 25, and it is the 166th day of 2026.

How to use this calculator

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3
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Frequently asked questions

What is the ISO-8601 week numbering rule?

Under ISO-8601, weeks start on Monday and week 1 is the week that contains the year's first Thursday, equivalently the week containing 4 January. This means the year's first calendar days can belong to the last week of the previous year, and a year has either 52 or 53 weeks. The rule is used across Europe and by most business and software systems for unambiguous week references.

Why can 1 January be in week 52 or 53?

Because ISO week 1 must contain the first Thursday, any early-January days that fall before that Thursday's Monday belong to the final week of the previous ISO year. For example, if 1 January is a Friday, it sits in the last week of the outgoing year. This is intentional: it keeps every ISO week a full seven days and tied to a single dominant year.

How many weeks are in a year?

An ISO year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday; otherwise it has 52. Most years have 52 weeks. The 53-week years, such as 2026, matter for payroll and retail calendars that budget by week, because they include one extra pay or reporting period compared with a 52-week year.

What is the day of the year used for?

The day of the year, also called the ordinal date, numbers each day from 1 on 1 January to 365 or 366 on 31 December. It is useful in logging, filenames, batch numbering and astronomy, and for quickly estimating the gap between two dates in the same year by subtracting one ordinal from the other. It complements the week number when you need a single running index.

Does the calculator use Sunday or Monday as the first day?

It uses Monday, following the ISO-8601 standard, so a week runs Monday through Sunday. This differs from the US convention where weeks are often shown starting on Sunday and numbered differently. If your organisation uses a Sunday-start or a different week-1 rule, the ISO number here may differ by one near the year's edges, so confirm which standard your systems expect.