Calculate ISO Week Number for Any Date Get the ISO week number for any date along with week ranges and quarter info.

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Week Number Calculator

Get the ISO week number for any date along with week ranges and quarter info.

Select a Date

Choose any date using the date picker.

View Week Number

See the ISO week number, US week number, and ISO notation.

See Details

View day of week, day of year, quarter, and the week's date range.

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What Is Week Number Calculator?

The Week Number Calculator determines the ISO 8601 week number for any given date. ISO weeks start on Monday, and the first week of the year is the one containing the first Thursday. This tool displays the ISO week number, the ISO year-week notation (YYYY-Wnn), the US week number (which uses Sunday as week start), the day of the week, the day of the year, the number of days remaining in the year, the quarter, and the Monday-to-Sunday date range for the week. It's widely used in European business environments, logistics, manufacturing, and project planning where week numbers are the standard way to reference time periods.

Why Use Week Number Calculator?

Instant ISO week number lookup
Shows both ISO and US week numbering
Full context: day of year, quarter, week range
ISO year-week notation for standard business use
Clean, information-dense display
Runs entirely client-side

Common Use Cases

European Business

Reference specific weeks in business communications using ISO week numbers.

Manufacturing

Track production schedules and batch numbers by week.

Logistics

Plan shipping and delivery schedules using week-based timelines.

Project Planning

Set milestones and deadlines by week number rather than specific dates.

Reporting

Align weekly reports and analytics with standardized week numbers.

Technical Guide

The ISO week number is calculated using the standard algorithm: 1) Copy the date and set it to the nearest Thursday (current date + 4 - current day of week, treating Sunday as 7). 2) Determine the year of that Thursday — this is the ISO year. 3) Calculate the ordinal day of that Thursday within its year. 4) Divide by 7 and ceiling to get the week number. The US week number uses a simpler calculation: (day of year + first day's offset) / 7, where weeks start on Sunday. The week range is computed by finding the Monday on or before the date and adding 6 days for Sunday. Quarter is simply Math.ceil(month / 3).

Tips & Best Practices

1
ISO weeks always start on Monday — the first week contains January 4
2
The ISO year in the week notation may differ from the calendar year near year boundaries
3
Use week numbers for sprint planning in agile development
4
The week range shows exactly which dates fall within that ISO week
5
Pair with the Calendar Generator to visualize weeks within a month

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Frequently Asked Questions

Q What is an ISO week?

ISO 8601 defines weeks as starting on Monday, with the first week of the year being the one containing the first Thursday of January.

Q Why is the ISO year sometimes different?

Near year boundaries, a date in late December might fall in ISO week 1 of the next year, or a date in early January might fall in week 52/53 of the previous year.

Q How is the US week number different?

The US week starts on Sunday and counts from January 1, while ISO weeks start on Monday with a different first-week rule.

Q Can a year have 53 ISO weeks?

Yes, some years have 53 ISO weeks. This happens when January 1 is a Thursday, or when it's a leap year and January 1 is a Wednesday.

Q What is the week notation format?

ISO week notation uses the format YYYY-Wnn, where YYYY is the ISO year and nn is the zero-padded week number (01-53).

About This Tool

Week Number Calculator is a free online tool by FreeToolkit.ai. All processing happens directly in your browser — your data never leaves your device. No registration or installation required.