What is an ISO week number?

It's the week-of-year number defined by the ISO-8601 standard: weeks run Monday through Sunday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing 4 January). Every date belongs to exactly one ISO week.

How does the ISO Week Number Calculator handle year-boundary dates?

It computes the correct ISO week-year, which can differ from the calendar year. For example, 1 January 2021 falls in ISO week 53 of week-year 2020, and some late-December dates fall in week 1 of the following week-year.

Do all years have 53 ISO weeks?

No. Most years have 52 ISO weeks; a year has 53 only when 1 January falls on a Thursday, or when it's a leap year and 1 January falls on a Wednesday. The calculator shows the correct count for whichever year you pick.

Can I convert a week number back to a date range?

Yes. Switch to Week → Date range mode, enter a year and a week number, and the tool returns the exact Monday start date and Sunday end date for that ISO week.

Is this the same as the week number in Excel or Google Sheets?

Only if you use the ISO week system (ISOWEEKNUM in Excel, or WEEKNUM with return type 21). The default WEEKNUM in most spreadsheets uses a Sunday-start convention that gives different results, especially near year boundaries.

Does the ISO Week Number Calculator upload the dates I enter?

No. All the date math happens locally in your browser. The dates and week numbers you enter never leave your device and nothing is sent to ArrayKit.

ISO Week Number Calculator Online

Find the ISO-8601 week number for any date, or the Monday–Sunday range for a given week, right in your browser.

The ISO Week Number Calculator runs entirely in your browser. Dates and week numbers you enter are calculated locally and never uploaded to ArrayKit.

Open the Date Calculator

About ISO Week Number Calculator

The ISO Week Number Calculator converts any calendar date into its ISO-8601 week number and week-year in one step. Pick a date and it shows you the exact week (like 2026-W03), the ISO week-year, the weekday, and the full Monday–Sunday range that week covers. Switch to the Week → Date range mode to go the other way: enter a year and a week number and get back the exact start and end dates. It correctly handles the ISO-8601 rule that week 1 is the week containing the year's first Thursday, so late-December and early-January dates land in the right week-year even when it differs from the calendar year, and it knows which years have 53 weeks instead of 52. Useful for sprint planning, payroll and billing cycles, shift schedules, and any spreadsheet or system that tracks work by week number. Everything runs locally in your browser — no dates are uploaded.

Features

Convert any date to its ISO-8601 week number and week-year
Shows the weekday (Monday–Sunday) for the selected date
Displays the full Monday–Sunday range for that ISO week
Reverse mode: enter a year and week number to get the date range
Correctly resolves the ISO week-year across year boundaries
Shows whether a given year has 52 or 53 ISO weeks
One-click 'Today' button to jump to the current week
Copy buttons for the week label, dates, and full range

How to use the ISO Week Number Calculator

Pick Date → Week mode and choose a date, or click Today
Read the ISO week number, week-year, and weekday
Switch to Week → Date range mode to go the other way
Enter a year and week number to see the Monday–Sunday range
Copy the week label or date range you need

Example

Input

2026-01-15

Output

ISO week: 2026-W03 (week 3)
Range: Mon 2026-01-12 – Sun 2026-01-18

15 January 2026 falls in ISO week 3, which runs Monday 12 January through Sunday 18 January.

Common errors & troubleshooting

A date in late December shows a week-year that doesn't match the calendar year. — This is correct ISO-8601 behavior: 31 December can belong to week 1 of the following week-year, and 1 January can belong to week 52 or 53 of the previous week-year, depending on which weekday they fall on.
Entering week 53 for a year gives an error. — Not every year has 53 ISO weeks — most have 52. The ISO Week Number Calculator tells you how many weeks a given year has so you know the valid range before you enter one.
The week number here doesn't match what my spreadsheet or calendar app shows. — Some tools use a US-style week numbering (Sunday-start, week 1 = the week containing 1 January) instead of ISO-8601. This calculator always follows the ISO-8601 standard: Monday-start weeks, week 1 contains the year's first Thursday.
The date range for a week looks shifted by a day. — ISO weeks always run Monday through Sunday. If you were expecting a Sunday-start or Saturday-start week, that's a different, non-ISO convention.

Frequently asked questions

What is an ISO week number?: It's the week-of-year number defined by the ISO-8601 standard: weeks run Monday through Sunday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing 4 January). Every date belongs to exactly one ISO week.
How does the ISO Week Number Calculator handle year-boundary dates?: It computes the correct ISO week-year, which can differ from the calendar year. For example, 1 January 2021 falls in ISO week 53 of week-year 2020, and some late-December dates fall in week 1 of the following week-year.
Do all years have 53 ISO weeks?: No. Most years have 52 ISO weeks; a year has 53 only when 1 January falls on a Thursday, or when it's a leap year and 1 January falls on a Wednesday. The calculator shows the correct count for whichever year you pick.
Can I convert a week number back to a date range?: Yes. Switch to Week → Date range mode, enter a year and a week number, and the tool returns the exact Monday start date and Sunday end date for that ISO week.
Is this the same as the week number in Excel or Google Sheets?: Only if you use the ISO week system (ISOWEEKNUM in Excel, or WEEKNUM with return type 21). The default WEEKNUM in most spreadsheets uses a Sunday-start convention that gives different results, especially near year boundaries.
Does the ISO Week Number Calculator upload the dates I enter?: No. All the date math happens locally in your browser. The dates and week numbers you enter never leave your device and nothing is sent to ArrayKit.

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