Week Number Calculator 2026 – ISO Week Number & Date Lookup
Find the ISO week number for any date, look up dates by week number, and see week start/end dates. Perfect for project planning and scheduling.
Updated for 2026
By the GlobalCalqulate team, founded by Pavan Kusunuri · About our editorial standards
Frequently Asked Questions
Clear answers to common questions to help you use this calculator confidently.
What is the current week number?
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What is the current week number?
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The current ISO week number changes every Monday. Use the calculator to find the exact week number for any date.
How are ISO week numbers calculated?
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How are ISO week numbers calculated?
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ISO week numbers follow ISO 8601: weeks start on Monday, week 1 contains the first Thursday of the year, and weeks run Monday through Sunday.
Why do some years have 53 weeks?
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Why do some years have 53 weeks?
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Years with 53 weeks occur when January 1 falls on a Thursday (common year) or on a Wednesday or Thursday (leap year).
What is the difference between Monday-start and Sunday-start weeks?
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What is the difference between Monday-start and Sunday-start weeks?
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ISO standard uses Monday start. Some US calendars use Sunday start. The calculator supports both.
How do I find the week number for a past date?
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How do I find the week number for a past date?
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Simply enter the date in the calculator and select 'Get week number from a date' mode.
Can I find dates for a specific week number?
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Can I find dates for a specific week number?
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Yes. Select 'Get date range from a week number' mode, enter the year and week number.
What is day of year?
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What is day of year?
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Day of year is the sequential day number from January 1 (day 1) to December 31 (day 365 or 366).
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