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πŸ—“οΈ Week Number Calculator

Enter a date to see the ISO 8601 week number and the US-style week number side by side, or enter a year and week number to find that week's start and end dates.

ISO 8601 Week
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US Week (Jan 1 based)
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Related Tool

Just need the ISO week number with a Korean UI? Use the Week Number Calculator (Korea-specific, ISO only) β†’

Need months/weeks/days between two dates? Use the Months Calculator β†’

GUIDE

Learn more

01

ISO 8601 week vs US week

The ISO 8601 international standard treats Monday as the start of the week and counts the week containing the year's first Thursday as week 1. The common US convention instead treats the week containing January 1st as week 1, often with Sunday as the start of the week. These two conventions can produce different week numbers, especially near the start and end of the year, so this tool shows both values side by side to avoid confusion.
02

Finding a date range from a week number (reverse lookup)

Enter a year and an ISO week number, and this tool uses the ISO 8601 rule that January 4th of a year always falls in week 1 to calculate that week's Monday and Sunday dates. For example, in 2026 January 1st is a Thursday, so week 1's Monday is December 29, 2025 and its Sunday is January 4, 2026.
03

When week numbers are useful

Week numbers are used across many tasks that manage dates on a weekly basis: project scheduling, weekly reports, weekly sales and performance analysis, and confirming delivery or shipment weeks. Since the standard varies by country or system, checking both conventions side by side helps avoid mistakes.

Frequently asked questions

Why do the ISO week and US week sometimes differ?
ISO 8601 treats Monday as the start of the week and counts the week with the first Thursday as week 1, while the US convention treats Sunday as the start and counts the week containing January 1st as week 1. Because the rules differ, the two numbers can be off by about a week near the start or end of the year.
What standard is used for the year + week β†’ date lookup?
It uses the ISO 8601 rule: January 4th of the given year always falls in week 1, so the tool finds that week's Monday and adds (week number - 1) x 7 days to find the target week's Monday and Sunday.
Can a year have 53 weeks?
Yes. Under ISO 8601, a year can have 53 weeks depending on conditions such as January 1st or December 31st falling on a Thursday.