Date calculator
Week Number.
Find the ISO week number for any date
Date math depth
How this week number page differs
Each calculator type uses the same interactive shell, so the page adds type-specific examples, edge cases, and canonical boundaries to avoid becoming a duplicate utility surface.
Best use case
Best for ISO-style planning when sprint, payroll, logistics, or reporting cycles refer to week numbers.
Worked example
ISO week 1 is the week containing the first Thursday of the year, so early January dates can belong to the previous ISO year.
Edge cases this page calls out
ISO weeks start Monday · Some years have 53 ISO weeks · For side-by-side ISO/US/simple weeks, use the dedicated week-number page
Richer companion route
The dedicated week-number page compares ISO, US Sunday-start, and simple week systems. This page stays indexable as the lightweight date-calculator variant, while the companion route handles deeper interactive intent.
Calculations are deterministic calendar math. User-entered dates stay in the browser component and are not turned into crawlable pages or structured data.
Week number calculator
The dedicated week-number page compares ISO, US Sunday-start, and simple week systems.
Open Week number calculator →Popular countdowns
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FAQ
What standard is used for week numbers?▾
ISO 8601. Week 1 is the week containing the first Thursday of the year. Weeks start on Monday.