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Week numbers divide the year into 52 or 53 numbered weeks — a system widely used in business, payroll, manufacturing and project planning, especially across Europe. "We ship in week 27" is precise and unambiguous once everyone counts weeks the same way. This tool tells you the week number for any date instantly.
Enter a date, and its week number appears immediately.
How is it calculated?
How weeks are numbered
Under the common ISO 8601 standard, weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week containing 4 January). This means the first few days of January can belong to the last week of the previous year, and late December can fall into week 1 of the next.
Why the ISO rule exists
A simple "count from 1 January" approach produces awkward partial weeks. The ISO rule ensures every week number covers a full seven-day Monday-to-Sunday span, which is why payroll and scheduling systems rely on it.
52 or 53 weeks?
Most years have 52 weeks, but some have 53 — when the year has 53 Thursdays. This happens every five or six years and is worth noting for annual planning and reporting.
Common uses
- Business and manufacturing: production and delivery schedules ("week 27").
- Payroll: weekly and fortnightly pay periods.
- Project management: sprint and milestone planning.
- Retail and logistics: seasonal planning by week.
For counting days between dates, a days-between-dates calculator is the companion tool.
Worked example
1 January 2026 falls in week 1, and 1 July 2026 falls in week 27 under the ISO 8601 standard (weeks starting Monday, week 1 containing the first Thursday). Note the edge behaviour: a date like 31 December can sometimes belong to week 1 of the following year, and 1 January can belong to the last week of the previous year — which is why a consistent standard matters when teams coordinate by week number.
FAQ
How is the week number of a date found?+
The tool applies the ISO 8601 standard: weeks start on Monday and week 1 contains the year's first Thursday. Just enter a date to get its week number.
How does ISO week numbering work?+
Weeks run Monday to Sunday, and week 1 is the one containing 4 January (the first Thursday). This keeps every week a full seven days, unlike a naive count from 1 January.
Can a year have 53 weeks?+
Yes — a year has 53 weeks when it contains 53 Thursdays, which happens every five or six years. Most years have 52 weeks.
Why can 1 January be in week 52 or 53?+
Because ISO week 1 is defined by the first Thursday, the first days of January can belong to the last week of the previous year if they fall before that Thursday.
Where are week numbers used?+
In business, manufacturing, payroll, logistics and project planning — especially in Europe. "Delivery in week 27" is a precise way to schedule without a specific date.
Do all countries use the same week numbering?+
Most use ISO 8601 (Monday start), but some (e.g. parts of North America) count weeks from Sunday and number differently. This tool uses the ISO standard.