Time & Date

Week Number Calculator

Find the ISO 8601 week number and related date information for any date.

Date

ISO Week Number

30W30

Week Year2,026

Day of Year203

QuarterQ3

Day of WeekWednesday

What is the Week Number Calculator?

A week number is the sequential count of weeks in a calendar year, starting from week 1. ISO 8601 (the international standard) defines week 1 as the week containing the first Thursday of January, which typically starts on a Monday. Week numbers range from 1 to 53, with most years having 52 or 53 weeks. Different regions and industries use week-based schedules for planning, reporting, and compliance.

How it works

The calculator takes any date you enter and applies the ISO 8601 algorithm: it finds the Monday of the week your date belongs to, then counts how many Mondays have passed since the Monday of week 1 (the week with January's first Thursday), plus one. The result is your week number. If a date falls before the first Thursday of January, it belongs to the last week of the previous year; if it falls after December's last Thursday, it may belong to week 1 of the next year.

weekNumber = ceil((dayOfYear + (4 - dayOfWeek)) / 7)

dayOfYear is the number of days since January 1st (1–366); dayOfWeek is the day of the week where Monday = 1 and Sunday = 7. This formula counts complete weeks and assigns partial weeks correctly according to ISO 8601.

Examples

Input	Result	Notes
1 January 2025	Week 1	January 1, 2025 is a Wednesday and falls within week 1 (Monday 30 Dec 2024 – Sunday 5 Jan 2025)
15 June 2025	Week 24	Mid-summer; useful for checking progress through the year in weekly intervals
31 December 2025	Week 1 (2026)	Year-end dates often belong to next year's week 1 under ISO 8601

How to use the Week Number Calculator

Enter any date using the date picker or type it directly (e.g., 2025-06-15 or June 15, 2025)

The calculator automatically determines the day of the week your date falls on

It applies the ISO 8601 week-numbering algorithm to find which week of the year the date belongs to

The week number is displayed (1–53); if it's at a year boundary, the correct year is shown

Use the week number in your payroll, project planning, or reporting systems

Compare multiple dates to find all dates within the same week

Benefits

Instantly resolve date-to-week confusion in payroll, project timelines, and reports without manual counting

Comply with ISO 8601 standard used across industries, governments, and data systems worldwide

Plan sprints, quarters, and schedules by week instead of remembering specific dates

Identify year-boundary edge cases where a date's week number belongs to a different calendar year

Eliminate rounding errors and timezone confusion when week numbers are critical for compliance

Tips & common mistakes

Common mistakes

Assuming week 1 always starts on January 1 — ISO 8601 week 1 is the week with January's first Thursday, so it may start in late December of the previous year

Confusing the day of the week with the week number — the day (Monday, Tuesday, etc.) is different from which week number it falls into

Using a non-ISO system by accident — many organizations use custom week-numbering; always confirm your system uses ISO 8601

Forgetting that some years have 53 weeks — most have 52, but years starting on Thursday or leap years starting on Wednesday may have 53

Tips

Bookmark this tool if you work with payroll, accounting, or project scheduling — it's faster than manual lookup tables

Use week numbers in meeting agendas and reports to ensure everyone refers to the same week (e.g., 'Q2 2025 Week 22' is unambiguous)

For international teams, specify 'ISO 8601 week number' to avoid confusion with regional or fiscal week systems

Check the year output for dates in late December or early January — the week number's year may differ from the calendar year

Frequently asked questions

Why does January 1 sometimes not be in week 1?

ISO 8601 defines week 1 as the week containing the first Thursday of January. If January 1 is a Monday, Tuesday, or Wednesday before the first Thursday, it belongs to the last week of the previous year.

Can a year have 53 weeks?

Yes. Most years have 52 weeks, but years starting on Thursday or leap years starting on Wednesday will have 53 weeks. This happens roughly every 5–6 years.

Is ISO 8601 week numbering used everywhere?

ISO 8601 is the international standard and is used in Europe, most of Asia, and many industries globally. The USA, Canada, and a few other regions sometimes use different systems; always confirm which standard your organization uses.

How do I find all dates in a specific week?

Once you know the week number, the week runs from the Monday of that week to the Sunday. For example, ISO 8601 week 20 in 2025 runs from Monday, May 12 to Sunday, May 18.

Why is week numbering important for payroll?

Payroll systems use week numbers to group timesheets and ensure consistent pay periods. A week-based system avoids confusion when month boundaries don't align with work schedules.

What if my organization uses a different week system?

Some companies use fiscal weeks or Sunday-start weeks. Confirm your organization's standard first; this tool provides ISO 8601 weeks, which is the global default.