Week Number Calculator

How to use this calculator

1. Enter a date (year, month, day).
2. We apply ISO week rules (weeks start Monday; Week 1 contains the first Thursday).
3. See the ISO week number, ISO year, and a formatted label like 2025-W06.

Inputs explained

Year

Calendar year for the date you’re checking.

Month

Calendar month number (1–12).

Day

Day of month (1–31, depending on month/leap year).

When to use it

• Coordinating releases or sprints that use ISO week references.
• Logistics and manufacturing schedules that run on week numbers.
• Content calendars or financial reporting aligned to ISO weeks.

Tips & cautions

• ISO weeks can shift into the next year; Dec 31 may be Week 1 of the next ISO year.
• If your organization uses Sunday-start weeks, note the difference before sharing.
• Combine with the Day of Week calculator for a full date + weekday + week-number snapshot.
• Uses ISO 8601 rules only; regional fiscal or retail week systems may differ.
• Assumes the Gregorian calendar.
• No built-in support for custom week numbering schemes.

Worked examples

Deep dive

FAQs

What calendar does this follow?

ISO 8601 weeks: Monday is the first day of the week and Week 1 contains the first Thursday of the ISO year.

Can I use it for US week numbers?

Most project tools follow ISO weeks globally. If you use a different convention, note the difference before sharing.

Why is December 31 sometimes in Week 1 of the next year?

ISO weeks are tied to weeks, not calendar years. If the last days of December contain the first Thursday of the next ISO year, they belong to that next ISO week year.

Does it handle leap years?

Yes. Leap days are counted in the day-of-year calculations that drive ISO week placement.

Do weeks start on Monday or Sunday?

ISO weeks start on Monday. If you need Sunday-start weeks, adjust expectations before sharing the result.

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