Week Calculator

About the Week Calculator

The Week Calculator converts between dates and ISO week numbers, finds the week of the year for the reference date on the page, and returns the matching date range when you start from a week number instead of a calendar date.

ISO weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday of January. That convention is used widely in business reporting, production planning, and payroll schedules because it avoids some of the ambiguity of month-based planning.

The page also shows how much of the year has passed, how many weeks remain, and the quarter the week belongs to, so you can use the same date in a few different planning contexts.

When This Page Helps

Week numbers are useful when a project, payroll cycle, or reporting period is easier to track by week than by date. Converting between dates and week numbers in the same result keeps the schedule consistent across systems that use different conventions.

How to Use the Inputs

1. View the week number and date range for the reference date automatically
2. Enter any date to find its ISO week number
3. Enter a week number and year to find the date range
4. See how many weeks remain in the year
5. Check which quarter the week falls in
6. Toggle between ISO (Mon start) and US (Sun start) conventions
7. Use the annual week calendar for reference

Example Calculation

Tips & Best Practices

• ISO weeks always start on Monday and end on Sunday
• Week 1 always includes January 4th — a quick way to check
• Most European and international businesses use ISO weeks
• Week numbers are handy for bi-weekly payroll schedules
• Q1 = Weeks 1-13, Q2 = 14-26, Q3 = 27-39, Q4 = 40-52
• Calendar apps often have an option to show week numbers

ISO 8601 Week Numbering

The ISO 8601 standard defines a clear, unambiguous week numbering system. Week 1 is defined as the week containing the year's first Thursday, or equivalently, the week that contains January 4. Weeks always start on Monday. This means January 1, 2 or 3 can fall in the last week of the previous year. The system ensures every day belongs to exactly one week of exactly one year.

Week Numbers in Industry

Manufacturing uses week numbers extensively for production planning, quality tracking, and supply chain management. "Ship by W26" means ship by the end of Week 26. Automotive, electronics, and FMCG industries all rely on week-based scheduling. Date stamps on products often use the format "YYWW" (for example, "0924" means Week 24 of 2009).

52 vs 53 Week Years

Most years have 52 ISO weeks, but some have 53. A year has 53 weeks when January 1 falls on Thursday, or when it's a leap year and January 1 falls on Wednesday or Thursday. On average, 53-week years occur every 5.6 years. Notable 53-week years include 1998, 2004, and 2009.

Frequently Asked Questions

• What is an ISO week?

  ISO 8601 defines weeks starting Monday. Week 1 is the week containing January 4th (or the first Thursday in January). Most of the world uses this standard.

• Can a year start in Week 1 or Week 52/53?

  Yes — January 1 might be in Week 52 or 53 of the previous ISO year. In some year transitions, the first few January dates still belong to Week 53 of the prior ISO year.

• Do all years have 52 weeks?

  Most have 52, but some years have 53 ISO weeks. This happens roughly every 5-6 years when the year starts on Thursday (or Thursday/Friday for leap years).

• What's the US convention?

  In the US, weeks often start Sunday and Week 1 is the week containing January 1. This can differ from ISO by up to a week.

• Why do businesses use week numbers?

  Week numbers are unambiguous ("Week 24" vs. "the second week of June") and align with standard reporting periods. Manufacturing, logistics, and payroll extensively use them.

• Which week number standard should I use?

  ISO 8601 is the international standard and is the safest choice when multiple regions or systems need to agree on the same week number.