Calculate week numbers, find dates by week, convert between ISO weeks and dates, and analyze weekly patterns.
The Week Calculator converts between dates and ISO week numbers, finds the current week of the year, 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.
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 one place keeps the schedule consistent across systems that use different conventions.
ISO Week Number: The week containing January 4th is Week 1. Weeks start Monday. Day of Year = (date - Jan 1) / 86400000 + 1. % of Year = Day of Year / (365 or 366) × 100. Weeks Remaining = Total Weeks - Current Week.
Result: Week 24 of 2025
June 9, 2025 is in ISO Week 24. This week runs from June 9 (Monday) to June 15 (Sunday). Day 160 of 365, which is 43.8% through the year.
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.
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" (e.g., "2524" means Week 24 of 2025).
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: 2004, 2009, 2015, 2020, 2026.
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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.
Yes — January 1 might be in Week 52 or 53 of the previous year. For example, January 1, 2021 was in Week 53 of 2020.
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).
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.
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.
ISO 8601 is the international standard and is the safest choice when multiple regions or systems need to agree on the same week number.