Week numbers are used in project management, manufacturing, logistics, and European business communication. Instead of saying 'the week of July 14,' teams say 'week 29,' which is unambiguous and concise.
Manufacturing and supply chain teams use week numbers for production scheduling. A factory in Germany producing parts for a car assembly plant in Spain might commit to delivery in week 32, meaning the week starting August 3, 2026. Both sides understand the exact timeframe without ambiguity.
Scrum teams use week numbers for sprint planning. A two-week sprint starting in week 28 ends in week 29. This is cleaner than saying 'the sprint starts July 13 and ends July 24' in every communication.
European businesses commonly use week numbers in invoices, purchase orders, and delivery schedules. If a Swedish supplier says delivery is in week 33, you need to know that means August 10-16, 2026.
Retailers use week numbers for sales tracking and inventory management. Comparing week 29 sales across years is more consistent than comparing 'the week of July 14,' which may fall in different weeks depending on the year.
The ISO 8601 standard defines week numbers based on the first Thursday of the year. Week 1 is the week containing the first Thursday, which is equivalent to the week containing January 4. This means January 1 may fall in week 52 or 53 of the previous year.
ISO weeks start on Monday and end on Sunday. This is different from the US system, where weeks typically start on Sunday. The tool shows both ISO and US week numbers for comparison.
The day of year (also called Julian day) counts how many days into the year a date falls. January 1 is day 1, and December 31 is day 365 (or 366 in a leap year). July 18, 2026 is day 199 of the year.
The tool also shows year progress (what percentage of the year has passed), the current quarter, and the full week at a glance with all seven dates.
Input
July 18, 2026 (Saturday)
Output
ISO week 29, US week 29, day 199 of 366, 54.4% of year complete
2026 is not a leap year, so 365 days total
Input
January 1, 2026 (Thursday)
Output
ISO week 1, US week 1, day 1 of 365
January 1 is a Thursday, so it falls in ISO week 1
Input
December 31, 2026 (Thursday)
Output
ISO week 53, US week 53, day 365 of 365
2026 has 53 ISO weeks because it ends on a Thursday
Input
February 29, 2024 (Thursday)
Output
ISO week 9, day 60 of 366
2024 is a leap year, so February has 29 days
Things to watch out for
What is the ISO week number?
The ISO 8601 standard defines week 1 as the week containing the first Thursday of the year, which is equivalent to the week containing January 4. ISO weeks start on Monday and end on Sunday. This system is used in most of the world outside the US.
Why does the US use a different week numbering system?
In the US, weeks traditionally start on Sunday rather than Monday. This can cause the US week number to differ from the ISO week number, particularly at the beginning and end of the year.
How many weeks are in a year?
Most years have 52 ISO weeks. A year has 53 ISO weeks if January 1 is a Thursday, or if it is a Wednesday in a leap year. This happens roughly every 5-6 years. 2026 has 53 weeks.
Can January 1 be in the previous year's week 52?
Yes. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year in the ISO system. ISO week 1 of the new year starts on the following Monday.
What is the day of year?
The day of year (also called Julian day) counts how many days have passed since January 1. January 1 is day 1, and December 31 is day 365 (or 366 in a leap year). July 18, 2026 is day 199.
How is year progress calculated?
Year progress is the day of year divided by the total days in the year, expressed as a percentage. On July 18, 2026 (day 199 of 365), the year is 54.5% complete.
What is the difference between ISO and US week numbers?
ISO weeks start on Monday and use the first-Thursday rule. US weeks typically start on Sunday. This means the same date can have different week numbers in the two systems, especially near January 1 and December 31.
Is the ISO week system used everywhere?
The ISO 8601 week numbering is standard in Europe, Asia, and most of the world. The US, Canada, and some other countries use different conventions. Always clarify which system you are using when communicating internationally.
Week numbers follow the ISO 8601 standard, which is the international standard used in most countries. US week numbers use a different convention (weeks starting on Sunday). Always clarify which system you are using in international communications.