Week Number
ISO week of the year
About This Calculator
The ISO 8601 standard defines week numbering: week 1 is the week containing the year's first Thursday. ISO weeks run Monday to Sunday, and a year always has 52 or 53 weeks. Week numbers are widely used in business scheduling, academic calendars, and international project planning.
Formula
ISO week: Week 1 = week containing January 4 (or first Thursday)
Week runs Monday through Sunday
Week number = ceil((dayOfYear + dayOfWeek(Jan 1) − 1) / 7) (approximate)
A year has 53 weeks if it starts on Thursday, or is a leap year starting on Wednesday
Example Calculation
What ISO week number is March 16, 2026?
- January 1, 2026 is a Thursday (Week 1)
- March 16 = day 75 of 2026
- Week = ceil(75/7) ≈ Week 12
- Jan 1 (Thu) means week 1 starts Jan 1, so March 16 = Week 11 (verify: March 9-15 = Week 11, March 16-22 = Week 12)
March 16, 2026 = ISO Week 12
Week Number Rules
| Day Jan 1 falls | Week 1 starts | 53-week year? |
|---|---|---|
| Monday | Jan 1 | No |
| Tuesday | Dec 31 (prev) | No |
| Wednesday | Dec 30 (prev) | No (unless leap year) |
| Thursday | Dec 29 (prev) | Yes |
| Friday | Jan 4 | No |
| Saturday | Jan 3 | No |
| Sunday | Jan 2 | No |
Frequently Asked Questions
Does everyone use ISO week numbering?
ISO 8601 weeks are standard in Europe and international business. The US commonly uses a system where Week 1 starts January 1, and weeks may start on Sunday. Always clarify which system is being used when sharing week numbers internationally.
Why can a year have 53 weeks?
A 52-week year has 364 days, but most years have 365 (or 366) days. The extra day(s) push part of the year into a 53rd week. This happens when January 1 is a Thursday, or when it's a leap year starting on Wednesday or Thursday.
What does 'fiscal week' mean?
Many companies define a fiscal calendar that doesn't align with the standard calendar. A company's 'Week 1' might start on a date other than January 1. Retail and manufacturing often use a '4-4-5' quarter system with 13-week quarters.
How do I convert between week number and date?
For ISO weeks: the first day (Monday) of week W in year Y = January 4 of year Y + (W-1) weeks − (dayOfWeek of Jan 4 − 1) days. This formula handles the edge cases where Week 1 starts in the previous December.