Week Number Calculator
Find the week number of any date instantly with our precision tech-noir suite.
Convert Date to Week
ISO 8601
W27
US Week
27
Quarter
Q3
Day of Year
185
Remaining
180
Convert Week to Dates
Year 2026 Overview
ISO 8601 Explained
The ISO 8601 week date system is an international standard for designating days and weeks. A year has 52 or 53 full weeks. Week 01 of any year is defined as the week with the first Thursday of the year in it.
- check_circleWeeks always start on a Monday.
- check_circleThe first week of the year is the one that contains January 4th.
- check_circleThe US system differs, often starting weeks on Sundays and defining Week 1 as the week containing January 1st.
Frequently Asked Questions
Week Numbers: The Calendar Businesses Run On
Who counts in weeks
“Delivery in week 24” is normal speech in several industries:
| Domain | How weeks are used |
|---|---|
| Manufacturing & supply chains | Production schedules and lead times by week |
| European business partners | Meetings proposed as 'KW 24' (Kalenderwoche) |
| Retail planning | Seasonal resets and promotions by week number |
| Project management | Sprint and milestone weeks |
| Payroll (weekly/fortnightly) | Pay periods anchored to week numbers |
ISO 8601: why week 1 isn't January 1st
The ISO standard defines week 1 as the week containing the year's first Thursday (equivalently: the week of January 4th), with weeks starting Monday. Consequences that surprise: December 29–31 can belong to week 1 of the next year, January 1–3 can sit in week 52/53 of the previous one, and some years have 53 weeks. US systems often count differently (week 1 = the week of Jan 1, Sunday start) — when a partner quotes a week number, knowing which system they use prevents a one-week shipping error.
Date arithmetic companions
Spans between two dates (in days, the unit contracts use) compute in the date difference calculator; age on a cutoff date in the age calculator; and cross-timezone scheduling for that week-24 call in the meeting planner.