Date math
Week Number Calculator
Find the ISO 8601 week number for today or any date.
ISO 8601 Mon-start week Instant
What is a Week Number Calculator?
A Week Number Calculator returns the ISO 8601 week number for any date — a standardized way to identify which week of the year a date belongs to. Under ISO 8601, weeks start on Monday, and week 1 is the week containing the year's first Thursday.
Week numbers are widely used in business, project management, manufacturing, and finance to report and schedule work at a weekly granularity — for example, "ship by week 42" or "Q3 financials due week 37."
Example: picking July 1, 2026 → Week 27 of 2026 — running from Monday, June 29 through Sunday, July 5.
How to Use the Week Number Calculator
Select any date from the date picker and click Find Week Number. The tool shows the ISO 8601 week number, the corresponding week year (which may differ from the calendar year near January 1), the day of the year, and the start and end dates of that week.
For the current week, tap the Today preset. To check how a year starts or ends, use the Jan 1 or Dec 31 presets. Week numbers near the year boundary often belong to the adjacent year under ISO 8601.
Common Uses
- Project scheduling – Report progress in "week 12 of 52" format
- Manufacturing date codes – Many products stamp "YYWW" (year + week) on packaging
- Finance and accounting – Weekly revenue, payroll, or close cadence
- Academic calendars – Track which week of the semester or term
- Shift rotation – Identify which week employees are on for rotating schedules
- Pregnancy and baby milestones – Gestational week tracking follows the same Monday-start convention
How ISO 8601 Week Numbering Works
Under ISO 8601, weeks always start on Monday. Week 1 of any year is the week that contains that year's first Thursday — equivalently, the week that contains January 4.
Because of this rule, the first few days of January can belong to week 52 or 53 of the
previous year, and the last few days of December can belong to week 1 of the
next year. This is why ISO week numbers are often written with a "week year,"
like 2026-W01-1 for Monday of week 1, 2026.
A year has 53 ISO weeks when January 1 falls on a Thursday, or when it's a leap year and January 1 falls on a Wednesday. All other years have 52 weeks. Examples of 53-week years in the 2020s: 2020, 2026.
Frequently Asked Questions
What is the current week number?
Tap the Today button above to see the current ISO week number. Week numbers run 1 to 52 (or 53 in some years), and reset each year.
What's the difference between ISO and US week numbering?
ISO 8601 weeks start on Monday and week 1 is the week containing the first Thursday of the year. The US convention often uses Sunday as the start of the week and counts week 1 as the one containing January 1. Broadcast and financial systems in the US may use their own conventions. This calculator uses ISO 8601 — the international standard.
How many weeks are in 2026?
2026 has 53 ISO weeks, because January 1, 2026 falls on a Thursday. Most years have 52 weeks; roughly one year in every 5–6 has 53.
Why does the calculator show a different year for some dates?
Under ISO 8601, the "week year" can differ from the calendar year near January 1 and December 31. For example, December 31, 2018 is ISO week 1 of 2019 because the week it belongs to contains Thursday, January 3, 2019.
What week of the year is my birthday?
Enter your next birthday date into the calculator to see its ISO week number. For a countdown to your birthday, use our Days Until Birthday Calculator.
Does the calculator show the day of the year too?
Yes. Along with the ISO week number and week year, the result includes the day of the year (1–366) and the Monday–Sunday range of the week that contains your date.